WISE finds the very first Earth Trojan asteroid

By Phil Plait | July 27, 2011 11:17 am

NASA’s Wide-field Infrared Survey Explorer (WISE) has found the very first asteroid that (more or less) shares an orbit with Earth! Called 2010 TK7, this asteroid is about 300 meters (roughly 1000 feet) across, and is the first in an up-to-now theoretical class of objects called Earth Trojans.

Here’s a WISE image of the little bugger:

Doesn’t look like much, does it? Of course, from 80 million km (50 million miles) from Earth it’s amazing we can see it at all. Moreover, given its position in the sky, it’s only up during the day as seen from Earth; it was only discovered because WISE orbits the Earth, so the sky is always dark. Also, WISE sees in the infrared, so warmer objects are easier to spot. This rock is probably around the freezing point of water or so, which, to an astronomer, is pretty warm.

So what makes this asteroid special?


If you have a large body (like the Sun) orbited by a smaller body (like the Earth), then there exist some points in space where, if you place a much smaller object (like an asteroid) it will remain there. Normally, if you put this smaller body in some random spot, the gravity of the two objects will alter its path, making its orbit unstable. But these five special places, called Lagrangian points, are stable. Well, kinda. Two are stable, in that if you put an object there it’ll tend to stay there even if you poke it gently — think of it like a small dip in flat table. Put a marble there and it’ll stay put, even if you push it a little. It’ll fall back into place in the center of the dip. In the diagram here, those are the L4 and L5 points.

The other three points (L1, 2 and 3) are stable as long as nothing perturbs any objects located in them; if you do poke an object orbiting there, it’ll tend to keep moving. Think of each these points as a bump in the table, where you can just barely balance a marble. It’ll stay there until you push it, then it rolls away. That’s called an unstable equilibrium, if you want to impress people at parties.

Anyway, the first real-world (real-Universe?) example of this was found in 1906, when an asteroid was spotted in Jupiter’s L4 point, 60° ahead of it in its orbit. It was named Achilles, which started the tradition of naming them after characters involved with the Trojan War (the asteroids ahead of Jupiter are named after Greeks, and the ones behind at the L5 point after Trojans). Since then, Trojan asteroids have been found for Mars as well as Neptune.

2010 TK7 is in Earth’s L4 point, and is the first Earth Trojan to be found. Since this point is 60° ahead of the Earth, from our point of view any object there is up during the day (or twilight), making it really hard to spot. And it isn’t just sitting out there in the L4 point, either. Remember the marble analogy? If you poke the marble, it’ll move around but settle into the bottom of the dip. That’s due to friction with the table. But in space there is no friction, and it’s possible for objects in the L4 and L5 points to move around those points, in a sense orbiting them. Technically, there are called Lissajous orbits (pronounced "LEE-sa-jzoo"), and are stable over long periods, though the shape of the actual orbit can change.

It’s all very weird, but the bottom line is that 2010 TK7 can sometimes be nearer the Earth than other times, though it never gets close enough to be a danger (it’s always many tens of millions of kilometers away). As soon as I heard about this, I wondered if it would make a good target for exploration; since it’s not moving much relative to Earth, it wouldn’t take much fuel to get there. Unfortunately, the Lissajous orbit of the asteroid takes it well above and below the Earth’s orbital plane (as shown in green in this not-to-scale diagram; click to odysseyenate), making a rendezvous difficult. That’s too bad, but there are plenty of other near-Earth asteroids that make good future targets.

And still, there may be other asteroids loitering around the L4 and L5 points, smaller or darker and therefore harder to spot. Hopefully more will be found as the WISE data is mined by astronomers.

Which brings up an interesting point: eventually, 2010 TK7 will have to be given a real name. I’ll toss in my two cents here: it should be named either Coeus or Crius, the sons of Gaia, the Greek goddess of the Earth*. As far as I can tell, those names have not been used yet for asteroids, so that’s a plus. We found one asteroid there, so we’re bound to find another, and since those guys were brothers it seems like an appropriate investment in the future of Trojan astronomy.

[UPDATE: More info can be found on Paul Weigert’s site, one of the co-discoverers of 2010 TK7 [Update update: I have been informed that the discovery credit goes to the WISE/NEOWISE team, so my apologies]. A preprint of their paper is also available (PDF).]


* Oceanus was also a child of Gaia, but that sounds like a planet with water on it. Probably best to leave that one for an exoplanet To Be Determined Later.

Image credits: NASA/JPL-Caltech/UCLA; Wikipedia


Related posts:

Dawn of a new Vesta
The WISE family comets
No, YU55 won’t destroy the Earth
When asteroids collide

CATEGORIZED UNDER: Astronomy, Cool stuff, Top Post

Comments (139)

  1. Dragonchild

    “TK7, why aren’t you at your. . . oh, never mind.”

  2. Chris

    So does this mean Earth is not a planet, but a dwarf planet? ;) I ask tongue-in-cheek (no flames please)- If this is the criteria used for Pluto (I am not sure the below is correct):

    For an object to be a planet, three requirements are defined by the IAU:

    It needs to be in orbit around the Sun – Yes, so maybe Pluto is a planet.
    It needs to have enough gravity to pull itself into a spherical shape – Pluto…check
    It needs to have “cleared the neighborhood” of its orbit – Uh oh. Here’s the rule breaker. According to this, Pluto is not a planet.

  3. Linda

    Ha! That is so cool!

  4. Wait, if it’s in the L4 spot, doesn’t that mean it appears to us to be 60 degrees away from the Sun? (Earth-Sun-L4 forms an equilateral triangle.) So why would it only be up during the day? Seems like there should be a roughly 4 hour gap (depending on latitude and time of year) between Sun-set and L4-set…

    Oh, and Chris, I know you may be tongue-in-cheek but to seriously answer the question… Pluto has not “cleared its orbit” in the sense that Pluto is not the overwhelming majority of the mass in its orbital zone. The Earth *is* the overwhelming majority of the mass of our orbital zone.

    Look up “Clearing the neighbourhood” on Wikipedia. (I find when I cite direct links I get relegated to moderation limbo so forgive me if I don’t link directly to the article.)

  5. Chris A.

    Umm, clicking the last image did nada–I think you forgot to embed the link, Phil.

  6. @Chris we have cleared the neighborhood. Other planets too have trojans. They are stable objects, not wandering rocks in our orbit.

  7. jsb16

    @Chris #2, I’m pretty sure the “cleared the neighborhood” requirement makes some mention of nearly equivalent-sized objects. That’s why Mars and Jupiter and Earth (all of which have Trojan asteroids) are still planets, but Pluto, an orbit with other plutoids, is not.

  8. Alan Williams

    Phil, Thank You for the analogy! 2 of my most-loaned-out books are Niven and Pournelle’s Mote stories, and the story-lines make extensive use of “Leading Trojan” and “Trailing Trojan.” I inevitably get asked questions about this, and while I can stumble around an explanation that “kinda-sorta” makes sense without having to resort to math, this lays it out on the table (as it were.)

    Your enthusiasm for science and science education is infectious, and I refer people to your site all the time. (Typhoid Plait?)

    Alan Williams

  9. Anon

    Aren’t there any probes at L4? I thought there were a couple of Sun observatorise there, though I might be confusing it with L1… I did read a plan to launch space stations to the L4 and L5 points. These asteroids would make nice mines to build those from.

  10. Toevallige Voorbijganger

    Why name a Trojan after the sons of a Greek goddess ? Makes no sense.

  11. Melissa Dow

    But neither of those mythological deities were involved in the Trojan war! (Or does that naming protocol only apply to Jovian asteroids?)

  12. andy

    Well asteroids in other types of 1:1 resonance with the Earth were known already, e.g. 3753 Cruithne which is sometimes described (inaccurately) as Earth’s second moon.

  13. The SOHO spacecraft is parked at L1 while the James Webb Space Telescope is (was?) going to sit at L2. As far as I know, there are no spacecraft at either L4 or L5.

  14. Alan Clark

    The statement that it would be up during the day is wrong – it could be 40 degrees above the horizon while the sun is 20 degrees below, so it would be well above the horizon during total darkness.

  15. davidlpf

    Quick we need some protection from it.

  16. That diagram looks like a still from this video http://vimeo.com/26949350 marked “Earth’s Trojan companion – Asteroid 2010 TK7: Video courtesy of Athabasca University, the University of Western Ontario and the Canada-France-Hawaii Telescope.”

    In the video, 2010 TK7’s orbit is roughly at a right angle to the ecliptic plane. The orbit is centered on a point which passes through L4 from time to time. This point oscillates from end to end of a long arc of Earth’s orbit having L4 at its center. So, over many orbits, 2010 TK7 traces the curved face of a deformed cylinder whose center point is L4.

  17. amphiox

    re #2;

    There are two details primarily to remember:

    1. Of all the objects floating around in earth’s orbital area, the Earth and Moon together constitute >99.99% of the total mass. Of all the objects floating around in Pluto’s orbital area, Pluto and Charon and the other moons constitute <0.1% of the total mass.

    2. The Lagrange points (earth's) are defined by the interaction of earth's gravity with the sun's gravity. In other words, everything that is situated within an L4 or L5 orbit is there because, essentially, earth's gravity forced it there. In short, Earth cleared the object from the earth-area orbit and dropped in into the “wastebasket” stable Lagrange point/area. Similarly, Pluto orbits in a resonance orbit with Neptune. And it is in that orbit because Neptune’s gravity forced it there, and keeps it there. Neptune cleared Pluto from its orbit.

    Basically, “clearing your orbit” means gravitationally dominating the orbit. Objects that are cleared from the orbit can either be ejected, absorbed, captured (turned into a moon), or put into a stable orbit whose character is defined by the gravity of the dominant mass.

    Note that if the mass of an object in a Lagrange area exceeds a certain percentage of the dominant mass (something around 10% or so), it will no longer be held in the Lagrange point stably, and will drift out. Trojan objects must always remain a minuscule fraction of the mass of the dominant body in the orbit.

  18. Jon

    Your first point about this object being the first discovered sharing Earth’s orbit doesn’t consider Earth’s companion asteroids such as Cruithne. What I think is interesting about TK7 is oscillate between L3 and L4.

  19. Yoxi

    It should, of course, be named Neville – Neville and Luna, a match made in heaven.

  20. gamercow

    This is very cool, but at the same time, potentially dangerous for future NASA missions. If there’s something at the L4 and L5 points, anything man-made there would be more prone to collisions.

  21. Uh oh. Theia’s little brother has come back to finish the job! Quick, hide!

  22. Will

    Just learned about different states of equilibrium in physics; about an hour later, Phil applies it to something awesome for me. It kinda makes me want to be an astronomer.

  23. Renee

    Hmmmmmm. Since the Earth has obviously failed to clear its orbit of other objects, it can no longer be classified as a planet, right? Hold on Pluto … here we come!

  24. Amos Newcombe

    “the asteroids ahead of Jupiter are named after Greeks, and the ones behind at the L5 point after Trojans”

    Is this really true? I thought I remember reading years ago that it was not.

  25. Carl

    Earth Trojans, Moons of Pluto, Exoplanets, Vesta’s surface, Enceladus rains on Saturn: lots of interesting and exciting discoveries recently. Too bad none of it involves human exploration and it won’t for a long time.

    What is the meaning of odysseyenate?

  26. Sam H

    @13: Only the ones we don’t know about ;). I have been recently engrossed in the best birthday present I’ve received in a while – Paul Davies’ Eerie Silence, which has offered me so many amazing new ideas (plus fresh takes on old ones) for extraterrestrial life I’m just loving every page. But anyway – in mentioning the take of some (such as Frank J. Tipler) on the Fermi Paradox in relation to the apparent absence of von Neumann machines and alien space probes hidden in our solar system, he made it clear that there are many places in the solar system where they could still be hiding. SETI searches have already been performed on L4 and L5 to no avail, but so far no one’s thought of sending signals over there in order to “activate” any probes that may have been dormant for eons.

    As for the L-points in relation to humanity, while the great cylinders and spheres that O’Neill dreamed up won’t even have the possibility of coming close fruition for a very long time, NASA should seriously consider sending a series of solar observatories to each L-point, including L3. In any future human Mars mission (or any deep space mission going beyond the Earth’s magnetic/gravitational influence), this could give crews a (relatively) early warning system for protection against otherwise potentially lethal solar events.

  27. mhhhh

    uff Question:

    Wasnt there are more spectacular one ?

    the one with the “U” move form, when viewed from earth.
    The one that once is slower than earth, then earth is getting from his back nearer, he gets “shoot” higher and moves slightly faster then earth. then “some” time later he “coming from the back” and gets pushed done (gets slower) and earth spins faster then him and when earth catches him the game continuous from the beginning ? (sorry couldnt find the link now)

    Hope someone understands me, else i will find a link tomorrow.

