When worlds collide

By Phil Plait | August 10, 2009 12:31 pm

The Spitzer Space Telescope has detected signs of an interplanetary smashup, and oh, what a colossal event it was… apparently, 100 light years away around the young star HD 172555, an object the size of the Moon slammed into a planet the size of Mercury!

First, the way cool animation they created portraying the event:

Whoa.

So how did they figure this out?

Astronomers used Spitzer to take infrared spectra of HD172555, a young star only a few million years old. By breaking up the light into a spectrum, you can determine what elements and molecules are in the light’s source (I’ve written about this sort of thing before). When astronomer Carey Lisse looked at the spectrum from the star, he got a shock: it was a mess. His team finally figured out what they were seeing: amorphous silica, and lots of it.

In other words, glass.

Glass? From a star?

The most likely explanation is that the glass is in the form of tektites, which are blobs of glassy material that form when something big hits something else big. The silica gets fused into glass. But that means that there was a pretty big impact that must have happened at that star, and that in turn means that two planet-sized objects must have had a very bad day. This was supported by the detection of other chemicals consistent with the aftermath of a massive collision.

The best fit to the data suggest that one object was planet-sized and the other Moon-sized, meaning the collision would have been at very high speed — several kilometers per second — and launched an unimaginable amount of material into space. Furthermore, it couldn’t have happened too long ago, or else the material would have dissipated and wouldn’t have been seen. It looks like this was a recent event, then, occurring maybe only a few thousand years ago!

And there’s another thing that I find personally very cool. Remember, HD 172555 is only 100 light years away. That is extremely close on a galactic scale (our galaxy is 100,000 light years across, so this star is our next door neighbor). It seems incredibly unlikely that this is a rare event in the galaxy, since this happened so close by and so recently. It’s far more likely that it happens all the time in young solar systems, which means it happened to us, too. Well, a long time ago, like 4 billion years ago when planet-sized objects were more common and still forming in our solar system.

But it lends credence to the idea that planetary-scale collisions do happen. We know this already, of course, because we’re pretty sure the Moon formed when a Mars-sized object smacked into us over 4 billion years ago, and we see evidence of vast impacts on other bodies in the solar system as well.

But to see evidence of it around another star, with different conditions, means this sort of thing is common, and is an inevitable step in the process of solar system formation. The Universe is a violent place, but every day it looks more likely that violence is necessary for us to be here at all.

I’m not sure there’s a lesson in there worth extrapolating, but it’s certainly an interesting thing to be reminded of.

P.S. The animation above is cool, but not a perfect representation of what happened. For example, the shock wave ring travels around the planet as shown, but when the ring converges on the point opposite the collision point, there would be a huge explosion and a vast plume of material launched into space. No one ever puts that in their animations, and I think it would be very cool! I need to get people who create physics-based simulations to make one that’s accurate, so it can be used in situations like this.

CATEGORIZED UNDER: Astronomy, DeathfromtheSkies!, NASA

Comments (85)

  1. Well, all I can say is A++ for NASA finding a small, rocky world so very close. Sure, it’s a mostly molten, misshapen blob for now, but give it a few billion years and another impact or two and there may be people on that rock looking through telescopes at our giant red, dying sun, and wondering if there was ever life over here.

  2. Evil Eye

    Why no recoil? All I see left is the original planet and some chunks.

    Not even an impact cloud? (Like a mushroom)

    Nothing at all? Just a bonk?…oops?

  3. Jason Wilson

    So, coupling some recent topics. By looking at the aftermath they’ve found would they be able to determine if this were ever potentially life-supporting planet?

  4. In other words, glass.

    Perhaps we could send optical engineers out to it to shape and polish it, creating a planet-sized lens. Imagine the telescope that would fit that baby!

  5. Utakata

    Once again I’m about to show my ignorance to science in this field…

    …thanks for the PS, but something really bothers me still about it. Do planets really bang into each other like billiard balls as depicted? I always thought gravity from both objects would have them swing into each other like lovers on a skating rink. You know…much more graceful and elegant like. Though still causing “unimaginary” catastrophic release of kenetic energy. I suspect it something to do with speed each object approaches each other. Either way, this inquring mind still needs to know. Thanks. :)

  6. Not that there was a lot of doubt, but is this the first really conclusive evidence that there are rocky worlds around stars other than our own?

  7. mgmoody42

    Wouldn’t there also be shock waves traveling through the center, or at least perpendicular to the tangent angle of impact, of the planet as well?

  8. @inverse

    If my vast knowledge wielded from my experience as an armchair astronomer is remotely accurate (which could be a stretch in and of itself) they’ve found evidence of rocky worlds out there, but they’re usually massive – 5-10 times the mass of earth or more, since that’s the amount of mass it would take to create the distortions on the orbiting star that we can see with our current telescopes (Kepler excluded). I don’t know of any Earth or Sub-earth exoplanets ever being discovered to date, though the good Doctor Plait is really the one to ask about that.

  9. Glass?

    It wasn’t the planet, it was the Crystal Sphere!
    ;)

    J/P=?

  10. adam

    I’ve always wondered if a mars sized object hit the earth and the debris formed the moon, what happened to the rest of the other mass?

  11. What I’ve always wondered about large impacts is what they’d look like in real-time. The Hollywood version always has the meteor/asteroid impactor spending several seconds in the atmosphere (with a long, fluffy contrail), but then the shock wave and ejecta spreading out at super mega hypersonic(tm) speed.

    I’ve always assumed that is backward (though never really known for sure). Anyone know of a meteor impact simulation/animation done in real time?

  12. Adam says: I’ve always wondered if a mars sized object hit the earth and the debris formed the moon, what happened to the rest of the other mass?

    The material gets thrown out in all different directions. Maybe some of it is leftover in the asteroid belt (won’t that be an interesting search…for asteroids with the the same chemical makeup of Earth) But there are a couple of things we don’t know (and may never know), such as the original mass and size of the Earth before the collision. Much of that material would have stayed in orbit then rained down on the new Earth/Moon pair. We definitely know the Moon has taken a pounding since that day.