    But yeah cool we got a “static” one.

  28. Mandydax

    If it’s at the third point of an equilateral triangle with the Earth and Sun, shouldn’t its distance be equidistant from each, 150M km (93M mi)? I could see the perturbation around the Legrange point varying a bit, but nearly 50%? Are you sure it isn’t at 80M mi, not km?

  29. truthspeaker

    Sam H Says:
    July 27th, 2011 at 1:47 pm

    @13: Only the ones we don’t know about ;) . I have been recently engrossed in the best birthday present I’ve received in a while – Paul Davies’ Eerie Silence, which has offered me so many amazing new ideas (plus fresh takes on old ones) for extraterrestrial life I’m just loving every page. But anyway – in mentioning the take of some (such as Frank J. Tipler) on the Fermi Paradox in relation to the apparent absence of von Neumann machines and alien space probes hidden in our solar system, he made it clear that there are many places in the solar system where they could still be hiding.

    This just makes me WTF.

    Even if other intelligent civilizations exist or existed in the universe, hell, even if one existed in our galaxy, why would it send probes to our solar system? Sending a probe to every yellow star in our galaxy would be a huge expense even a for a civilization far more advanced than ours.

    Even if there is, or was, other intelligent life in our galaxy I wouldn’t expect to find their artifacts in our solar system.

    We are a very small needle in a very large haystack.

  30. John W.

    Jon, Cruithne has a horseshoe-orbit. It doesn’t “orbit” just one of the two trojan points, but bounces back and forth around both. (The classic example of this is Janus and Epimetheus around Saturn.)

    Amos: There is one interloper in each camp. I believe Patrocolus is in the Trojan group and Hektor is in the Greek. (Maybe because they were both killed wearing Achilles armor? I don’t know.)

    As far as danger to spacecraft: there’s certainly more risk of collisions if the Lagrange points have clusters of bodies in them, but remember: space is mind-bogglingly big (borrowing from Douglas Adams). This is even more true of the L4 and L5 points since they’re not stable in the conventional sense (the various potentials don’t yield a local minimum there), but because Coriolis forces cause objects to orbit them. This means that trojan bodies are spread out around the Lagrange points quite a bit. So while this can enhance collision rates, it’s not necessarily a serious threat.

  31. J. D. Mack

    Wait a second. There’s a rock in space with no atmosphere and it’s around 0 degrees Celsius? Why? Is sunlight sufficient to warm its surface to that high a temperature?

  32. Andrés

    Why the vertical (relative to that orientation of course) trails? Something to do with the sensor? Just a visual artifact?

  33. amphiox

    Even if other intelligent civilizations exist or existed in the universe, hell, even if one existed in our galaxy, why would it send probes to our solar system? Sending a probe to every yellow star in our galaxy would be a huge expense even a for a civilization far more advanced than ours.

    The problem goes away with von Neumann probes, because they are self replicating. A single von Neumann probe, directed to simply go to the nearest star indiscriminately, do some science, then replicate itself a few times from local resources, and have each daughter probe go indiscriminately to the nearest stars and repeat the process, would fill the entire galaxy with probes in only a few hundred million years, even if the probes travel sublight.

    It’s a version of the Fermi Paradox (in which instead of self-replicating probes, you have self-replicating aliens colonizing directly).

    The question of course, is whether/why any alien civilization would want to produce such probes.

  34. So this explains the dust I’m always eating?

    Who will claim* the 300-metre real estate?

  35. dschol:

    Wait, if it’s in the L4 spot, doesn’t that mean it appears to us to be 60 degrees away from the Sun? (Earth-Sun-L4 forms an equilateral triangle.) So why would it only be up during the day? Seems like there should be a roughly 4 hour gap (depending on latitude and time of year) between Sun-set and L4-set…

    I agree. Somewhat. Given the L4 location, it rises and sets before the sun, not after. You need to see it before sunrise.

    Still, given how small it is, there may be too much glare from the predawn Sun to see it through the atmosphere.

  36. BA:

    As soon as I heard about this, I wondered if it would make a good target for exploration; since it’s not moving much relative to Earth, it wouldn’t take much fuel to get there. Unfortunately, the Lissajous orbit of the asteroid takes it well above and below the Earth’s orbital plane

    So, why not place an exploration ship near the L4 point, and examine everything in the area? And L5, while you’re at it. You don’t really think that 2010 TK7 is the only thing out there?

  37. Pete Jackson

    @4,14,36: You’re all right, it should not be difficult to do ground-based observations of the L4 and L5 areas at 60 degrees elongation from the Sun. After all, Venus’ greatest elongation is 47 degrees and it is plenty dark while it is still high in the sky. There are some caveats, however. First, at high northern or southern latitudes, you need to observe in the evening sky in the spring and the morning sky in the fall. And the zodiacal light will add sky brightness.

  38. Nitpick: the unstable Lagrange points are like saddles, not local maxima. If you push objects VERY CAREFULLY along a specific curve they’ll move back to the LAgrange points, but in any other direction they’ll fall away down the saddle.

  39. Brian Davis

    John W. said: “This is even more true of the L4 and L5 points since they’re not stable in the conventional sense (the various potentials don’t yield a local minimum there), but because Coriolis forces cause objects to orbit them”

    I wanted to repeat that, as it’s something that seems to come up pretty regularly, and in this case (a rare case!) Phil gets it… a little bit off.

    L4/L5 are *not* potential minima. They don’t act like dimples in a table or a surface at all. In fact they are in a sense potential *maxima*, and are only stable in that the Coriolis force modifies the situation. So perversely, if there is enough stuff “hanging out” in the vicinity of L4/L5, these are not stable positions due to drag forces (drag reducing velocity, velocity reducing Coriolis force, leading to loss of some objects).

    That’s not just academic (in that I’m a teacher pointing out a picky little point), but a significant issue in figuring out how you get things into and out of L4/L5.

  40. Tom

    You say “The other three points (L1, 2 and 3) are stable as long as nothing perturbs any objects located” this should read “are equilibrium points where the forces on any objects there are balanced…” Your usage of stable here is wrong.

    The definition of unstable, as you later say these points are, is exactly that perturbations move the objects away. You have a nice write up here, and it’s technical enough that you should use the correct definitions of stable and unstable.

  41. Fred X. Quimby

    I call shotgun!

  42. CB

    @ Sam H

    But anyway – in mentioning the take of some (such as Frank J. Tipler) on the Fermi Paradox in relation to the apparent absence of von Neumann machines and alien space probes hidden in our solar system, he made it clear that there are many places in the solar system where they could still be hiding.

    Sounds like an interesting book. But what an understatement that is!

    I like the Fermi Paradox because it’s an interesting question. But what bothers me is when some people to take it to be a real paradox, as in a contradiction between two ideas that implies that alien civilizations are nonexistent or exceedingly rare or that they inevitably (?) destroy themselves somehow.

    This is based entirely on taking “If there is life out there, why haven’t we seen the evidence yet?” to imply “If there is life out there, why isn’t there any evidence to see?” Which is preposterous! The idea that we’ve exhaustively searched our solar system for anything that could possibly be of alien origin and nope, haven’t found any so it’s not there, is completely divorced from reality.

    The alien von Neumann probe wouldn’t even have to be hiding! For example, think of the picture showing Pluto’s newly discovered 4th moon. There could be a gigantic observatory that dwarfs the biggest of our ground-based telescopes, studying our entire solar system, sitting right where we were looking but we can’t see it!

  43. David Criswell

    I’ve always suspected that Earth has some additional tiny moons that haven’t been discovered yet — but my professors always pooh-poohed that idea, without really giving a good reason why.

    Now it turns out that an Earth Trojan has gone undetected until now. This strengthens my belief that Earth has some miniature natural satellites awaiting discovery.

  44. Ross Cunniff

    Cool! We should send something to take a look-see.

    Actually, if we are *serious* about Solar System exploration, we should establish communication relays at the Earth-Sun L4 and/or L5. On average, would only add 16 minutes round-trip to deep space communications, and one of Earth, L4, and L5 should be far enough away from the apparent position of the Sun at any given time from any given outer Solar System point.

  45. Torbjörn Larsson, OM

    That’s too bad, but there are plenty of other near-Earth asteroids that make good future targets.

    Huh? There aren’t many good known NEO targets, at least if you want to use current technology and target an asteroid that is larger than the craft. Current feasible target estimates range between two for trips within 2025, and a handful for later visits:

    “But just 17 of the objects reachable by humans are larger than 164 feet, and only 15 are accessible within the next several decades. Of those 15, only three fit within the 180-day round-trip limit.”

    Which btw is why you can hear some suggest a telescope in orbit at Venus range, looking out towards Earth.

    You can do it (with a modified CEV or a manned Dragon), but it is not as if it is a smörgåsbord (“smorgasbord”) to choose from.

  46. Grand Lunar

    I knew of Jupiter and Neptune Trojans, but I never knew that Mars had trojans as well!

    For Earth, I felt it was only a matter of time. I wonder if Venus might have some as well.

    It would be nice for us to get out to those locations and do some prospecting.

    Also, it would be a nice place for a fuel depot.

  47. Troy

    I wonder why such objects can’t be searched for in areas near or above the arctic circle in the land of the noontime dark (or midnight sun)? Must be the plane of the sun is out of view as well as the sun.
    Naming them after Coeus and Crius isn’t too bad but I think the theme of war camps from Jupiter is a good one. So perhaps figures of the American Civil war such as Grant and Lee, Lincoln and Davis.

  48. Ben

    Phil, it appears that your odysseyenation algorithm has malfunctioned. A click on the pic has no effect.

  49. Messier Tidy Upper

    I thought we already knew of at least one Earth trojan – Cruithne :

    http://en.wikipedia.org/wiki/Cruithne_(asteroid)

    Plus I recall reading a Asimov book mentioning another “quasi-moon” of Earth’s in such an orbit – named Toro from memory – and also the BA saying something about several such “quasi-moons” or Earth trojans in an article on our planet once.

    Still a great discovery though! :-)

  50. Messier Tidy Upper

    I agree with (#2) Chris’es assessment too :

    So does this mean Earth is not a planet, but a dwarf planet? I ask tongue-in-cheek (no flames please)- If this is the criteria used for Pluto (I am not sure the below is correct):
    For an object to be a planet, three requirements are defined by the IAU:
    It needs to be in orbit around the Sun – Yes, so maybe Pluto is a planet.
    It needs to have enough gravity to pull itself into a spherical shape – Pluto…check
    It needs to have “cleared the neighborhood” of its orbit – Uh oh. Here’s the rule breaker. According to this, Pluto is not a planet.

    The IAU definition is illogical and bad scence in my view and like many astronomers and people globally I utterly reject it on the grounds that they simply got it inexcusably wrong.

    Dwarf planets are still planets just as dwarf stars (like our Sun) are still stars and dwarf people are still people.

    @4. dcsohl :

    Oh, and Chris, I know you may be tongue-in-cheek but to seriously answer the question… Pluto has not “cleared its orbit” in the sense that Pluto is not the overwhelming majority of the mass in its orbital zone. The Earth *is* the overwhelming majority of the mass of our orbital zone.

    Which is an unfair criteria because Earth orbits in much closer to the Sun and thus has a much smaller orbit. Put Pluto where Earth is and it would become a planet because it wouldn’t have so much ground to cover. Put Earth where Pluto is and it would cease to be a planet because it too would be unable to clear the other ice dwarfs -and the same applies even for Jupiter!

    The IAU definition thus fails the reductio ad absurdum test of logic as well as the Occam’s Razor one.

    @6. Craven :

    @Chris we have cleared the neighborhood. Other planets too have trojans. They are stable objects, not wandering rocks in our orbit.

    Actually the trojans are wandering rocks they just wander within a specific area! ;-)

    How does that matter anyhow? If an orbit isn’t clear then it isn’t clear -and if you apply the IAU’s rubbish definition strictly NO planet in our solar system has a clear orbit as all are crossed by sun-grazing comets and most by asteroids.

    So how clear is “clear” becomes the next question- and why discriminate against Pluto and not Jupiter or Earth?

    That’s where the IAU apologists start hand-waving and saying well these rocks are toosmall to count or whatever diving planets into sinister sounding uber-and unter types and so. But that fails Occam’s razor test where unnecessary and superflous complications should be avoided if at all possible -and the IAU definition requires numerous needless “epicycle and deferent” type additions and explanations to keep it working justlike the old Ptolemaic (Earth-centred model) did.

    In brief the IAU definition utterly fails and should be rejected on scientific and logical grounds.