  13. andy

    I don’t know of any Earth or Sub-earth exoplanets ever being discovered to date

    The innermost planet orbiting the millisecond pulsar PSR B1257+12 has a mass approximately twice that of our moon. There are two additional planets with masses 4 times that of the Earth. The system was the first extrasolar planetary system to be confirmed, several years before the discovery of the hot Jupiter orbiting 51 Pegasi began the current radial velocity planets bonanza.

    Unfortunately it seems that the media are a bunch of fusion chauvinists and seem eager to herald each new several-Earth-masses planet as the “smallest exoplanet yet”. Oh well.

  14. Mike Skoog

    “because we’re pretty sure the Moon formed when a Mars-sized object smacked into us over 4 billion years ago”

    I thought this was basically accepted as fact these days.

  15. Joe Meils

    Finally! 30 years after watching the movie at a Saturday Afternoon Matinee, I get to see the event that George Pal didn’t have the budget to portray on screen. (I can’t tell you how cheated I felt when I saw them shoot up flames, and cut away from the impact during “When Worlds Collide.” Someone should take that scene, and intercut this animation with it… “WWC: The Special Edition!” Kids, you don’t have to throw popcorn bags at the screen anymore!

  16. Alan

    I hope the inhabitants of that planet had their towels ready (courtesy Douglas Adams).

  17. Lucky I was in my underground bunker when this happened!

  18. Paul M.

    What they’re not saying is that they also saw Kryptonite in the spectrum…

  19. Whoa! Science totally kicks butt! This is so cool!

    See, Planet X was real, it was just in the wrong solar system!

  20. Peter

    “It seems incredibly unlikely that this is a rare event in the galaxy, since this happened so close by and so recently.” – Sorry, but this is a statistical non-sequitur. The distance and timing of a singular event don’t allow for any conclusion about the genereal probability for such an event.

  21. Thanny

    I think this is an excellent example of why the IAU’s attempt at the definition of a planet is too flawed to be taken seriously.

    Despite the label in the video, according to the IAU, that was not a planet! It had obviously not “cleared its orbital neighborhood”. Yet if you grant no other objects, it’s not a planet before the collision, but is a planet afterwards. Any definition that works like that is not a definition at all.

  22. Lukas

    I was wondering the same thing as #14 ( Jim Seymour): this animation is obviously not realtime. Is there anything out there that shows the impact energy travelling through the impacted body at a (i assume) more realistic (less than a couple % of c) speed?

    Also what #7 (Utakata) said.. would those bodies just run into each other head on?

  23. Norbert

    As others noted, the video is obviously not realtime, actually looks like it’s 700x faster with respect to the speed of the impactor and say 50x with respect to the propagation of the shock wave.
    The Moon-sized objects travels its radius in ~~1s in the video, when in fact it should be something like (Moon radius)/(Mercury escape velocity) ~~ 3500 km/5 km/s = 700s.
    Also, the shock wave should travel with the speed of an earthquake wave, say 20km/s, which means that it would take (Mercury radius)/(shock speed) ~~ 4900km /20 km/s = 245s.
    (Data from Wolfram Alpha and Wikipedia)
    More realistic video would be much cooler.

  24. Stephen

    comment deleted

  25. Stephen

    oops- mental typo

  26. amphiox

    #3, 28, etc:

    This is a very young system – the process of planet formation is not yet complete. It is therefore quite unlikely that either of the planetoids involved in the collision, or any other planetoid currently in the system, would be suitable for life. Their stage of development is equivalent to earth’s back in the early phase of the aptly named ‘Hadean’ era.

    The IAU planet definition essentially only applies to mature planetary systems, not ones still in the process of formation, like this one. It is through collisions and mergers (and other gravitational effects) that the larger planetoids both grow and clear their orbits. Essentially the mercury sized object is “clearing” the moon sized object out of its orbital neighborhood by blasting it apart and probably merging with the majority of it (and ejecting the remaining fragments). And of course, something even bigger might later do the same to the mercury sized object.

    I remember that a major pillar of the ‘Rare Earth’ hypotheses is that our large moon is very important to earth’s long-term habitability (a claim I question), and that the collision that produced our moon was a very rare one-off event, a claim I’ve always really questioned, and this observation I think puts that to rest.

  27. coolstar

    Thanks Peter and Norbert! Is the word “tektite” really the best way to describe the remnants of
    a collision in another solar system? While those remnants and terrestrial tektites seem to be similar (impossible to prove that they’re identical) in chemical composition, the word tektite makes me, at least, think of the way some impact debris on the earth is aerodynamically shaped by the earth’s atmosphere ( I realize that perhaps ALL tektites don’t share that property).

  28. I am a big fan of this page! Keep up the good work. :)

    15 : Most recent models have much of the deep interior Theia, the Mars-sized impactor purported to have hit the Earth and spun off the Moon, as merging with the Earth’s core, and the most of the rest getting blasted into circumterrestrial orbit. Some must have gone into circumsolar orbit as well.

    25: Dang, I knew I forgot to look for something. Anybody got a good thermal emission spectrum of kryptonite for me to use?

    30: The animation is indeed only an approximation of what happened, and was calculated for a direct head on collision, but contains much of the important physics of the event.

    33: Nice reply! Your reasoning sounds sound – except for your last point, I think. Making a large moon around Earth does require a relatively special pool shot – a slow, glancing blow by Theia. Any fast shots put most of the spalled off material into solar orbit.

  29. Peter @ 27

    Maybe, but it isn’t a singular event. This is the second one we know of in the last 4 billion years within 100 light years.

  30. Alan

    I agree Amphiox,
    Doesn’t the Pluto/Charon system show that large moons are not absolutely rare.

    Does it take a very large planet and relatively small moons to make a ring system ?

  31. I think that the video gets lots of the detailed physics wrong.

    Suppose that the relative velocity was 10 km/sec, and the diameter of the larger body was 2400 km (i.e., Mercury sized). Then. to go 1 diameter takes 240 seconds, or 4 minutes, which I would expect would set the time scale for the collision, so the video is simply too fast. In the few minutes or so before the impact, the tidal force from the larger body (assuming roughly similar densities) would begin to disrupt the smaller body (well inside the Roche limit, after all), and also deform the larger body. I would expect a jet at the antipodes, but also that the shock wave would reflect off the internal layers, causing ejections in other directions. That may be what they are trying to show, as it looks like the entire crust and upper mantle of the larger body was removed.

    Finally, if either body had an intact core left after all of this, it would ring through normal modes like crazy, for days or weeks, looking more like a raindrop than a planet. THAT, I would like to see.

  32. Mang

    Ok, so when do we get to hear the Earth shattering kaboom :)

    (Sorry couldn’t resist).

  33. Tom

    Peter @27: Modern statistics is perfectly able to draw draw conclusions from singular events based on our large body of prior knowledge. This is one of the big claims to fame of the Bayesian revolution.

    In the case at hand, informally, we can say that this observation increases the plausibility of models that predict frequent massive collisions of this form, and decreases the plausibility (to zero!) of models that say they never happen.

    The details of the collision may also be used to increase or reduce the posterior plausibility of various models of planetary formation.

  34. Levi in NY

    Sweet! Explosions = awesome. Mega-explosions = mega-awesome. Scientists call that the Hyneman-Savage Principle. I hope both planets were uninhabited at the time of impact though.

    As others pointed out, you missed one of the biggest news stories here: this is strong evidence for extrasolar terrestrial planets, and systems with multiple terrestrial planets at that. Which makes this mega-explosion mega-mega-awesome (tera-awesome?).

  35. Markk

    #27 not true. One event can reset Bayesian expectations a lot. Given that there are millions of young solar systems in our view and our current formation theories have an expected probability that a collision event will occur per system per time interval, then the possibility that one occurred in our neighborhood in the last few thousand years has a definite predicted probability.

    This particular detection likely caused no real issues with current theories but it certainly could have made a difference if the probability was really low. It sets a lower limit on the expectation of rarity of these events. In reality it isn’t one such event anyway, it is one outcome over all the stars we have studied the spectrum on to this level of detail. That is the number of “events” we are looking at.

  36. Josh

    What if the Asgard blew it up because it was the replicator homeworld?

  37. CMLisse (#35): I can’t believe I wrote your name as Casey. I fixed that. And thanks! This was a cool press release.

  38. Canadian Astronomy

    Jaw. Hits. Floor!

    The fact that we can detect and infer this is so amazing. Incredible!

  39. LouAlbano

    I felt a great disturbance in the Force, as if millions of voices suddenly cried out in terror and were suddenly silenced. I fear something terrible has happened.

  40. Ron

    “The Miracle Planet – Violent Past” has a great impact simulation between a modern day Earth and a 1500 km asteroid that’s moving at 700,000 kph. It doesn’t have any plume coming out the far side, but it is, none the less quite awesome to watch. The actual impact takes about 10 seconds so the time scale seems like it is more or less real-time.

  41. StevoR

    @ 10. somecallmejim Says:

    @inverse : If my vast knowledge wielded from my experience as an armchair astronomer is remotely accurate (which could be a stretch in and of itself) they’ve found evidence of rocky worlds out there, but they’re usually massive – 5-10 times the mass of earth or more, since that’s the amount of mass it would take to create the distortions on the orbiting star that we can see with our current telescopes (Kepler excluded). I don’t know of any Earth or Sub-earth exoplanets ever being discovered to date, though the good Doctor Plait is really the one to ask about that.

    & also @ 16. andy Says:

    … The innermost planet orbiting the millisecond pulsar PSR B1257+12 has a mass approximately twice that of our moon. There are two additional planets with masses 4 times that of the Earth. The system was the first extrasolar planetary system to be confirmed, several years before the discovery of the hot Jupiter orbiting 51 Pegasi began the current radial velocity planets bonanza. Unfortunately it seems that the media are a bunch of fusion chauvinists and seem eager to herald each new several-Earth-masses planet as the “smallest exoplanet yet”. Oh well.

    Yes – the pulsar planets are definitely small in mass & probably rocky – although they are possibly carbon-based planets rather than silicon and thus have tarry surfaces over diamond mantles & cores! :

    ***

    PSR B 1257+12 : Discovered in 1991 these pulsar planets were the first ever found. Four very low mass worlds orbit a pulsar with the inner three spaced like a half-sized model of our inner solar system and the outermost just 1/5th Pluto’s mass* at a distance equivalent to the asteroid belt in our system.

    ***

    @ 28. Thanny Says:

    I think this is an excellent example of why the IAU’s attempt at the definition of a planet is too flawed to be taken seriously.

    Despite the label in the video, according to the IAU, that was not a planet! It had obviously not “cleared its orbital neighborhood”. Yet if you grant no other objects, it’s not a planet before the collision, but is a planet afterwards. Any definition that works like that is not a definition at all.

    Absolutely right! I think the IAU got things very badly wrong and this is just one example of how absurd & unworkable the IAU definition – foisted on the world undemocratically and unscientifically truly is.

    Another absurdity is that if you put Earth out where Pluto is it wouldn’t be a planet anymore – and vice-versa putting Pluto where Earth is would then make it a planet. Plus the definition makes double planets as imagined by many Sf writers (for instance “Eau & Roche” in Robert L. Forward’s novel ‘Flight of the Dragonfly’ impossible by definition.

    I won’t get started on the dozens of other issues with the ridiculous, stupid and politically rather than scientifically motivated IAU anti-Pluto dictat … Except to say that the IAU needs to change their definition ASAP to a more reasonable one which removes the “orbital clearing” baloney – because the longer they leave it the worse it makes themselves – and by extension astronomers generally look. :-(

    —–

    * NB. Note there that one confirmed pulsar exoplanet – PSR B 1257+12 e ,the smallest and outermost of them – has just 1/5th Pluto’s mass – so again saying Pluto ain’t a “planet” is inconsistent with reality and downright wrong.

    Also our Sun is a “dwarf” star – like 90% of other stars. Do we take its “dwarf star” status as meaning it ain’t a proper star? Of course not! Surely then, logically and consistently, a dwarf planet too remains a proper planet!

    Frankly, the IAU definition is just too dumb for words and not only does it need to be reversed pronto but also those responsible for making it should be expelled from the IAU for their stupidity too!

  42. Plutonium being from Pluto

    Channelling Dr Suess :

    A planet’s a planet no matter how small
    Those IAU fools can’t change that at all!