  51. Messier Tidy Upper

    @7. jsb16 :

    @Chris #2, I’m pretty sure the “cleared the neighborhood” requirement makes some mention of nearly equivalent-sized objects. That’s why Mars and Jupiter and Earth (all of which have Trojan asteroids) are still planets, but Pluto, an orbit with other plutoids, is not.

    But then what about this exoplanetary system :

    http://kencroswell.com/HD45364.html

    around the orange dwarf star named HD 45364 where two gas giants cross orbits? Are each of those gas giants now to be considered dwarf planets?

    What too about the possibility of the double planets – and one’s that end up on collision courses through orbital chaos like Earth and the Mars-sized world that struck our Earth leading to the formation of the Moon? Do such worlds lose planetary status when there’re on collision course but have it before that?

    The IAU definition leads to absurd consequences that make no sense.

    Just as it makes no sense to strip Pluto – and Eris and the other ice dwarfs – of their proper planetary status.

    As Ken Croswell also explains further here :

    http://kencroswell.com/NinthRockFromTheSun.html

    we do indeed need a better definition than the IAU one which restores Pluto’s planethood.

  52. Mike

    Agreeing with #5, Chris, here: the link for to enodysseyenate the last image was a bit of a fail must have disappeared into the quantum nothingness from which it was formed.

  53. Clint

    “Lissajous orbits” ?

    Celestial mechanics being named by Jar Jar Binks?

  54. Al Viro

    @34: IOW, launching a worm. Which suggests a solution to paradox – any
    civilization dumb and obnoxious enough to pull that off gets LARTed out
    of existence by sufficiently annoyed neighbors. Come to think of that,
    for *really* scary picture consider a botnet out of such probes, acting
    as galaxy-wide distributed antenna and spamming the living hell out of
    the entire Local Group…

  55. Tom

    We got a rock!

    But shouldn’t we have found something like this before now? Correct me if I’m wrong, but wouldn’t an object at an L-point always be 60 degrees ahead of or trailing the sun in the night sky? So at the best time of year it would rise/set around 3 hours before/after the sun, which would be still high enough to snap some images on consecutive nights before it got too low or the sky to bright, and then check the images for anything that moves?

  56. Robin

    Lissajous curves are generated from a set of parametric equations, all of the form x=a(bt+φ). One place they’re commonly seen is in oscilloscopes.

    Looking at the last picture in your post, Phil, reminds me again of how beautiful and deceivingly complex physics is, and how that it is displayed in the Cosmos before our eyes. At first blush, looking at Newton’s Laws of Motion coupled with with his Law of Universal Gravitation, it wouldn’t seem their solutions could be so complex (metaphorically, not necessarily with respect to real numbers). Yet, I imagine watching years of T7’s orbits around L4 trace out is nothing less than awe inspiring in its complexity. Thankfully we have the tools to see the beauty hidden by the short sample period of our lifetimes.

  57. mfumbesi

    Beautifully explained. I wish I could attend an Astronomy class and you be the lecture (or professor). I never really understood the L-point concept (well to be honest I never bothered to read up either) but your explanation flows like chocolate milk. Thank you, thank you.

  58. Alan Clark

    John ” There is one interloper in each camp. I believe Patrocolus is in the Trojan group and Hektor is in the Greek. (Maybe because they were both killed wearing Achilles armor? I don’t know.)”

    The actual reason is that Patrocolus was taken prisoner by the Trojans, and Hektor by the Greeks.

  59. @27 You are correct. Cruithne’s orbit librates in a tadpole shape (as you described) relative to Earth’s position.

    A tadpole libration is a compact case of a common horseshoe libration. A spectacular mutual horseshoe libration in the Solar System is that of Janus and Epimetheus. These Saturnian moons, of similar size, actually exchange their orbits every four years, repeating a libration cycle every eight years. By this time, each moon has orbited Saturn 4212 times.

    A simple search of Cruithne, Janus and Epimetheus will provide more information.

  60. Nigel Depledge

    Chris (2) said:

    It needs to be in orbit around the Sun – Yes, so maybe Pluto is a planet.

    Yup.

    It needs to have enough gravity to pull itself into a spherical shape – Pluto…check

    Yup.

    It needs to have “cleared the neighborhood” of its orbit – Uh oh. Here’s the rule breaker. According to this, Pluto is not a planet.

    Not quite.

    Put more simply, it just needs to gravitationally dominate the region of its orbit.

    Pluto most certainly does not do this.

    Jupiter has many known Trojans, but because they are at Jupiter’s L4 and L5 points, they are gravitationally dominated by Jupiter.

    Neptune’s orbit is crossed by several KBOs (including Pluto), but Neptune exerts enough influence that none of them ever gets close to Neptune itself (AFAICT), and Neptune has established an orbital resonance with objects such as Pluto.

    Now, you can argue about Pluto until you are blue in the face, but it is up to the IAU how they define the terms of their profession. Unless you are a professional astronomer, you should leave them to it.

    If you are a professional astronomer, the correct forum for discussing Pluto’s status is a meeting of the IAU, not intenet discussion fora.

  61. My educated guess –
    odysseyenate – verb – created by combining odyssey, a long wandering voyage, with the addition of the suffix -nate, meaning to make or do

    odysseyenate – (0 dis e uh nate) – To odyssey.

  62. Ian S

    Messier:
    “…just as dwarf stars (like our Sun) are still stars and dwarf people are still people.”

    and yet both your examples are called Dwarf, why can’t we use that term for planets?

    “Put Earth where Pluto is and it would cease to be a planet because it too would be unable to clear the other ice dwarfs -and the same applies even for Jupiter!”

    I’ve seen you claim this many times but I have never seen you produce a calculation that shows this? How have you reached the conclusion that jupiter couldn’t clear plutos orbit?

    “NO planet in our solar system has a clear orbit as all are crossed by sun-grazing comets and most by asteroids.”

    By definition any object that crosses an orbit does not share that orbit. Having a clear orbit does not require nothing ever crosses that orbit.

    I know you want pluto to be given equal status to jupiter but really your arguments against the IAU definition is more flawed than you claim the definition to be.

  63. walter franklin

    so bottom lin what u are saying this is the asteroid thats said gonna smash into the earth in 2012 to end it all?

  64. hhEb09'1

    @MandyDax, @Tom, the earth trojan is supposedly at 80 million km now, whereas the earth-sun lagrange point is at 150 million km. Over the next hundred years, it might get as close to earth as 24 million km. In other words it gets almost as far from the lagrange point as earth is from the sun.

  65. JimTKirk

    I had hoped that the STEREO A & B satellites would spot something as they passed the L4 and L5 points, but nary a peep. I guess they never came close enough.

  66. Good choice of names but Uranus may demand a paternity test.

  67. Ian W

    It could be named “Kuik” after model Tiiu Kuik ( TK ). lol.

  68. Messier Tidy Upper

    @62. Nigel Depledge :

    Put more simply, it just needs to gravitationally dominate the region of its orbit. Pluto most certainly does not do this.

    Yes it does – hello : Pluto has no less than four moons, (that’s 3 more than our Earth boasts, two more than Mars has and 4 more than moon-less planets Mercury and Venus btw.) and one of them is huge! ;-)

    Of course, we can argue about how far away from itself an object’s gravity needs to dominate but that is raising another question. Raising questions that require plenty of other arbitrary explanations and definitions is a problem in science and scientific definitions where Occam’s razor says superflous questions and requiremenst should be cut. That definitions and sciencitif ideas should be kept as simple as possible. The IAU definition fails on that count. There are several simpler alternatives that in contrast would pass that test.

    Now, you can argue about Pluto until you are blue in the face, but it is up to the IAU how they define the terms of their profession. Unless you are a professional astronomer, you should leave them to it.If you are a professional astronomer, the correct forum for discussing Pluto’s status is a meeting of the IAU, not intenet discussion fora.

    Three words for you there :

    Fallacy of Authority

    Do you really think the IAU are infallible and incapable of getting things wrong on occassion?

    I don’t.

    I heard there was a lot of nasty politics and a very undemocratic decision in the Prague meeting which did NOT actually include all the relevant astronomers such as Alarn Stern who described the IAU’s decision as idiotic. I agree with Alan Stern on that -and withKen Croswelland many other astronomers as well.

    Sometimes authorities – even scientific ones – make mistakes and need to be challenged and even rejected. This is one of those times in my view.

    @65. Ian S :

    Messier :“…just as dwarf stars (like our Sun) are still stars and dwarf people are still people.”
    and yet both your examples are called Dwarf, why can’t we use that term for planets?

    I’m fine with calling Pluto and Eris ice dwarf *type* planets just as we have gas giant planets like Jupiter and Saturn and terrestrial “rock dwarf” planets like Earth and Mars – this should be a division of the major types between planets however NOT used to deny that planets are planets – and planets incl. Jupiter, Earth and Pluto.

    Planets is abroad category justas animal is. A honeybee is an animal of the insect ctageory even though its small and very different in nature to an alligator or a blue whale. This is thesame sort of deal. Its like the IAU has classified “bees” as plants instead of animals.

    “Put Earth where Pluto is and it would cease to be a planet because it too would be unable to clear the other ice dwarfs -and the same applies even for Jupiter!”
    I’ve seen you claim this many times but I have never seen you produce a calculation that shows this? How have you reached the conclusion that jupiter couldn’t clear plutos orbit?

    I’ve read that in several places and various sources. If you are disputing it where are your calculations showing that it could?

    “NO planet in our solar system has a clear orbit as all are crossed by sun-grazing comets and most by asteroids.”
    By definition any object that crosses an orbit does not share that orbit.

    Hmm.. really? isn’t this whole post about a trojan asteroid sharing Earths orbit as also happens to Jupiter and Neptune and others? Plus didn’t (#61.) Shawn Urban just mention Saturn’s co-orbital moons Janus and Epimethus?

    Having a clear orbit does not require nothing ever crosses that orbit.

    Doesn’t it? Says who? Why? What’s a clear orbit and how clear exactly doe sitneed to be? There’s more question raised and needing further unneccesary elaboration by the IAU definition that a better alternative definition that exclduses teh whole “orbital clearnace” nonsense avoids.

    I know you want pluto to be given equal status to jupiter but really your arguments against the IAU definition is more flawed than you claim the definition to be.

    As you’d expect itotally disagree with that assertion there. Oh & I don’t want Pluto given equal status to Jupiter – I’m not claiming Pluto is a gas giant planet just that it is a planet. ;-)

  69. JediBear

    Dwarf Planet is indeed a (small) lexical problem because we already had those. Earth was one, for example.

    But planet/not a planet is a wholly arbitrary distinction, and there is really no argument that Pluto should or shouldn’t be considered a planet. There’s no value in it being or not being a planet.

    That said, many astronomers (rightly) see Pluto’s demotion as putting it back in its proper place — correcting the initial mistake of classifying a body that’s merely one of a swarm of similar miniscule objects as a noteworthy body.

    Ceres got a similar demotion, and you don’t hear a lot of complaining about that. As a matter of fact, Ceres is far more notable than Pluto as it actually stands out as the largest and roundest of the Asteroids. Where Pluto is only the first of the Plutoids that we found, and that a matter of pure happenstance.

    I’m more annoyed by the solar-centric features of the classification system, and its idea that “(modifier) planet” should be seen as distinct from “planet” rather than subsumed within it.

  70. JediBear

    It’s not really accurate to say that Pluto has moons. Rather, Pluto is (barely) the largest part of a gravitationally-bound system of similar objects. Leaning on the IAU’s misclassification of Charon as a moon in order to argue its classification of Pluto as a minor planet is dirty pool.

  71. Chris Winter

    Toevallige Voorbijganger wrote: “Why name a Trojan after the sons of a Greek goddess ? Makes no sense.”

    It makes some sense to me. Both L4 and L5 are “Trojan points” — but only bodies at the L4 position get Greek names. (I assume the protocol that applies to Jupiter will also apply to the other planets.)

    Also, for historical reasons there are no Trojans around to object. ;-)

    Maybe you should just wave it off as one more example of the sloppiness of the English language. After all, this is the language in which you park on a driveway and drive on a parkway.

    http://english-zone.com/language/english.html

  72. Matt K

    Better names: “Magnum” & “Ecstasy”. Those are also names of Trojans (condoms).

  73. andy

    Everyone got on perfectly fine with the term “minor planet” which was used for various small objects that weren’t planets… guess we’ll just overlook that eh?

  74. Chris A.

    @Messier Tidy Upper:
    Read Steven Soter’s “What is a Planet?” (arxiv.org/ftp/astro-ph/papers/0608/0608359.pdf) and then we can discuss why Pluto isn’t a planet. Especially note Fig. 3.

    Briefly, Soter notes that the ratio of object mass to the combined mass of everything else in its orbital zone (“mu”) shows a definite gap between the planets and the dwarfs. Such a gap bespeaks of a physical process which differentiates between the two categories.

    Scientific taxonomy is at its best when quantifiable attributes illuminate a clear boundary between categories, as opposed to arbitrary cut-offs (e.g. the dividing line between large islands and small continents).

  75. DigitalAxis

    @23. Renee and @72. Messier Tidy Upper

    The Earth constitutes 98.7% of the mass of the objects that cross the Earth’s orbit. The moon takes up basically the rest of that. More to the point, if you zapped it some-odd degrees ahead in its orbit, the Earth would be just fine and continue to orbit… maybe with a few more collisions, but its orbit would not change. If you did that to 2010 TK7 its orbit would change DRASTICALLY because it would no longer be at Earth’s Lagrange point. That’s gravitational dominance for you.
    (Likewise, if you remove Earth entirely, TK7’s orbit will change dramatically because it won’t be getting regular tugs from the Earth. If you remove TK7, the Earth’s orbit won’t change.)

    Shift Neptune elsewhere in its orbit, it would be fine and basically nothing would happen. Shift Pluto, and it would get out of its 2:3 resonance with Neptune. Neptune would start pulling at the wrong times and change its orbit.

    As for MessierTidierUpper:
    “Put Earth where Pluto is and it would cease to be a planet because it too would be unable to clear the other ice dwarfs -and the same applies even for Jupiter!”
    I’ve seen you claim this many times but I have never seen you produce a calculation that shows this? How have you reached the conclusion that jupiter couldn’t clear plutos orbit?

    I’ve read that in several places and various sources. If you are disputing it where are your calculations showing that it could?

    Here’s some rough numbers (I don’t have good enough information to dispute the bit with the Earth, except that the mass of the Earth is apparently larger than the entire Kuiper Belt, combined, which should tell you just what range of objects we’re talking about)

    Let’s plop Jupiter in there. Let’s even make the mass of the Kuiper Belt 4 Earth masses, which is the highest number I’ve seen and is now apparently too high. Jupiter is 318 Earth masses, so it would contain 98.7% of the mass out there, just like the Earth does in our orbit. Well, the Earth dominates everything in its orbit just fine. Yes, you’ll have to account for the increased SIZE of the orbit, so it’ll take a bit longer to clear… but it should force everything to eventually end up in resonances, even if they are 1:1 trojan or quasimoon resonances.
    {edit: What about Neptune? At 17 Earth Masses it’s ALREADY controlling the orbits of a large portion of the Kuiper Belt- Cubewandos, Plutinos, etc. Jupiter could certainly do that).

    As for your resonant planets, there is one critical difference between the planets and Neptune/Pluto: If you move Pluto, nothing happens to Neptune; if you move Neptune, lots of things happen to Pluto. In your resonant planets (see also GJ 876), moving EITHER planet will almost certainly affect the other, as the original paper (Correira et al. 2009) points out.
    Now, we can define a timespan and a definition of ‘effect’, but in every case Neptune has more effect on Pluto than Pluto has on Neptune.
    Ultimately, I think the intent of the decision is to restrict the full-blown planet designation to conspicuously large bodies in a solar system. When you’ve got hundreds of objects in similar orbits with similar (if usually smaller) masses, neither 1 Ceres nor 134340 Pluto really stand out like Neptune or Mercury do. Have we taken anything FROM Ceres or Pluto? No, they’re still fascinating worlds, and should be right up there with the 8 planets and the larger moons as objects of interest and study.

  76. Anon

    Oh look, it’s THIS discussion again…

  77. CRB

    dude, seriously–THANK YOU. thank you, in particular, for adding pictures explaining what you mean, because when I read an article about this on Yahoo, their explanation went right over my head, and they did NOT provide even a little diagram explaining things. not one.

    mind you, I can generally figure things out, but sometimes (sleep deprivation doesn’t help a bit) I need a little boost. thank you for supplying that.

  78. Dragonchild

    @51. Messier Tidy Upper –
    A “Trojan” has a very specific definition in that it’s located at either the L4 or L5 point of a planet’s orbit. Cruithne has 1:1 orbital resonance with Earth, but its actual orbit is quite different and clearly independent of the Lagrangian points. Cruithne being located at a Lagrangian would be happenstance and transient.

  79. icemith

    @ 58; and also in Phil’s original blog above:

    ” …called Lissajous orbits.”

    For those not familiar with the actual “shape” of a “Lissajous patterns”, Australians have had for probably 50 years a representation on their television screens, namely, the icon for the ABC TV service, (Australian Broadcasting Corporation, originally Commission, being a Government Instrumentality, and funded from general revenue, albeit rather frugally. I wonder if it still costs 8 cents a day, for every citizen, but paid for by every taxpayer.)

    I tried to find an example in my font book, but the nearest I could come to it was to imagine an “Infinity” sign with a extra loop, making it three loops. Some pretzels have that shape. If you google “The ABC”, it is on the home page.

    Ivan.

  80. CB

    @ MTU, as always your objections to the IAU definition are ridiculous, and only demonstrate your deliberate lack of understanding of the matter so as to make the IAU definition seem ridiculous. But it backfires.

    Seriously, why claim to understand the definition well enough to claim it’s silly, and talk about comets or this newly discovered Earth Trojan compromising Earth’s planetary status? Because all that really says is that either 1) you don’t understand anything about the IAU definition or 2) you’re saying ridiculously wrong things on purpose as part of a disinformation campaign aimed at restoring Pluto for emotional reasons.

    Like take the argument that if you magically moved Earth or Jupiter to Pluto’s orbit today, they wouldn’t be planets — as if this has any bearing on reality! If Earth had instead formed in Pluto’s orbit, then it would be a planet because it would have necessarily accreted all the mass that causes Pluto to not be a planet into itself. In fact there isn’t enough other mass in Pluto’s orbit for earth to have formed there, much less Jupiter. Gee, maybe this is telling us something about planetary formation?

    But to address one of your actual reasonable questions, “How clear exactly does it need to be?”, the answer is: there is no reason and no need to draw a line saying exactly how clear it has to be, because there’s a five order of magnitude as in 10^5 or 10,000x gap. You don’t need to and shouldn’t draw an infinitely precise line when spilling a dump truck full of paint would still amply separate the two sets of objects! You don’t quibble about where exactly North America and Asia end when the existence of the Pacific Ocean between them makes it clear they are distinct!

    @ JediBear: The Ceres argument is completely logical, but will not sway a single person angry over the “demotion” of Pluto because they don’t care about Ceres or the observation that it’s just a rather large asteroid, since they never had any emotional attachment to it. “Go ahead and make it a planet”, they’ll say.

  81. Anon

    I think this discussion shows once again how silly the new IAU definition is. There’s nothing wrong with the thing itself – it just leaves too much room for irrational people who want Pluto to be a planet because they grew up like that. I’d prefer a more exact definition:

    I. The object has achieved hydrostatic equilibrium (is round), or probably was in equilibrium before an impact deformed it (like Vesta).
    II. The object must directly orbit the Sun.
    III. The object must be more massive than the most massive known moon (currently Ganymede).
    IV. The object must not fuse hydrogen into helium (obviously).

    If all four apply, it’s a planet. If only III doesn’t apply, it’s a dwarf planet. If only I applies, it’s a moon. If none apply, it’s a dwarf moon. If II doesn’t apply because the object orbits a common barycenter with another object, but the barycenter isn’t inside either object, it’s a double (dwarf) planet, like Pluto-Charon.

    There, no room for argument but enough room for change if necessary.

  82. Aidan Karley

    Several things :
    @0 (The article) – Good to see Paul Wiegert getting attention for this. He’s been doing interesting work on the stability of orbits for a looong time.
    – Cruithne gets mentioned.
    – His work on the conditions for stability of planets in multi-star systems is interesting but probably scares off too many SF writers – they’d make interesting places for a story.
    @47 – the smörgåsbord came across just fine ; evidently Discovery Magazine cares more about it’s non-anglo readers than Slashdot does. Well, that’s my €0.02 anyway.
    @55 Chris : “Lissajous orbits” – A Lissajous figure is what you get on an oscilloscope formed by the interaction of two sinusiodal curves which may be of the same frequency but out of phase (in which case you get a simple ellipse), or of different frequencies and therefore constantly varying phase difference. You can generate the first by hooking up your oscilloscope so that the X signal comes from the current into a capacitor and the Y signal from the voltage across the capacitor ; for the latter, you need two signal generators. Lissajous figures are easy to generate and look really “sciencey”, so you’ll see them on the display screens in a multiplicity of low-budget SF programmes from pre-StarTrek to … well probably tomorrow (for all likely values of “today”).
    In this case, the input sinusoids would probably be (in approximate order of strength) solar gravitational force ; terrestrial gravity ; Venerian (Venusian? gravity) ; Martian gravity ; Jovian gravity. The last 3 would probably be of comparable strength. So you’d gete a complex figure.
    I don’t have the precise figures to hand, but when I asked my braincell “how wide a range on the sky do the orbits of the major planets occupy when seen from the Sun”, the answer “about 20 degrees” popped out. Which is enough to throw quite a bit of out-of-plane force into what is pushing 2010TK7 around, leading to the out-of-plane motions.
    @49 Troy : The authors are Canadians, using the Canada-France-Hawaii telescope ; so surely they’d use figures from a Canadian civil war (I’m not sure if there has been one, though terrorist invasions form America tried to start one in the late 1700s?), a French civil war (plenty of choices, starting with the war Julius Caesar exploited), a Hawaiian one (probably some choice there too). Why would they need to use foreign names?

  83. Aidan Karley

    @83 icemith : “I tried to find an example in my font book, but the nearest I could come to it was to imagine an “Infinity” sign with a extra loop, making it three loops.” That would be a specific case of the general class of Lissajous figures. If I recall the math, that’s the sort of figure that would be generated by an X input like “k.sin(3Ø)” and a y input like “l.sin(Ø)”.

    There are, of course, infinitely many Lissajous figures.

    Playing around with them is one of the first things I did when I got my sweaty little paws onto an oscilloscope. I’d be astonished if I were the only person on this forum who could say that.

  84. Aidan Karley

    @85 Anon says “III. The object must be more massive than the most massive known moon (currently Ganymede).”
    This is the sort of arbitrary choice that leaves a logical hostage to fortune. Say your definition were adopted, and in another 50 years we have improved exoplanet detection so much that we can detect an object in orbit around 51 Peg (name ; hat ; pull) that is less massive than Ganymede. Then the Internet would be full of people arguing over whether it’s a planet or a dwarf planet.
    OR, more credibly, we find a trans-Neptunian object composed of two objects in a mutual orbit, one 1.5x Ganymede’s mass, the other 2.5x. Are they both planets? Are they both dwarf planets? Since the barycentre of the system would lay outside either object, are they both moons?
    I, II, and IV are reasonable points ; III is a hostage to fortune. Actually, point II has some possible problems too, but they get wildly improbable. “Fleet of Worlds”, anyone?

  85. Anony

    If we find more Earth Trojans, I vote we start with the name “ultra-sensitive” and go from there…

  86. Anon

    @88. AIdan Karley: The ‘most massive moon’ could either be our most massive moon or that system’s most massive moon. The choice is up to the IAU – there are pros and cons for both sides.
    As to that TNO, it would be a double planet. Any other objects orbiting the barycenter would be moons (assuming they are so tiny that they don’t have much effect on the barycenter and wouldn’t be able to move it outside the primary object if the secondary (1.5 Ganymass) were to be removed, like the situation with Nix, Hydra or P4).

  87. ceramicfundamentalist

    i can’t believe i’m weighing in on this, but why not just have a tier system with one variable that determines the status of an astronomical object? if it’s too small for hydrostatic equilibirum then it’s a small body. if it’s big enough for hydrostatic equilibrium but too small to ignite fusion then it’s a planet. if it’s big enough to ignite fusion then it’s a star. we’d have many planets in our solar system then, even planets orbiting other planets, regardless of where barycenters are. i think the confusion arises when the definition uses multiple variables that are intended on one hand to preserve the historical status of some objects (like grouping the eight current planets into the same category despite the fact they show incredible variation) while also trying to introduce some scientific rigour into that definition (like excluding pluto because it is only one of many similar “plutos”).

    imagine for a moment this weird creature: a non-fusioning object which has achieved hydrostatic equilibrium but still has lots of topographical relief, ie. high mountains and low valleys (i believe that mars would be the best example of this in our solar system, but some astronomical objects out there must certainly be more exaggerated). this object has a companion and the two revolve around a common barycentre that just by coincidence happens to be sometimes inside and sometimes outside the first body due to the extreme topographical relief. how would you define the two objects: planets, satellites, other? with a one-variable categorization scheme it is simple. with multiple categorization schemes (such as we now have) it gets complicated, and new, arbitrary rules have to be created for objects that fall between the cracks.