    Also memo to Neil DeGrasse Tyson -

    The public and esp. those schoolkids that you abuse and mock who support Pluto are correct and you are wrong. Now accept it, stop being so nasty and making astronomers look like utter a-holes & just get over yourself!

    NB. As a proud Plutonian I can certainly assure you my homeworld is in fact a planet! ;-)

    @ 45. Canadian Astronomy Says:

    Jaw. Hits. Floor!

    The fact that we can detect and infer this is so amazing. Incredible!

    Seconded by me. This is just so awesome. 8)

    @ 46. LouAlbano Says:

    I felt a great disturbance in the Force, as if millions of voices suddenly cried out in terror and were suddenly silenced. I fear something terrible has happened.

    Hmm … If so you’ll be sensing something *other* than this Mercury-sized & Moon-sized collision around the forming planetary system of HD172555 I’d say. Its very unlikely there are any sentient beings in the system at all let alone on either of the doomed planets.

    … Unless there are a few alien races visiting to witness this collision & its aftermath for themselves! ;-)

    @ 8. inverse Says:

    Not that there was a lot of doubt, but is this the first really conclusive evidence that there are rocky worlds around stars other than our own?

    No. As others here have pointed out we already know of a few low-mass worlds that are most probably rocky.

    Planetary models seem to suggest – please correct me if I’m wrong folks – that the dividing line for Mini-Neptune’s vs SuperEarths (or SuperVenus’) is around the 5-7 Earth mass mark with planets above that being probable Neptune-like gas, ice or hot ice giants and planets below that being probable rocky superEarth’s although this is theory rather than observation based. To be honest I think we don’t really know what a 5 or 7 Earth mass planet is like.

    We also have evidence going as far back as the 1983 (?) IRAS (Infra Red Astronomical Satellite) studies of Vega, Fomalhaut and Beta Pictoris (among others) revealing dusty disks of material in process of forming rocky planets around those stars.

    More recently, we’ve found dusty material around white dwarf stars where asteroids or perhaps even whole rocky planets have fallen in towards the stellar corpse and been shredded by is gravity or possible collisions. For example, the white dwarf star “named” GD 362 has a dusty disk of material around as noted in the July 09 issue of ‘Astronomy’ magazine’ which featured an article on this as its cover story.

    Mind you, this latest finding sure is good direct observational evidence too! :-)

  43. Chris Reed

    And just think that a couple of years ago, we were stunned and happy to detect the first planets around other stars… Now we’re detecting planetary collisions.

  44. Funkopolis

    I suddenly crave a video game, er, serious scientific simulation, that lets you just throw planets at each other and see the results in animations like this. Someone get on that. Er, for science.

  45. Peder

    48. StevoR Says:
    Another absurdity is that if you put Earth out where Pluto is it wouldn’t be a planet anymore – and vice-versa putting Pluto where Earth is would then make it a planet.

    Except if you put Earth where Pluto is it surely would clear the orbital path and hence be a planet again…

  46. Marko Rauhamaa

    Here’s how you can simulate an asteroid impact in real time.

    Take a huge beach ball with a one-meter diameter. Then, take a small grain of sand (1 mm across) between your thumb and forefinger. The beach ball is Earth, the grain of sand is a humongous asteroid (> 10 km).

    Now, hold the grain of sand at a distance of one meter from the beach ball. Slowly inch your hand closer to the beach ball, so that after 20 minutes the sand grain touches the ball.

    Witness the end of all humanity and most of the mammal species.

  47. Thor

    Ooh
    Er
    So that’s what I did last eon when drunk?
    Ooh. Sorry everyone.

  48. Christian Treczoks

    If you are doing that scientifically correct simulation and animation that you thought of, then please include the aftermath – what happens to the parts that flew off the planet and formed this kind of halo? How much of it gets “blown away”, how much returns to the planet – and in which timeframe?

  49. firemancarl

    Stuff like this happened 4 billion years ago? I was pretty sure that it was 6000 years ago……

  50. And just think that a couple of years ago, we were stunned and happy to detect the first planets around other stars… Now we’re detecting planetary collisions…

  51. Ken

    I love the video, but have a small nit – maybe. The surfaces of the bodies look a little old for a system still in the equivalent of the Hadean. The smaller body even shows Moon-like craters including one with distinct impact rays. Surely the crusts would not be thick enough at this stage in the system to support such features?

    (Actually I had two small nits, but others have already covered the timing issues.)

  52. amphiox

    #47 StevoR:
    I assume that when you say “earth” you mean “earth and moon”, because if you swapped earth and pluto and left the moon in place, the moon would become the planet, and, in all likelihood, pluto would become the moon! (or be ejected from the orbit)

    #41 Peder:
    I’m not sure if earth would actually be able to clear the orbital area around Pluto, because there is a lot more stuff, and a lot more space in an outer orbit like that of Pluto/other inner Kuiper belt objects, than there is in an inner orbit like earth’s. One of the weaknesses of the IAU definition is the fact that the mass requirement for orbital clearance changes depending on distance from the star. (And, as I previously noted, also depends on the age of the solar system – at the very beginning, even Jupiter hadn’t cleared its orbital neighborhood, yet)

    On the other hand there is a clear qualitative and quantitative difference, easily observed, between objects that have cleared their orbital neighborhood, and those that haven’t. To develop a definition that does not acknowledge this plain fact in some way would in my mind be equally unsatisfactory.

    Ideally, I think we ought to have a classification system that can accommodate multiple subtypes to truly account for the great diversity of planetary types that are doubtless out there, but we don’t yet know enough about the potential varieties of planets to do this.

  53. stardriver

    Yea, what he said!!! ^

  54. sam

    WOW !
    Looks like the exact same planet disrupting details that were shown on one of the Star Trek shows when the “Genesis” torpedo slammed into a lifeless planet and created new life – ” The Genesis Effect” ( movie is ” The Search for Spock ??? ). Now, who’s copying off of who, MGM from Nasa or Nasa from MGM. If the latter, does this mean we didn’t really go to the moon?
    By the way … why is it we had so much technology in the 60′s to go to the moon, but havn’t done squat since ? Did someone steal or technology or did we misplace it. Millions + of dollars to send remote control buggies to Mars …. Mattel and Hasbro have been making remote control buggies for years. Another person has made a remote control lawnmower. – Whats up ??
    I’m not getting it !