  88. amphiox

    The IAU definition leads to absurd consequences that make no sense.

    Well, that’s pretty much standard fare for any compromise position, which is what the IAU definition ultimately was. (Perhaps we could just go back to the first IAU proposed definition, the one that included Pluto, Charon, Ceres, and several others, to a total of 13 planets?)

    And in this case, if I remember the sequence of events right, consensus for compromise was reached more or less by having a sufficient number of dissenters walk out….

    But really, names are just names, categories just categories. Pluto will still be the same Pluto no matter what we call it. The only thing that matters for a classification system is that it be useful, and enough people agree on using it so that everyone knows what everyone else is talking about, when using the same terms (a criteria which, I suppose, the IAU definition also sort of fails).

    But at some point in our ongoing exoplanet search, we’re going to be forced to redefine planet anyways (the IAU definition does not apply to any exoplanets, at present).

  89. CR

    I’m so TIRED of the “planet/not a planet/what’s a planet?” pedantry that I find the multiple silly condom jokes are actually more interesting.

    But not as interesting as the new discovery of this asteroid. It’ll be cool to see how many more might be out there…

  90. J.D.Mack at #32 “Wait a second. There’s a rock in space with no atmosphere and it’s around 0 degrees Celsius? Why? Is sunlight sufficient to warm its surface to that high a temperature?”

    Sorry if I missed an answer to this in the dwarf discussion but yes, 0°C makes sense.

    Using very round numbers the average temperature of the Earth’s surface is 15 °C which is 30 °C above the temperature it would have without the greenhouse effect. However, the Earth is quite reflective (has a high albedo) so the equilibrium temperature it would have without an atmosphere is cooler than that of a darkly coloured body at the same distance from the Sun.

    Looking at it in a slightly different way, a dark coloured lump of rock would have about the same emissivity for the long-wave thermal infrared (around 10 µm) as an airless Earth but would have a higher absorptivity for solar radiation (mostly visible light and short-wave infrared (around 1 to 2 µm) plus a bit of UV). Therefore the rock would absorb more of the solar radiation arriving on its surface so have to be slightly warmer to get rid of the same amount to maintain equilibrium.

    Of course, without an atmosphere the Earth’s albedo would be dramatically different, too. The clouds would fall on to the ground then sublime away into space. The ice caps would also sublime away. Vegetation would degrade pretty quickly. The bare earth and rocks would darken under the effect of solar radiation. Etc.

  91. Nigel Depledge

    @ MTU (72) –
    D’you know, I can’t be bothered with that any more.

    I have stated my case, and it has not changed your mind. I do not agree with your reasoning. As far as I am concerned, you have not made a convincing case.

    To my mind, Pluto is just another KBO. Yes, it was identified before we even knew about the Kuiper Belt, but so what? I consider it the prerogative of the IAU – right or wrong – to define their terms; and you clearly do not. Let’s leave it there.

  92. Davis

    Massive steroids hitting the Earth will be an eternal menace for us. Humanity should be more prepared to skip out into space in case of an imminent annihilation. http://www.youtube.com/watch?v=GSkxPghXTCg

  93. ND

    Pluto is no longer a planet because Chuck Norris used it as a bowling ball. End of debate.

  94. Galen

    Is the animation of Cruithne’s orbit found here -> http://en.wikipedia.org/wiki/3753_Cruithne

    incorrect? It shows a horseshoe orbit around just one Lagrange point.

  95. amphiox

    Massive steroids hitting the Earth will be an eternal menace for us.

    It’s the ‘roid rages triggering WWIII that is what’s going to get us, in the end….

  96. Ryan

    It should be named ♪Trojan Man♪, to be sung as in the commercials whenever uttered, since Earth is a fertile planet.

  97. Messier Tidy Upper

    @94. amphiox : Yep. Some good points there. :-)

    Astronomers are still searching for Edgeworth-Kuiper Cometary belt and have yet to find anything else larger than Pluto. There is the possibility that they’ll find something as large as Mercury or even Earth (or larger) out there – which won’t have cleared its orbit and will (or should anyhow) reopen the question of what’s a planet.

    Plus there’s the possibility we *won’t* find anything larger in which case Pluto, Eris, Makemake and a handful of others are the largest and most significant bodies in that region and thus should be recognised as different to the others there which should also re-open the question.

    The only thing that matters for a classification system is that it be useful, and enough people agree on using it so that everyone knows what everyone else is talking about, when using the same terms (a criteria which, I suppose, the IAU definition also sort of fails).

    Indeed. It does fail. Many people consider the IAU definition so flawed they refuse to use it. I’m one. Common usage will hopefully eventually see the IAU definition become an embarrassing footnote for future generations to laugh at.

    But at some point in our ongoing exoplanet search, we’re going to be forced to redefine planet anyways (the IAU definition does not apply to any exoplanets, at present).

    Absolutely. Technically speaking we haven’t discovered *any* planets around other stars – just exoplanets which remain undefined and are specifically excluded from planetary status by the IAU’s ludicrous decree.

    (Perhaps we could just go back to the first IAU proposed definition, the one that included Pluto, Charon, Ceres, and several others, to a total of 13 planets?)

    Yes, that would certainly be an improvement. :-)

    Or we could define a planet as an object NOT orbiting a planet directly and not shining by nuclear fusion over a certain set size such as the size of Ceres or thereabouts. (Eg. 450 km or so.)

    Or we could use some other better alternative definition too.

    @85. Anon :

    I think this discussion shows once again how silly the new IAU definition is.

    Yup. Indeed it does. :-)

    There’s nothing wrong with the thing itself – it just leaves too much room for irrational people who want Pluto to be a planet because they grew up like that.

    Nope. People are hardly being “irrational” when they notice that an object that has always been a planet is now being wrongly stripped of that distinction based on what amounts to the very dubious arguments that its too small and in a zone of other similar objects.

    Pluto is large enough to be round, hold an atmosphere complete with seasonal weather, be geologically active and boasts four moons and maybe rings too. The argument that because it is no longer a lone oddball but exists amidst a zone of similar worlds is as silly as saying that because Earth exists in a zone of similar “rock dwarfs” or Jupiter exists in a zone of similar gas giants all those should be disqualified from planet-hood too.

    My objections to the IAU definition are based on logic and having a definition that makes good sense which the IAU definition does NOT.

    I’d prefer a more exact definition:

    I’d prefer a better definition which is much broader and allows for more possibilites esp. in light of exoplanetary findings showing worlds are often weirder than we’d expected.

    Galaxy-Star-Planet is kinda the astronomical quivalent of Animal-Vegetable-
    Mineral and should be correspondingly broad categories in my view.

    I. The object has achieved hydrostatic equilibrium (is round), or probably was in equilibrium before an impact deformed it (like Vesta).
    II. The object must directly orbit the Sun.

    I’d agree with your condition (I) there but would say no to your condition (II) because we now know of exoplanets that are free-floating within galactic and probably intergalactic space. These would NOT orbit a star – let alone specifically our Sun as the IAU definition hubristically demands in violation of the Copernican principle! – but still be planets in most folks eyes.

    III. The object must be more massive than the most massive known moon (currently Ganymede).

    Er .. WHY?? As (#88.) Aidan Karley has pointed out that’s an arbitrary and superflous criterion. Why would you want that included when it would for instance render Mercury a non-planet? (You did know Ganymede is larger than Mercury right?)

    Only reason I can imagine for that would be if you are one of those irrational Pluto-haters! :P

    IV. The object must not fuse hydrogen into helium (obviously).If all four apply, it’s a planet. If only III doesn’t apply, it’s a dwarf planet. If only I applies, it’s a moon. If none apply, it’s a dwarf moon. If II doesn’t apply because the object orbits a common barycenter with another object, but the barycenter isn’t inside either object, it’s a double (dwarf) planet, like Pluto-Charon. There, no room for argument but enough room for change if necessary.

    Dwarf moon? Hmm … that’s a new one. Your definition there is fairly close to what my preferred choice would be :

    A) An object is a planet if it has enough gravity enough to be round or ellipsoidal for fast rotators..

    B) ..And it isn’t so massive that it has ever been self-luminous via nuclear fusion ..

    C) .. And it doesn’t orbit another planet.

    Three simple rules for planethood that are usually fairly clear-cutand easy to determine.
    Do you or anyone else object to this definition on any lines other than it not being the IAU’s (eventual & dubious) choice?

  98. Messier Tidy Upper

    @95. CR : ”I’m so TIRED of the “planet/not a planet/what’s a planet?” pedantry that I find the multiple silly condom jokes are actually more interesting.”

    Well nobody is forcing you to read and partcipate in that discussion if you don’t want to, CR. If you don’t care about it then rather than being rud eand shouting dismissively I suggest you just skip past and engage in the comments and threads which you *do* find interesting.

    We’re all individuals and get interested in different things to different extents. This is something that I and others feel strongly about and matters to us. I’d regretfully listen to the BA if he asked me to stop discussing this topic because this his blog and his rules. It’s not your blog however so don’t expect me to listen when you whinge about it. Oh & FYI it wasn’t me that raised this topic in this thread in the first place.
    ******

    @99. Nigel Depledge :

    @ MTU (72) –
    D’you know, I can’t be bothered with that any more. I have stated my case, and it has not changed your mind. I do not agree with your reasoning. As far as I am concerned, you have not made a convincing case.

    Sorry tohera you think that. I’m certainly not convinced at allby your case either.

    To my mind, Pluto is just another KBO. Yes, it was identified before we even knew about the Kuiper Belt, but so what?

    Pluto is the brightest, second largest and most significant world in the Edgeworth –Kuiper belt. It is a complex, amazing, fascinating little planet in its own right. To dismiss it as nota planet is, I think, ridiculous and downright wrong. That’s what.
    You are, of course, entitled to your opinion to the contrary.

    I consider it the prerogative of the IAU – right or wrong – to define their terms; and you clearly do not. Let’s leave it there.

    I think the IAU should be told they’ve made a mistake when they have madea mistake. I think they should be humble and reasonable enough to correct their mistakes. I hope to see this happen. I’m happy to agree to disagree with you on this and leave it there. But I do reserve the right to express my views as I see them and argue my case too.

  99. Messier Tidy Upper

    @84. CB :

    @ MTU, as always your objections to the IAU definition are ridiculous, and only demonstrate your deliberate lack of understanding of the matter so as to make the IAU definition seem ridiculous.

    *Seem* ridiculous? The IAU definition *is* ridiculous.

    I think you are confusing ‘understanding’ with ‘disagreement’ – I understand the IAU definition – and I reject it for the many good reasons I have frequently given you.

    Seriously, why claim to understand the definition well enough to claim it’s silly, and talk about comets or this newly discovered Earth Trojan compromising Earth’s planetary status? Because all that really says is that either 1) you don’t understand anything about the IAU definition or 2) you’re saying ridiculously wrong things on purpose as part of a disinformation campaign aimed at restoring Pluto for emotional reasons.

    I have said nothing that is factually wrong. You may disagree with theinterpretations or implications I have drawn but that’s a matter of opinion NOT fact.

    Like take the argument that if you magically moved Earth or Jupiter to Pluto’s orbit today, they wouldn’t be planets — as if this has any bearing on reality! If Earth had instead formed in Pluto’s orbit, then it would be a planet because it would have necessarily accreted all the mass that causes Pluto to not be a planet into itself!

    Do the words ‘hypothetical’ or ‘thought experiment’ mean anything to you?

    Of course in some star and planet systems they may turn out not to be so hypothetical like, say, HD HD 45364 (see #53 here – I notice you have’nt mentioned that) or even Fomalhaut b which is still inside a disk of planetesimal material yet is far more massive than Jupiter. (Admittedly the star is much younger but still.)

    In fact there isn’t enough other mass in Pluto’s orbit for earth to have formed there, much less Jupiter. Gee, maybe this is telling us something about planetary formation?

    Maybe there isn’t – today. There may well have been earlier – and, too, exoplanets have shown us that where planets form isn’t necessarily where they end up orbiting.

    But to address one of your actual reasonable questions,..

    Actually, I think all my questions are reasonable ones! ;-)

    .. “How clear exactly does it need to be?”, the answer is: there is no reason and no need to draw a line saying exactly how clear it has to be, because there’s a five order of magnitude as in 10^5 or 10,000x gap. You don’t need to and shouldn’t draw an infinitely precise line when spilling a dump truck full of paint would still amply separate the two sets of objects! You don’t quibble about where exactly North America and Asia end when the existence of the Pacific Ocean between them makes it clear they are distinct!

    Hmm.. yet once those two land masses *were* connected ino one by the Bering land bridge.