  55. Tadd M.

    Quote

    “because we’re pretty sure the Moon formed when a Mars-sized object smacked into us over 4 billion years ago”

    I thought this was basically accepted as fact these days.

    Unquote

    Actually, There are a lot of theories of how the moon was formed. they agree that that is the most probable theory.

  56. Dr. Hal Levison did the calculations years ago for Earth in Pluto’s orbit and determined definitively that it would not clear that orbit. As several people here have said, the IAU definition is biased in favor of objects closer to their parent stars. The further an object is from its parent star, the bigger the orbit it will have to clear.

    The IAU GA is going on right now. Anyone who wants to see the planet definition discussion reopened should email the IAU executive committee and Working Group on Planetary System Nomenclaure. Email addresses are available at http://www.iau.org

  57. andy

    The IAU resolution is a (very?) flawed attempt to set out what is actually a fairly reasonable way of describing the solar system. You have planets and you have stuff in “asteroid” belts. And sorry to say, the view of the solar system today is rather different from the view back in 1930 when Pluto was discovered. Lose a planet, gain the Edgeworth-Kuiper Belt and the scattered disk…

    As for those who keep harping on about how the orbital location is absurd, who mourns for Triton? Getting booted out of potential (dwarf) planethood just because it got captured into Neptune orbit. So unfair…

  58. I mourn for Triton.

    (weeps)

  59. Mike Mullen

    So now we know what really happened to the Death Star, I never bought into that Rebellion propaganda film, destroyed by a single torpedo down an exhaust port, hah! Clearly a terrible navigation error, probably using Imperial measurements instead of metric. :)

  60. Plutonium being from Pluto

    @ 65. somecallmejim Says:

    I mourn for Triton. (weeps)

    Bit early for that! Don’t mourn too much too soon – Triton still has millennia left to its life – plus it will be transformed into a great set of Neptunean rings! Oh,wait you meant as a planet not a moon? ;-)

    @ 51. Peder Says:

    Except if you put Earth where Pluto is it surely would clear the orbital path and hence be a planet again…

    Bzzzt! Wrong – as Amphiox pointed out that’s just not the case.

    @ 59. amphiox Says:

    #47 StevoR: I assume that when you say “earth” you mean “earth and moon”, because if you swapped earth and pluto and left the moon in place, the moon would become the planet, and, in all likelihood, pluto would become the moon! (or be ejected from the orbit)

    Well maybe … Or we could have two planets for the price of one : Pluto & our Moon. Three counting Charon as half of a double planet! ;-)

    Or Pluto could be at the Lagrangrian L1 point in astable orbit with Earth & Moon – making for a double double planet! (Earth-Moon, Pluto-Charon) Wonder if a sort of three body suituation there would be stable? Might make things interesting for Pluto’s other small moons Nix and Hydra and its likely rings too! ;-)

    @ 63 Laurel Kornfeld :

    (BTW. Love your Pluto blog & thanks again for posting my “12 reasons why Pluto is a planet” there! Much appreciated. :-) )

    I have just sent the IAU* the following letter by email – everybody who agrees with this please feel free to copy and send as well:

    ***
    Dear IAU administration

    I am writing to request and strongly urge you to please revise the current IAU definition of planet which is logically flawed and inconsistent. I find the third “orbital clearing” criterion in the current IAU definition fails the ‘reductio ad absurdum’ logic test having ridiculous consequences.

    Among these flaws is the bias towards planets orbiting closer to their primary. For instance, if the Earth orbited where Pluto does, it would no longer be considered a (“classical”) planet and were Pluto to orbit where Earth is it then would be considered a planet rather than merely a “dwarf planet”. Moreover, all planets in our solar system would be rendered dwarf planets by a strict use of the orbital clearing criterion since sun-grazing comets and asteroids cross the orbits of all planets making them not strictly clear and thus not strictly “planets”!

    The current IAU definition is also absurdly limiting in excluding objects that clearly would be termed planets for instance were exoplanets found in the ‘double planet’ configuration of two planets of even superJovian mass then neither would be classified as a planet nor – by current IAU definition – could we ever have colliding or intersecting planets. There are already examples of exoplanets that are accepted as being planets yet breech the strict IAU definition – notably:

    1.Fomalhaut b which hasn’t “cleared its orbit” but circles within a protoplanetary disk. (The same situation also applies to the planets around HR 8799.)

    2. HD 45364 b & c which are analogous to the Neptune-Pluto planetary pairing in intersecting orbits in a 3:2 resonance.

    3. The outermost and smallest “pulsar planet” PSR B1257+12 e which has a mass only 1/5th of Pluto’s and yet is termed a planet.

    The orbital clearing criterion is also vague, arbitrary and poorly defined – it adds an unnecessary and confusing level of complexity to a definition which should be basic and easily determined.

    There is also the inconsistency of having dwarf stars accepted as “proper” stars and yet dwarf planets rejected by the IAU as “proper” planets. It seems ludicrous to suggest that dwarf stars – which include our Sun and 90% of all stars are NOT “proper” (‘classical?’) stars yet this is the logical extension of the “dwarf planets are not “proper planets” current definition and exactly analogous.

    I think these logical flaws and inconsistencies make the current IAU definition unscientific and, in the long term, unworkable. I consequently strongly urge and request that the current definition of the term “planet” be reconsidered and changed to a more rational, more inclusive and scientifically justifiable definition. I suggest something like the original 2006 Prague definition which excluded the illogical and absurd “orbital clearing” criteria, eg. defining a planet as an object which:

    a) Has enough gravity to be in hydrostatic equilibrium (i.e. is round or if rapidly spinning a rotationally flattened spheroid.)

    b) Is incapable of ever being self-luminous shining by nuclear fusion. (I.e. is not a star or brown dwarf.)

    c) Is not directly orbiting another planet. (I.e. is not a moon.)

    The classifications of planets so defined could then be further sub-divided into other specific types based on composition (rocky or terrestrial, gas giant, ice dwarf) and orbit (Hot Jupiter, Hot Neptune, Pulsar planet) and so forth.