    The point is that in a scientific definition Occam’s razor says we should avoid superflous questions and problems being raised. A scientifically elegant solution is a simple as posible and as clear as possible. The fact that the IAU definition raises unnecessary questions like “how clear is clear” is a point against that and means better clearer definitions that do NOT raise this issue are logically and scientifically superior to it.

    @ JediBear: The Ceres argument is completely logical, but will not sway a single person angry over the “demotion” of Pluto because they don’t care about Ceres or the observation that it’s just a rather large asteroid, since they never had any emotional attachment to it. “Go ahead and make it a planet”, they’ll say.

    Bzzt. Wrong again. Ceres isn’t just “a rather large asteroid”; it is – or was considered *the* largest asteroid and is a planet of its own. Isaac Asimov among others has called it the “4-&-a-halfth planet” :

    “… he had left out a planet. It was not his fault; everyone leaves it out. I leave it out myself when I list the nine planets, because it is the four-and-a-halfth planet. I’m referring to Ceres; a small but respectable world that doesn’t deserve the neglect it receives.”
    – Page 63, chapter 5 “The World Ceres” in ‘The Tragedy of the Moon’ by Isaac Asimov, Mercury Press, 1973.

    I agree with Asimov here. Ceres is notably spherical and is, almost certainly, geologically differentiated. It ain’t no ordinary asteroid but rather a tiny but remarkable icy little planet in its own right. Can’t wait to see it close up when Dawn visits Ceres in a year or so.

    @79. DigitalAxis :

    ..[Snipped for length.] Shift Neptune elsewhere in its orbit, it would be fine and basically nothing would happen. Shift Pluto, and it would get out of its 2:3 resonance with Neptune. Neptune would start pulling at the wrong times and change its orbit. .. [snip.] .. it [Jupiter in the Edgeworth-KuiperBelt] should force everything to eventually end up in resonances, even if they are 1:1 trojan or quasimoon resonances.

    Planets interact gravitational and change each others orbits. We see this in many exoplanetary systems (eg. Gliese 876) and in the history of our own solar system. Does this stop planets being planets? If Jupiter shifts Saturn, Neptune and Ouranos around – as it apparently did leading to the Late Heavy Bombardment – does that mean that those outer non-dominant planets lose their planethood? Of course not! That’s another example you’ve found for me where the IAU definition fails the reductio ad absurdum test of logic.

    Ultimately, I think the intent of the decision is to restrict the full-blown planet designation to conspicuously large bodies in a solar system. When you’ve got hundreds of objects in similar orbits with similar (if usually smaller) masses, neither 1 Ceres nor 134340 Pluto really stand out like Neptune or Mercury do.

    I disagree. Ceres and Pluto *do* stand out – although what “stands out” is, natch, a subjective measure anyhow as is “conspicously large”.

    Interesting you note the intent there was to impose a more restrictive, less inclusive (anti-Plutoand ice dwarf worlds) definition of “planet” in the first place. Inessence, itsemes you’re admitting the IAU decided to strip Pluto of planet-hood first and then came up with a definition that would (falsely) justify this. IOW, Pluto was facing a IAU panel predjudiced against it and determined to demote it however bad the definition was otherwise right from the start.

    @78. Chris A :

    Messier Tidy Upper: Read Steven Soter’s “What is a Planet?” [paper] and then we can discuss why Pluto isn’t a planet. Especially note Fig. 3. Briefly, Soter notes that the ratio of object mass to the combined mass of everything else in its orbital zone (“mu”) shows a definite gap between the planets and the dwarfs. Such a gap bespeaks of a physical process which differentiates between the two categories.

    I’ve read it before and I totally disagree with it. That’s the one where Soter talks about “uberplanets” and “unterplanets” which translates in essence as planets “they like” and “planets they don’t.”

    Yes, its true that ice dwarf planets are much smaller by orders of magnitude than the others. Jupiter is way larger than Pluto – and Earth. But think of the mammals as a taxonomic class. We have blue whales, we have humans and we have mice. The ratios of size, mass etc ..there are huge too. Mice are very different and much more numerous than blue whales, they live in a different place and have very different natures. But all these are still mammals. We don’t say that blue whales are mamals but mice are not. In the same way planets include the equivalent of the blue whales (Jupiter) humans (Earth) and mice (Pluto) – and the IAU definition is doing the equivalent of ruling that mice are non-mamals because of their small size and the fact that there tend to be lots of them!

    A planet is a planet no matter how small it may be and no matter how many other planets there are like it. Just like with animals and humans. :-)

  100. Messier Tidy Upper

    Planets interact gravitationally and change each others orbits on occassions. We see this in many exoplanetary systems (eg. Gliese 876) and in the history of our own solar system. It doesn’t stop them being planets!

    See :

    http://en.wikipedia.org/wiki/Gliese_876

    for that first Laplacian resonant system discovered outside our solar system – discovered ten years ago this year. :-)

    (Already! :-o Seems like yesterday. Kinda. All these wonderful new found planets inside our solar system and around other suns as well.)

    Of course in some star and planet systems they may turn out not to be so hypothetical like, say, HD 45364 (see #53 here) or even Fomalhaut b which is still inside a disk of planetesimal material yet is far more massive than Jupiter. (Admittedly the star is much younger but still.)

    See :

    http://stars.astro.illinois.edu/sow/fomalhaut.html

    for the details on Fomalhaut b which is triple Jupiter’s mass and is located 115 AU from its Altair-like sun – vastly further than Pluto’s mere 30 AU and further out than Eris too the planet formerly known as “Xena” reaching 97 AU at its maximum distance.

    yet once those two land masses [Asia and the Americas] *were* connected into one by the Bering land bridge.

    See :

    http://en.wikipedia.org/wiki/Beringia

    for more about that. Turns out it existed more than just once. :-)

  101. andy

    Once again MTU demonstrates why it is not worth the time and effort of arguing.

    MTU appears to be labouring under a quasi-religious belief that the only reason that anyone would disagree with his/her beloved Pluto being a planet is because the IAU says so. Anyone who disagrees with Pluto’s planethood is, in MTU’s mind, an IAU shill who accepts the IAU definition 100%. In MTU’s worldview there are only the two categories: the righteous who believe that Pluto is a planet, and the evil ones who believe the IAU utterly. Every single time this argument comes up it is clear that MTU is utterly unable to comprehend the possibility that people might have arguments for Pluto’s non-planethood based on anything other than “the IAU says so”, nor that people who disagree that Pluto is a planet can also be in disagreement with the IAU planet definition (either in entirety or in various aspects of it).

    And I’m not even going to start on MTU’s utterly ridiculous representation of the Soter paper.

    Essentially regarding the planethood issue, MTU is a troll. Whether it is worth engaging with the trolls is your call.

    I certainly can’t be bothered any more.

  102. Messier Tidy Upper

    @112. andy : Your comment there is one big ad hominem fallacy and lacks any rational discussion. You provide no actual evidence, real arguments or sources to back up your views there only a false strawman caricature of my comments here combined with personal insults against me.

    You are violating the BA’s rule here by being a jerk.

    You owe me an apology.

  103. realta fuar

    The idea that there are a LOT of earth Trojans to be discovered is almost certainly wrong.
    A 2002 paper by Morais & Morbidelli in Icarus predicts about 17 larger than 100 meters in diameter. There haven’t been good ground based surveys of earth’s L4 and L5 points for very good reasons: the area is HUGE, around 3500 sq. degrees, AND only observable for a few hours near morning and evening twilight (and thus at relatively large air mass) , plus the potential targets are faint.

  104. Nigel Depledge

    Oh, curses, I was afraid I’d get dragged back into this somehow. . .

    MTU (109) said:

    To dismiss it as nota planet is, I think, ridiculous and downright wrong.

    Well, it would be if Pluto were in any way being dismissed.

    I’ve never heard anyone comment along the lines of “oh, that’s only a dwarf planet / comet /asteroid / KBO, not a real planet”. It’s hardly as if New Horizons was re-tasked to fly to a different object after Pluto was reclassified or anything, is it?

    So, how is categorising Pluto alongside objects it actually resembles “dismissing” it?

  105. Nigel Depledge

    MTU (114) said:

    Your comment there is one big ad hominem fallacy and lacks any rational discussion.

    Actually, it’s not an ad hom. It’s a lot of opinion – admittedly not backed up with references, but it’s not too hard to scroll back through your preceding posts on this relatively short thread – but it’s not an ad hominem.

    An ad hominem would be the attempt to discredit your argument by pointing out some flaw or weakness about you that has no bearing on the issue. So, to make a silly example, and ad hominem might be “MTU likes blue cheese, therefore he is not to be trusted when it comes to the status of Pluto”.

    To be frank, I can sympathise to some extent with Andy (112). You do rant when it comes to Pluto, and you repeat the same arguments in whichever thread the topic arises in.

    To date, though, you have failed to:
    1. Convince me that there is anything substantially wrong with the IAU definition;
    2. Convince me that your idea of a planet is in any way better than the IAU’s (to be frank, I think the IAU and your defintions have approximately the same plausibility and utility, except that the IAU’s has the benefit that you can see all of their planets with the naked eye or a modest-sized ‘scope, whereas you’d need a pretty damn’ big ‘scope to see all the planets under your definition);
    3. Convince me that the IAU has no right to define the term “planet”.

  106. Messier Tidy Upper

    @ ^ Nigel Depledge : I’m sorry you don’t find the arguments I’ve presented here for the ice dwarfs being planets too convincing & I’m puzzled as to *why* you don’t find them convincing.

    As for the IAU’s right to define planet, I’m not arguing that they have no rght to do but more that they made a huge mistake in their definition which I think is absurd and a dreadful mistake that needs correcting.

    Actually, it’s not an ad hom. It’s a lot of opinion

    He called me a troll which is an insult & an ad hominem as well as contemptously dismissing me and my case in favour of a better definition of ‘planet’ that includes the ice dwarf worlds as not worth debating.

    You do rant when it comes to Pluto, ..

    Do I? It’s an issue I feel very strongly about and I express my veiws as I’m allowed to when the issue comes up. I guess whether you describe that as “ranting” or somehow a bad thing is subjective – I think you are being unfair there.

    .. and you repeat the same arguments in whichever thread the topic arises in.

    I repeat some arguments because they are valid and I think work. Yes, I guess theere’s a little repetiton but again is that necessarily a bad thing? I certainly don’t think it makes me a “troll” or anything. I’m not expressing things just to upset people and I keep discussion as polite and well reasoned as I can and don’t engage in abuse of others.

    @116. Nigel Depledge :

    MTU (109) said: “To dismiss it as not a planet is, I think, ridiculous and downright wrong.”
    Well, it would be if Pluto were in any way being dismissed. I’ve never heard anyone comment along the lines of “oh, that’s only a dwarf planet / comet /asteroid / KBO, not a real planet”. It’s hardly as if New Horizons was re-tasked to fly to a different object after Pluto was reclassified or anything, is it? So, how is categorising Pluto alongside objects it actually resembles “dismissing” it?

    When you call what clearly *is* a planet NOT a planet that’s dismissing it methinks.

    Dwarf planets are small planets – not “Non-planets” just as small animals are *still* animals not vegetables or minerals and just as small human individuals are *still* human individuals not “non-people.” Is that notion really so hard to grasp?

  107. Nigel Depledge

    MTU (118) said:

    He called me a troll which is an insult & an ad hominem as well as contemptously dismissing me and my case in favour of a better definition of ‘planet’ that includes the ice dwarf worlds as not worth debating.

    OK, calling you a troll may be insulting, but it is still not an argument ad hominem.

    As I pointed out earlier, an ad hom is not a simple insult, it is a rhetorical trick whereby you attack an aspect of the person (or some other argument they have made) that is irrelevant to the discussion at hand, and then extend the outcome of your attack to pretend that the person’s present argument is unworthy, but without actually addressing this argument.

    Now, while it is true that the commenter in #112 does not address your argument, he also does not make an ad hominem argument.

  108. Nigel Depledge

    MTU (118) said:

    @ ^ Nigel Depledge : I’m sorry you don’t find the arguments I’ve presented here for the ice dwarfs being planets too convincing & I’m puzzled as to *why* you don’t find them convincing.

    There are several aspects to it.

    First, your definition would have any old lump of rock or ice that was big enough to be round classified as a planet, as long as it does not orbit another planet. In and of itself, this is not a problem, but it becomes one in the context of my other points, for which see below.

    Second, our solar system has discontinuities in the kinds of objects that we see. We see the four inner planets. We see the four gas giants. We see many asteroids, of which all the bigger ones orbit in the main belt (IIUC). We see many comets, in two classes, and these seem to originate either in the Kuiper Belt or in the as-yet-hypothetical Oort cloud. We see objects that orbit in the Kuiper Belt itself. Pluto, while it was the first KBO to be discovered, resembles other KBOs far more than it resembles the inner or giant planets. Your definition of “planet” ignores many of the discontinuities that we observe, and would allow some KBOs to count as planets but would exclude others. If we came to the system from scratch and tried to classify the objects we see, we might come up with 6 principle categories (the Sun, Inner planet, giant planet, asteroid, KBO, comet). To draw an analogy from biological taxonomy, it’s as if you are trying to classify arthropods and ignore how many legs they each have.

    Third, you have argued that the fact that Pluto has moons should distinguish it from other KBOs, but this also does not convince me because (1) neither Venus nor Mercury has any moons, (2) we have no idea how many KBOs have moons, so it may turn out not to be a distinguishing feature after all, and (3) IIUC, some irregularly-shaped asteroids also have moons.

    Fourth, you have repeatedly claimed that the IAU’s definition is illogical but you have not shown this to be so. It seems to me perfectly logical to try to define the types of solar system objects so that the definitions reflect the categories into which they naturally fall.

    So, I conclude that, while your definition of “planet” is better than simply “this list of nine objects”, it is just as arbitrary and arguably not as useful as the IAU’s definition.

    Besides, I quite like the idea of being able to view all 8 planets through a modest-sized ‘scope (not that I have ever seen Ouranos or Neptune myself).

    As for the IAU’s right to define planet, I’m not arguing that they have no rght to do but more that they made a huge mistake in their definition which I think is absurd and a dreadful mistake that needs correcting.

    So, does this mean that they have the right to define the term as long as you agree with their definition?

    Or that they have the right as long as all astronomers agree with the outcome? I gather that there was some fierce dissension in the meeting at which the definition was reached, so I think it unlikely that professional astronomers will ever arrive at a unanimously-agreed defintion.

    So, what puts you in a position to judge the definitions arrived at by the IAU? In other words, you say they’ve made a mistake, but why is your opinion germaine to the matter?

    By way of analogy, would you also consider yourself at liberty to fiercely argue against the Biochemical Society’s definition of “phosphatase” if you disagreed with it, for example?

  109. Nigel Depledge

    MTU (118) said:

    I repeat some arguments because they are valid and I think work. Yes, I guess theere’s a little repetiton but again is that necessarily a bad thing? I certainly don’t think it makes me a “troll” or anything. I’m not expressing things just to upset people and I keep discussion as polite and well reasoned as I can and don’t engage in abuse of others.

    No, I don’t think I would call you a troll (I sympathised with #112 to some extent, not in its entirety).

    However, there are issues with the arguments you make (see my preceding comment above), and I don’t think you address these well, if at all. Thus, when I see the same argument being trotted out in a different thread, I either must raise the same objections all over again or simply ignore it. It does get tiresome.

    At the end of the day, and without undermining any of the foregoing, there are two key points that I think you have not adequately dealt with:
    1. What is wrong with the IAU’s definition; and
    2. What gives you the right to judge their decision?

    If your defintion of “planet” were clearly better than the IAU’s, then I’d probably be inclined to agree with you, but your definition is just as arbitrary as theirs and it ignores the discontinuity between the 8 IAU planets and such objects as Ceres, Eris, Pluto and Sedna (i.e. these last are but the largest known examples of collections of objects, whereas the 8 IAU planets are distinctive).

  110. Nigel Depledge

    MTU (118) said:

    When you call what clearly *is* a planet NOT a planet that’s dismissing it methinks.

    But Pluto is not clearly a planet.

    It is different from any of the 8 IAU planets, and – AFAWCT – it resembles all the other KBOs.

    What you seem to be doing is analogous to calling a spider an insect and then wondering why many people disagree with you. Pluto was only ever a planet by historical precedent, because no other KBOs were known when Pluto was discovered. Subsequently, Pluto was far and away the largest known object in the Kuiper Belt for several decades so its status remained mostly unquestioned. But then we started discovering other objects that are similar in size or mass to Pluto, and it became apparent that we needed a precise definition of the term “planet”.

    Surely to fail to respond to new discoveries would be irrational?

    I have just remembered, in fact, that some of the arguments that Pluto should not be classified as a planet were first mooted before we had discovered any other large KBOs, because they had merit even before we knew about objects like Eris and Sedna.

    Dwarf planets are small planets – not “Non-planets” just as small animals are *still* animals not vegetables or minerals and just as small human individuals are *still* human individuals not “non-people.” Is that notion really so hard to grasp?

    Well, the notion is simple enough until you encounter an organism that goes by the generic name Euglena (which is both animal and plant, sort of), or until you try to identify members of two species that have only just diverged (such as the willow warbler and the chiff-chaff). There is no obvious criterion to distinguish Pluto from other KBOs and align it with the 8 IAU planets. You seem to feel that classifying Pluto alongside objects that it resembles in some way diminishes it – or is it the word “dwarf” you object to? But Pluto is what it is and our system of classifying objects in the solar system should reflect the real discontinuities that exist.

  111. andy

    Hey, MTU, scroll back up to comment 108. There you wrote the following:

    Do you or anyone else object to this definition on any lines other than it not being the IAU’s (eventual & dubious) choice?

    Do you disavow this comment MTU? Or was I right that you are unable to comprehend that anyone might disagree with your views on criteria for planethood other than for the reason that the IAU says so?

    Oh yes. and as for that apology you claim I owe, yes I admit I failed to make the case adequately for you being a troll. Nevertheless, if we add the point that you disrupt pretty much every vaguely-related thread with your pro-Pluto, anti-IAU screeds, I think the conclusion still stands.

    Have a nice day :-)

  112. Nigel Depledge

    MTU (110) said:

    The point is that in a scientific definition Occam’s razor says we should avoid superflous questions and problems being raised. A scientifically elegant solution is a simple as posible and as clear as possible. The fact that the IAU definition raises unnecessary questions like “how clear is clear” is a point against that and means better clearer definitions that do NOT raise this issue are logically and scientifically superior to it.

    But your definition includes the “how round is round” question, and you conveniently ignore this question then.

    Also, having a definition that includes the largest examples of large classes of small objects as planets is perhaps more illogical than having to face the “how clear is clear” question. Given that the IAU is defining planets only for our solar system, we are confident that all of the 8 IAU planets represent more than 99% of the mass (counting their satellites as part of this mass) in their orbital region. We also know that Pluto (including Charon, Nix, Hydra and the new moon) is substantially less than 50% of the mass in its orbital region. So, because we are only defining planets for our solar system, it really doesn’t matter where we draw the line, because there is a natural discontinuity.

    To draw again on the analogy someone posted earlier (but, I hope, making a better job of it), it’s akin to asking where exactly is the border between the UK and Australia. The answer is it really doesn’t matter.

    Your objection to the IAU’s definition on the “how clear is clear” question is really no more logical than you claim the definition to be.

  113. Nigel Depledge

    MTU (110) said:

    Yes, its true that ice dwarf planets are much smaller by orders of magnitude than the others. Jupiter is way larger than Pluto – and Earth. But think of the mammals as a taxonomic class. We have blue whales, we have humans and we have mice. The ratios of size, mass etc ..there are huge too. Mice are very different and much more numerous than blue whales, they live in a different place and have very different natures. But all these are still mammals. We don’t say that blue whales are mamals but mice are not.

    But there is a continuum of mammal sizes from shrews (often smaller than mice) to the blue whale.

    OTOH, there is a glaring discontinuity in the mass ratios when you compare the 8 IAU planets to the dwarfs.

    Pluto isn’t being recategorised because it’s too small, it’s being recategorised because it is more like the other KBOs than it is like the 8 IAU planets.

    In the same way planets include the equivalent of the blue whales (Jupiter) humans (Earth) and mice (Pluto) – and the IAU definition is doing the equivalent of ruling that mice are non-mamals because of their small size and the fact that there tend to be lots of them!

    Utter rubbish. Your analogy fails, because you are talking about size alone in mammal classification and yet the key factor in planetary status is the mass ratio of an object to everything else in its orbital region.

    However, your analogy fails for a more fundamental reason, too. Mammals have a common origin. It seems probable that the 8 IAU planets formed through processes that are fundamentally different from the processes that formed the KBOs. This likely difference in origin should at least be considered when defining solar system objects.

    Addressing your other point – actually, there were lots and lots of blue whales before we hunted them to the verge of extinction. But there is only one Jupiter.

    Ceres, Pluto, Eris and so on remain the largest known examples of quite large classes of objects (KBOs in the case of Pluto and Eris; asteroids in the case of Ceres).

    If an Earth-sized KBO were ever to be discovered, would you say that had to be a planet? Even if it formed through a different process from that which formed the Earth? And even if all of its properties apart from mass and diameter classified it clearly as a KBO?

    Sorry, MTU, but your preferred definition of “planet” would have us classify things separately based purely on how big they are (i.e. distinguishing Ceres from the other asteroids, and my hypothetical Earth-sized KBO from the other KBOs), rather than on what else they resemble. Your appeals to biological classification fail to acknowledge that biological classification demands more than superficial resemblances before things are classified as close relatives. To stretch the analogy a bit, you would include the Komodo Dragon as a mammal, because it is the roughly same mass as a lion.

    What you have failed to address is that the 8 IAU planets are distinctive, but Pluto is just another KBO.

  114. Nigel Depledge

    MTU (110) said:

    I disagree. Ceres and Pluto *do* stand out – although what “stands out” is, natch, a subjective measure anyhow as is “conspicously large”.

    Er, no, not really. Isn’t Sedna nearly the same size as Pluto (at least, I think this is the object formerly known as 2003UB313, which was initially thought to be larger than Pluto)?

    To take your own analogy and turn it upon you, you are saying that the blue whale, being the largest mammal, should be classified as something other than a mammal because it is bigger than any of the others.

    Pluto and Ceres are not qualitatively different from other objects of their kind. And that’s the whole point.

    Interesting you note the intent there was to impose a more restrictive, less inclusive (anti-Pluto and ice dwarf worlds) definition of “planet” in the first place. In essence, itsemes you’re admitting the IAU decided to strip Pluto of planet-hood first and then came up with a definition that would (falsely) justify this. IOW, Pluto was facing a IAU panel predjudiced against it and determined to demote it however bad the definition was otherwise right from the start.

    Wow, this paragraph is loaded with biased language. However, if you strip out the “Pluto is a planet” spin and look at what was actually being said, it’s more along the lines of “the IAU recognised that there were issues with Pluto being classed as a planet. For example, it resembles all the other known KBOs and shares a general orbital region with them. Unlike the other planets, Pluto’s orbit is quite eccentric, and Neptune’s gravity has forced Pluto into an orbital resonance, which cannot be said of any other planet. And so on. For these reasons, it was recognised that any meaningful definition of “planet” must exclude Pluto.”

  115. Nigel Depledge

    MTU (108) said:

    Astronomers are still searching for Edgeworth-Kuiper Cometary belt and have yet to find anything else larger than Pluto. There is the possibility that they’ll find something as large as Mercury or even Earth (or larger) out there – which won’t have cleared its orbit and will (or should anyhow) reopen the question of what’s a planet.

    Why?

    Why should a very, very large dirty snowball re-open the question of what a planet is?

    Plus there’s the possibility we *won’t* find anything larger in which case Pluto, Eris, Makemake and a handful of others are the largest and most significant bodies in that region and thus should be recognised as different to the others there which should also re-open the question.

    You say this as if your point were self-evident, but it is not. Viewed uncharitably, you seem to be avoiding making an actual argument.

    Why should the largest 2 or 3 objects of a class be recognised as different from the other objects in that class?

    Seriously, your objections to the IAU definition are getting closer and closer to a simple ” ‘Cos I don’t want Pluto’s official designation to have changed!”

  116. Nigel Depledge

    MTU (108) said:

    Indeed. It does fail.

    You have failed to demonstrate this.

    Many people consider the IAU definition so flawed they refuse to use it. I’m one.

    So? Why should professional astronomers care what you think?

    Common usage will hopefully eventually see the IAU definition become an embarrassing footnote for future generations to laugh at.

    Either that or the objections will be what future generations laugh at.

    Seriously, you have not shown that anything at all is wrong with having a definition of “planet” that places Pluto with all the other KBOs and Ceres with all the other asteroids.

  117. Nigel Depledge

    MTU (108) said:

    Nope. People are hardly being “irrational” when they notice that an object that has always been a planet is now being wrongly stripped of that distinction based on what amounts to the very dubious arguments that its too small and in a zone of other similar objects.

    If it were true that the arguments were indeed dubious, or that one of the arguments was that Pluto is “too small”, then you might be right.