    I think that this simple, clear, logical and scientifically useful inclusive definition of the term “planet” which would include the current dwarf planets is vastly superior to the current scientifically and logically flawed, highly controversial and publicly unpopular and definition.

    I beg you to you to consider this definition and above all to change the current one to a more rational and reasonable one which I strongly believe should count Pluto and Eris as full planets.

    Sincerely:

    Steven C. Raine
    (w my email, phone & address)

    ***

    * Sent to their general email address & to IAU General Secretary, Dr. Karel A. van der Hucht & IAU Executive Assistant, Vivien A. Reuter. Hope those are the right people – were the main ones listed on the contact addresses anyway. I am also going to post (snail mail) a copy too.

  61. StevoR

    Not part of my letter but does anyone, anyone at all use that dumb term “classical planet(s)” anyway? If so, then I haven’t seen it outside – very occasionally – the IAUs unreasonably demoting Pluto debate.

    Besides, to me anyhow, the term “classical planet” would be far better used to describe the planets originally known to the ancient civilisations and earliest astronomers – the five “wandering stars” easily visible to the unaided eye of Venus, Jupiter, Mars, Mercury & Saturn.

    Also not included in the letter are a few other points like the fact that Pluto has three moons, rings and an atmosphere which is more than other planets such as Mercury can boast – so if Pluto isn’t a planet why should we consider Mercury one? Of course, once we start arbitrarily ruling out planets w/o good reason* as happened to Pluto then where do we stop?

    Plus there’s the whole incredibly anti-democratic and ideologically driven Prague IAU conference “vote” which excluded relevant experts like Alan Stern and was essentially an un-scientific dictator-style coup by the Pluto-bashers with only one side being given a chance. August 24th 2006 was, I think history will one day state as fact, the IAU’s most shameful and darkest day which hurt the reputation of astronomy as collateral damage along with disgracing that of the IAU.

    * No, Pluto being located in the Edgeworth-Kuiper Belt and small is NOT in my view a good reason. At all.

  62. StevoR

    @ 59 Amphiox :

    On the other hand there is a clear qualitative and quantitative difference, easily observed, between objects that have cleared their orbital neighborhood, and those that haven’t. To develop a definition that does not acknowledge this plain fact in some way would in my mind be equally unsatisfactory.

    Regarding your first sentence there: Is it? Really?

    See the case of the exoplanetary system HD 45364 b & c which are analogous to the Neptune-Pluto planetary pairing in intersecting orbits in a 3:2 resonance. See Ken Croswell’s article : http://kencroswell.com/HD45364.html for more.

    Or for that matter the imaged exoplanets of Fomalhautb & Hr 8799 which are still located in their protoplanetary disks. Seems to me this distinction is a *lot* less clear than you imply.

    If Earth was located where Pluto is and behaving the same as Pluto would it cease being a planet in your eyes? Really?

    Regarding your second sentence there : Why would you think it unsatisfactory?

    Having a “Cleared orbit” is a function of a given planets time of existence and distance from its sun and thus is dependent on a whole number of factors extrinsic to its basic nature. (Eg. age, distance, the amount of other material present originally, gravity, how much a material is tossed its way by other planets in the system, etc .. ) I do not see why it does or should matter all that much.

    IMHO “orbital clearing” is very much a secondary and contigent trait rather than a defining one.

    @ 58. Ken Says:

    I love the video, but have a small nit – maybe. The surfaces of the bodies look a little old for a system still in the equivalent of the Hadean. The smaller body even shows Moon-like craters including one with distinct impact rays. Surely the crusts would not be thick enough at this stage in the system to support such features?

    Back on topic here – I agree & thought that too. :-)

    I also wondered why the planets wasn’t already molten at least in patches from the pounding they’d been getting as they formed – and why the gravity didn’t stretch both these exo-Mercury & exo-Lunar planets out more as they approached.

  63. Allan

    And are all these complaints about Pluto’s status driven by the fact it was discovered by an American? Really, who cares. Scientifically it makes more sense for it to not be considered a planet. Pluto is not alive and does not have feelings so the concept of “unfairness” is surely somewhat silly. I think there is more for the IAU to do than waste their time reopening this dead can of worms.

  64. StevoR

    @ 70. Allan Says:

    And are all these complaints about Pluto’s status driven by the fact it was discovered by an American?

    No. I, for one, am an Aussie.

    OTOH, more than a few people have a sneaking suspicion that an anti-American bias was one factor influencing the Pluto-bashers.

    Really, who cares.

    Actually lots of people do!
    Again, for one, me.

    Scientifically it makes more sense for it to not be considered a planet.

    Wrong, wrong, wrong & wrong. For a large number of reasons which I’ve already gone into a number of times.

    How we define “planet” does matter – and the IAU definition which was drawn up specifically to exclude Pluto from planethood is an appallingly bad one from a scientific and logical viewpoint as you can see from my posts 67- 69 earlier.

    Pluto is not alive and does not have feelings so the concept of “unfairness” is surely somewhat silly.

    Pluto may be an inanimate object but people are not and this matters to people. Fairness and justice is a fundamental human principle and this also matters. Doing science too is a human activity when it is unfair it reflects badly on science and scientists.

    Treating Pluto fairly is thus anything but silly – and your question there makes me wonder why do you seem to want Pluto treated unfairly. What is your problem with our solar systems Ninth Planet?

    I think there is more for the IAU to do than waste their time reopening this dead can of worms.

    As I’m sure you won’t be surprised to hear by now, I couldn’t DISAGREE more.

    The anti-Pluto decison by the IAU makes them – quite rightly – look stupid and ridiculous. This tars astronomers and astronomy by association – astronomers should be able to say what a planet is and isn’t and have it make logical sense. At the moment they can’t do that. The Pluto debate has caused confusion and controversy and reflected poorly on the astronomical community.

    Thus it *does* need to be addressed properly and the IAU’s disgraceful error in defining planet really must be corrected as soon as possible. I’d even go as far as saying the people who made the anti-Pluto decison should be made accountable and penalised in some way for bringing astronomy into disrepute. Their expulsion from the IAU and a ban from any further IAU votes could be considered appropriate.

    But the main thing is that the IAU really do need to admit and correct their mistake. The longer they leave it the worse they – and, speaking here as an amateur astronomer myself , we – look and suffer.