    But, actually, you have not shown that the IAU’s definition is based on any “dubious” argument. Pluto is not “too small”, it is simply too small a proportion of the mass in its orbital region. Notice that your caricature of the argument is based on absolute size, whereas the real argument itself is based on the proportion of the mass in the orbital region that is represented by the object in question.

    But then, your own argument (at least, the one you state most often) that Pluto, Eris, Ceres and a few others should be called planets is based on absolute size, so for you to call an argument that you perceive to be based on absolute size illogical is the height of hypocrisy.

    Pluto is large enough to be round,

    This is trivial, and a necessary component of any worthwhile definition, to avoid including grains of dust as “planets”.

    hold an atmosphere complete with seasonal weather,

    Actually, IIRC, Pluto’s entire atmosphere appears to be seasonal. Plus, of course, this point is irrelevant since Titan ( a moon by any measure) has a thick atmosphere and Mercury (a planet by any measure) has no atmosphere to speak of.

    be geologically active

    So what? The most geologically active body in the solar system is a moon (Io). And most of the IAU planets are not geologically active (Only Earth and Venus show signs of extant or recent vulcanism). This, again, is no measure of what it is to be a planet.

    and boasts four moons and maybe rings too.

    Some asteroids have moons. Quite small ones, too (small asteroids, I mean, which are not large enouh to be gravitationally round). Some planets have no moons (Mercury and Venus). Again, the presence of moons is not a measure of planethood. Also, we have no idea how many KBOs have moons. It might be common, or it may indeed be rare among KBOs, but it is certainly not relevant. And “maybe rings” is not really any kind of an argument, is it?

    The argument that because it is no longer a lone oddball but exists amidst a zone of similar worlds is as silly as saying that because Earth exists in a zone of similar “rock dwarfs” or Jupiter exists in a zone of similar gas giants all those should be disqualified from planet-hood too.

    Well, if anyone had made this argument, you would be correct, but this is another strawman.

    The important phrase is the “orbital vicinity” (or whatever the precise IAU wording is, but this catches the gist of it). Mercury is by far the biggest object in the vicinity of its orbit. Venus, Earth, Mars, Jupiter, Saturn, Ouranos, Neptune, ditto.

    This is simply not true for Ceres, Pluto, Eris, Sedna etc.

    The eight IAU planets are distinctive within their orbital regions. Pluto is not, except as probably the largest known KBO. Ceres also is not, except as the largest asteroid.

    You seem to wish for the largest members of certain types of objects to be classed as planets by virtue of nothing else but their being the biggest examples of those objects.

    My objections to the IAU definition are based on logic

    If this is so you have yet to show it.

    AFAICT, your objection is based on emotion and historical precedent, and your arguments are mere rationalisations to justify your preference.

    and having a definition that makes good sense which the IAU definition does NOT.

    The IAU definition recognises natural discontinuities in the kinds of objects that exist in our solar system. That sounds like good sense to me.

    And in what way, pray tell, does it make sense to isolate the largest examples of large classes of objects and put them into a different category? Or would you really choose to separate the blue whale from the category “mammals” because it is the largest example of a mammal?

    If anything, MTU, your increasingly strident and irrational argumentation on this topic is driving me further into believing that the IAU did the right thing.

  118. Nigel Depledge

    MTU (108) said:

    Er .. WHY?? As (#88.) Aidan Karley has pointed out that’s an arbitrary and superflous criterion. Why would you want that included when it would for instance render Mercury a non-planet? (You did know Ganymede is larger than Mercury right?)

    Ganymede is larger than Mercury (5262 km vs 4880 km diameter respectively), but Mercury is about twice the mass of Ganymede (3.30 x 10^23 kg vs 1.48 x 10^23 kg respectively). The commenter to whom you responded here stated “more massive” not “larger”.

    Source: http://www.nineplanets.org

  119. Nigel Depledge

    MTU (108) said:

    . . . Three simple rules for planethood that are usually fairly clear-cutand easy to determine.
    Do you or anyone else object to this definition on any lines other than it not being the IAU’s (eventual & dubious) choice?

    Yes. You segregate large KBOs from small KBOs by size alone, and do the same thing for asteroids. Thus, under your definition, the largest KBOs, although still orbiting in the Kuiper Belt, would be classed as planets not as KBOs. And the same would count for asteroids. Ceres would be disinguished from the other asteroids by virtue of size alone.

    Surely a more logical definition would class objects according to their intrinsic properties, not by acting as a mere sieve?

    The “large enough to be round” criterion, while necessary to exclude all the smallest bits of junk, is problematic in other ways. The composition of the object would influence whether or not it became round under hydrostatic pressure due to the different strength of the materials. Thus, icy bodies, which are less rigid and more ductile than rocky ones, could have quite small objects classed as planets under your definition. Whereas rocky bodies, which are more rigid and less ductile, would need to be larger to achieve that roundness. So, your definition would have some KBOs counting as planets whereas larger and more massive asteroids might not count, because they are rigid enough and not quite massive enough to become round.

    Thus, you would split the KBOs and asteroids into two categories each – those large enough to be round (and therefore to count as planets) versus those not large enough to be round – but with a different dividing line for each of the two classes of objects. The fact that all KBOs have roughly the same composition and origin (as far as we can tell), and the fact that all asteroids have roughly the same composition and origin (as far as we can tell) would be swept under the rug because they are inconvenient for you.

    OTOH, the IAU definition recognises that Ceres is more like the other main belt asteroids than it is like any of the 8 major planets, and recognises that Pluto, Eris, Sedna et al. are more like the other KBOs (as far as we can tell) than they are like any of the 8 major planets except by size.

    But there are seven moons that are larger (both in diameter and by mass) than Pluto (all four Galilean satellites of Jupiter; Titan, Triton and the Moon), so does Pluto’s size really make it a distinctive object? Ceres is smaller still (although at a diameter of 970 km, it is still larger than Saturn’s second-biggest moon Rhea). Mercury, the smallest IAU planet, is 2.1 times Pluto’s diameter and 25 times its mass. The argument that Pluto should count as a planet because of its size does not really stand up to scrutiny.

    Additionally, if we don’t have some criterion other than size to distinguish a planet from a not-planet, we have to get involved in the “how round is round” question. All the known objects might fit neatly into either “round” or “not round” but one can easily envisage objects as-yet-undiscovered that confuse the issue. Maybe not main-belt asteroids (as we have probably already spotted all the biggest ones), but there are almost certainly many KBOs that we have yet to discover (possibly including some pretty large ones), and it is really hard to get decent data on the KBOs as they are so far away.

  120. Nigel Depledge

    MTU (110) said:

    *Seem* ridiculous? The IAU definition *is* ridiculous.

    No, it is not.

    You attempt to ridicule it, but your arguments along these lines are based either on strawmen (“Pluto is too small”) or on deliberate obtuseness (“how clear is clear?”).

    I think you are confusing ‘understanding’ with ‘disagreement’ – I understand the IAU definition – and I reject it for the many good reasons I have frequently given you.

    I have yet to see you give one good reason for rejecting it.

    You argue that the “gravitational dominance” criterion is illogical, but you have not shown this to be so.

    You propose that anything large enough to be round (apart from moons) should count as a planet, but you totally avoid addressing the meaningless segregation this rule would create among KBOs and among asteroids.

    You have argued by analogy to biological classification but your analogy fails for the reasons I have discussed in comment #125.

    You bring superficial factors like the presence of moons into the arena, but fail to address the fundamentals (such as context, composition and probable origin).

    In short, you have failed to make a convincing case.

  121. Nigel Depledge

    MTU (110) said:

    I have said nothing that is factually wrong.

    You have claimed that the IAU is dismissing Pluto as “too small” to be a planet. They did not do this.

  122. Nigel Depledge

    MTU (110) said:

    I agree with Asimov here. Ceres is notably spherical and is, almost certainly, geologically differentiated. It ain’t no ordinary asteroid but rather a tiny but remarkable icy little planet in its own right. Can’t wait to see it close up when Dawn visits Ceres in a year or so.

    It most likely formed alongside the other main-belt asteroids. I expect its composition will be largely similar to that of the other main-belt asteroids, even if it has differentiated to some extent. It lives alongside the other main-belt asteroids, and its only distinction seems to be that it is the only main-belt asteroid that is large enough to be round. Why not just call it an asteroid? Or leave it as a dwarf planet (which I suppose recognises that it’s the biggest main-belt asteroid without giving it the distinction of being called a full-fledged planet).

  123. Nigel DePledge says: “Now, you can argue about Pluto until you are blue in the face, but it is up to the IAU how they define the terms of their profession. Unless you are a professional astronomer, you should leave them to it.

    If you are a professional astronomer, the correct forum for discussing Pluto’s status is a meeting of the IAU, not intenet discussion fora.”

    This claim is essentially an appeal to authority and therefore a logical fallacy. The notion of a self-appointed group of experts accountable to no one making final decisions just because of who they are or claim to be is characteristic of religion, not science. What if the IAU voted that relativity is wrong or that the sky is green? Things are not true in science because someone says so. Furthermore, most of the four percent of the IAU who voted on this definition are not planetary scientists, and their decision was opposed in a formal petition by an equal number of professional astronomers led by none other than New Horizons Principal Investigator Dr. Alan Stern.

    Just because someone is a professional astronomer does not mean he or she is any sort of expert on planets. Conversely, there are many planetary scientists who are not IAU members and do not want to join the organization, especially in light of the 2006 debacle. Should their voices not count? What about the many well-informed amateur astronomers who focus on planetary science? Shouldn’t their opinions count too?

    Astronomy is not “owned” by the IAU, and anyone who studies it and has an interest in it has a right to take part in discussions of astronomical definitions. Through Internet fora, ideas are exchanged, which enhance and enrich the overall discussion. A sound rational argument deserves to be heard no matter who makes it.

    Interestingly, Dr. Hal Levison, a dynamicist, did calculate that if Earth were at Pluto’s orbit, it would not clear that orbit any more than Pluto does.

  124. “That said, many astronomers (rightly) see Pluto’s demotion as putting it back in its proper place — correcting the initial mistake of classifying a body that’s merely one of a swarm of similar miniscule objects as a noteworthy body.

    Ceres got a similar demotion, and you don’t hear a lot of complaining about that. As a matter of fact, Ceres is far more notable than Pluto as it actually stands out as the largest and roundest of the Asteroids. Where Pluto is only the first of the Plutoids that we found, and that a matter of pure happenstance.”

    Ceres and Pluto are NOT just the largest ina swarm of similar miniscule objects. Roundness is significant not in and of itself but because it means an object is shaped by its own gravity rather than by chemical bonds. This is what distinguishes complex worlds like Ceres, Pluto, and possibly even Vesta and Pallas from all those other miniscule objects. The problem with the IAU definition is that it is solely based on dynamics–how the object impacts other objects around it–while ignoring geophysics, meaning the composition and characteristics of the object itself. Putting objects large enough to be rounded by their own gravity–and the difference in threshhold for rocky and icy objects is not that great–in the same category as miniscule rocks is disingenuous because it blurs many important distinctions between these two types of objects. This is why proponents of a geophysical planet definition absolutely do view Ceres as a planet and view the nearly spherical Vesta and Pallas as an intermediate category sometimes known as protoplanets.

    When Ceres was demoted, it was not known to be round; therefore, one can excuse 19th century astronomers for making this mistake. Their telescopes could not resolve Ceres into a disk. Today, we know better. We know that being in hydrostatic equilibrium, Ceres is a complex world that is differentiated into core, mantle, and crust, just like Earth. The same is true for Pluto. Both are geologically active worlds that could host subsurface oceans. None of these things are true for asteroids or most KBOs. And Pluto is actually estimated at 70-75 percent rock, so it is hardly a ball of ice.

    What many people don’t know is that the person who initially coined the term dwarf planet is Dr. Alan Stern, back in 1991. He intended it to refer to a third class of planets in addition to terrestrials and jovians, small planets large enough to be rounded by their own gravity but not large enough to gravitationally dominate their orbits. He never intended for dwarf planets to not be considered planets at all.

    There is still a lot we don’t know about planet formation. We do know that there was migration among planets in the early solar system, so at least some are not currently in the location where they formed. Exoplanet systems show a much wider range of planet formation scenarios than our solar system does.

    The notion that the eight biggest planets are clearly different from the smaller ones only holds true if one focuses on gravitational dominance to the exclusion of all else. Compositionally, Earth actually has more in common with Pluto than with Jupiter. Jupiter is largely composed of hydrogen and helium, much like the Sun. It has its own “mini-solar system” going, and it has no solid surface. In contrast, Pluto does have a solid surface; it has nitrogen in its atmosphere, just like Earth does, and it is the only other solar system planet with a moon formed via a giant impact.

  125. Adan Rodriguez

    I say name them Themistoicles and Leonidas.

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