  65. andy

    HD 45364 has absolutely nothing to do with the Pluto/Neptune case apart from being in a 3:2 resonance. There is no evidence that the outer planet in that system is located in a belt of similar sized objects. Unlike Pluto.

    The Fomalhaut and HR 8799 planets are orders of magnitude more massive than the protoplanetary discs, and are in fact NOT embedded within the main parts of the discs, which lie beyond the orbits of the outermost planets in these systems – the planets orbit within the cleared region interior to the disc. Not to say there isn’t a population of objects that intersects the orbits of these planets (rather like the centaurs in our solar system, or Jupiter’s Trojan asteroids), but again the planets are orders of magnitude more massive than the population. This is unlike the case of Pluto and Ceres which share their location in the solar system with objects that have similar masses.

    The outermost object in the pulsar planet system has a complex discovery history – it was described as a possible fourth planet at the time when the evidence suggested a mass range which got up to 100 times that of Earth. Subsequent evidence revised this mass estimate downwards. It has not been described as a planet when the mass was constrained to be less than 1/5 Pluto – it was described as a possible asteroid or comet, not a planet. (Besides, it is not entirely clear whether the object exists at all or is just the effect of interstellar absorption)

    If Earth was located where Pluto is and behaving the same as Pluto would it cease being a planet in your eyes? Really?

    Yes actually. If it were in a belt of roughly Earth-sized objects at similar orbital distances then yes I would not call this a swarm of planets at this distance, but some kind of belt (though such a massive belt might be better regarded as an oligarch-stage planet-forming region). Would you consider Earth not to be a planet if it were in orbit around a massive gas giant planet? If you want to pursue this line of reasoning, please try and convince us that the major satellites of the gas giant systems should be regarded as planets.

    What is your problem with our solar systems Ninth Planet?

    That it’s a member of an “asteroid” belt, which has historically been a criterion for removing planetary status. Really you can split hairs over the IAU’s decision, which is poorly worded, but you can’t escape that fact.

  66. “It seems incredibly unlikely that this is a rare event in the galaxy, since this happened so close by and so recently.”

    It may not be bad astronomy, but that seems like bad statistics and a circular argument. To say that it’s not rare because it happened nearby is no different than to say it happened nearby because it’s not rare. It’s a sample of one.

  67. Torbjörn Larsson, OM

    Every once in a while someone makes an excellent and noteworthy observation from the funniest evidence.

    I like that.

    The distance and timing of a singular event don’t allow for any conclusion about the genereal probability for such an event.

    a sample of one.

    Not that I am especially good at statistics, but I don’t think either of these are true.

    Besides already mentioned bayesian expectations, which can be used in testable hypotheses, frequentist probabilities apply as well. As Markk notes, the sample space of solar systems is huge, it is not “a sample of one”. An event in a finite sample space is just any subset of it, and there is no such thing as a “singular event”. (Unless you count the empty set, and then again you have a perfectly definable probability.)

    Now there can be problems with probabilities if your event is sparse in all of sample space. If something is guaranteed to happen only once or twice, is the frequency definable (verifiable, testable)? But this is not such a case, since we have no theoretical or empirical reason to believe this is such a rare event. (And if we had, we would already know what we are trying to determine…)

  68. Torbjörn Larsson, OM

    Ideally, I think we ought to have a classification system that can accommodate multiple subtypes to truly account for the great diversity of planetary types that are doubtless out there, but we don’t yet know enough about the potential varieties of planets to do this.

    Yes, and even then it will be a lot of conflict, just look at the biologists that have upwards of 26 (!) different suggestions for definitions of species.

    Btw, funny thing is that there are already several systems, and AFAIU the exoplanet definition doesn’t even have a clearing condition! It’s all misdirection from the malcontents.

    What they should be arguing about is how the current definition of brown dwarf for practical purposes mislabels planets as dwarfs (and perhaps vice versa). At least, that is what I think is happening with the IIRC 13 solar mass limit. That is a real theft, … , um, loan for future adjustment!

  69. andy

    13 Jupiter masses doesn’t work. Architecture of several systems indicates that you can have objects above this limit which are planets, and objects below this limit which aren’t. Objects like 2M1207b and VB 10b have suspiciously high mass ratios compared to their parent star, suggesting they may well be small brown dwarfs. On the other hand, the orbital configurations of systems like HW Virginis, BD+20 2457 and HAT-P-13 suggest that the massive objects in these systems are probably planetary in nature.

    Relying on intrinsic properties of the object leads to mess and confusion. Considerations of the architecture of the host system lead to far fewer problems and a clearer picture of what is going on. (134340) Pluto is not a planet. ;-p

  70. StevoR

    @ 72 Andy :

    HD 45364 has absolutely nothing to do with the Pluto/Neptune case apart from being in a 3:2 resonance.

    And the 3:2 resonance means … ?

    Let nme help you out here – it means that these planets have NOT cleared their orbits -& thus by IAU diktat they are NOT planets – which lets face it is just absurd – like the IAU definition.

    The Fomalhaut and HR 8799 planets are orders of magnitude more massive than the protoplanetary discs, and are in fact NOT embedded within the main parts of the discs, … Not to say there isn’t a population of objects that intersects the orbits of these planets (rather like the centaurs in our solar system, or Jupiter’s Trojan asteroids), but again the planets are orders of magnitude more massive than the population.

    Again, you seem to be missing – or just refusing to see – my point entirely that their orbits are NOT cleared thus like Pluto they aren’t planets if you apply the IAU decree. That these objects have many times Jupiter’s mass but don’t qulaify basedonthe IAU’s anti-Pluto law shows again how ridiculous and unworkable it is.

    Your piece of sophistry there over what constititutes a “cleared orbit” just adds tothis revealing how problematic the vague and hard to define “cleared orbit” malarkey is.

    So what *is* meant by clear – how many asteroids or comets or small planets or large planets can cross or interescet or be in that orbit before it’s considered “clear”?

    How far out must the orbit be cleared to?

    At what point do you & the IAU acknowledge how large the space is and how that makes it impossible for an planet to clear everything from its orbit?

    Why include such a complicating, unnecesary and time and space dependent criterion in adefinition which should be basic and easy to apply?

    What do we calla gas giant planet -or even an Earth mass one – that orbits in such a non-cleared orbit – an asteroid? A dwarf planet? An Edgeworth-Kuiperbelt obect? Yeah right ..! :roll: That would just be plain dumb -but then that’s the IAU ruling for you.

    The outermost object in the pulsar planet system has a complex discovery history – it .. has not been described as a planet when the mass was constrained to be less than 1/5 Pluto

    Factually incorrect. That’s how I’ve seen it described in ‘Astronomy’ magazine among other places.

    This also raises the issue of what if a tiny planet with say the mass of Pluto or less ha sacleared orbit. It then *is* a planet depending on where it is -which again is unfair and inconsistent.

    It all adds up to the logical and rational scientific conclusion that whether an orbit is clear or not is very much a *secondary* minor trait and NOT a defining feature.

    Me : “If Earth was located where Pluto is and behaving the same as Pluto would it cease being a planet in your eyes? Really?”

    Andy : Yes actually. If it were in a belt of roughly Earth-sized objects at similar orbital distances then yes I would not call this a swarm of planets at this distance, but some kind of belt.

    Well that’s just ridiculous.

    BTW. Would you describe our inner solar system as a “belt” making Earth, Venus, Mars and Mercury all dwarf (non)planets? Your argument leads to that conclusion. It is just that absurd.

    Would you consider Earth not to be a planet if it were in orbit around a massive gas giant planet?

    In that case it would be a Moon.

    The three defining charcateristics I’d use for planet are pretty clear – and easily determined & defined :

    a) If an object doesn’t & has never been able to shine by its own natural nuclear fusion.

    (Thus Not a star or brown dwarf.)

    b) If an object is in hydrostatic equilibrium ie. if its shape is controlled by its own gravity into a sphere or for fats spinning objects oblate spheroids.

    (Thus not an asteroid or comet.)

    &

    c) If an object is NOT directly orbiting a planet or brown dwarf

    (Thus not a Moon.)

    Then its a planet – whether it has the mass of Ceres or the mass of Jupiter.

    Simple. Easy to tell. And inclusive.

    We can then divide the planetary category down into many types based on other factors – mass &composition (gas giant, rocky terrestrial planet or ice dwarf), orbital position (Hot Jupiter, Cold Jupiter, Cometary Eccentric orbiter) and even atmospheric conditions (Earth-like, Mars-like, Venus-like, etc ..)

    Honestly, doesn’t this strike you as a far better and more scientifically and logically approproiate idea than the IAU’s definition with its illogical and unworkable “Orbital clearning” nonsense which was thrown inat the last minutye deliberately toexclude Pluto in an undemocratic and highlyu dubious manner by people whose motivations are also dubious?

    Pluto is indeed a planet – and I think the IAU and you are one day just going to have to accept that reality. You may as well start now! ;-)

  71. CWCrosby42

    Adam asked: I’ve always wondered if a mars sized object hit the earth and the debris formed the moon, what happened to the rest of the other mass?

    A moon’s worth was ejected and formed the Moon. Some of the mass undoubtedly ended up in the sun, and some of it went out into space – perhaps to land on other planetary bodies. Some is certainly ‘out there’ somewhere. The rest was incorporated into the planet, Earth and by differentiation spread throughout the planet. Lighter elements in the crust, heavier to the core.

  72. andy

    Again, you seem to be missing – or just refusing to see – my point entirely that their orbits are NOT cleared thus like Pluto they aren’t planets if you apply the IAU decree. That these objects have many times Jupiter’s mass but don’t qulaify basedonthe IAU’s anti-Pluto law shows again how ridiculous and unworkable it is.

    The point you seem to be missing – or just refusing to see – is that I don’t adhere to the IAU definition, nor do I consider it to be workable. As you seem so eager to point out, the IAU definition can be made to exclude all the major planets depending on how pedantically you adhere to the wording. In case you didn’t notice, HD 45364′s planets do not qualify under the IAU definition because they aren’t orbiting the Sun.

    So what *is* meant by clear – how many asteroids or comets or small planets or large planets can cross or interescet or be in that orbit before it’s considered “clear”?

    How far out must the orbit be cleared to?

    At what point do you & the IAU acknowledge how large the space is and how that makes it impossible for an planet to clear everything from its orbit?

    This is precisely the points you have missed because you are assuming I am working with the IAU definition. For a start I exclude objects in circumplanetary orbit (i.e. moons). I also exclude mean motion resonances, as these follow rather different stability rules from orbits which are located close to, but not in the resonance. Basically I am saying that members of asteroid belts should not be regarded as planets. Mercury, Venus, the Earth and Mars are clearly NOT members of an asteroid belt – they each exceed by several orders of magnitude the total mass of all the other objects that cross their orbits. Ceres and Pluto on the other hand are clearly members of “asteroid belts”. This is not a difficult concept to grasp, even if it may be somewhat more qualitative than quantitative. And just to spell it out in case it isn’t obvious, HD 45364′s configuration is nothing like the asteroid belt, or Neptune and the Plutinos, because there is no large population of similar-mass objects in one of the orbits.

    Honestly, doesn’t this strike you as a far better and more scientifically and logically approproiate idea than the IAU’s definition with its illogical and unworkable “Orbital clearning” nonsense which was thrown inat the last minutye deliberately toexclude Pluto in an undemocratic and highlyu dubious manner by people whose motivations are also dubious?

    No. Point 1 of your definition leads to extremely nonintuitive descriptions of several extrasolar systems, which contain objects exceeding the deuterium fusion limit in orbital configurations that look like planetary systems. The configuration of the systems around BD+20 2457, HW Virginis and HAT-P-13 (among others) make sense when you consider the companions to be planets, less so when you designate one (or both) of the companions as brown dwarfs.

    Incidentally the politics of how the definition was arrived at are irrelevant to the question of how well the definition applies to nature: I don’t particularly care if it was arrived at democratically or not. The definition stands or falls on its own merits… in fact I believe it is pretty badly flawed, even if it is based on dynamical principles I agree with. And could you specify exactly what you mean by “dubious” motivations… care to point out some individuals and specifically state what their “dubious” behaviour was?

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