A nearby star may have more planets than we do!

By Phil Plait | April 6, 2012 10:52 am

HD 10180 is a star that’s nearly the Sun’s twin: it’s very close in mass, temperature, brightness, and even chemical content of our friendly neighborhood star. But in this case of stellar sibling rivalry, HD 10180 may have the upper hand: a new analysis of observations of the star indicate it may have nine planets!

In a new report accepted for publication in the journal Astronomy and Astrophysics, an astronomer re-analyzed data of the star taken with the High Accuracy Radial Velocity Planet
Searcher (HARPS), an exquisitely high-precision camera mounted on a 3.6 meter telescope in Chile. HARPS has been observing HD 10180 for years; the star is a mere 130 light years away, making it bright and easy to study. The observations look to see if the star exhibits a periodic shift in its light: a Doppler shift as planets circle it, tugging it one way and another.

Six clear Doppler shift signals were found in the original analysis: six planets, five of which have masses ranging from 12 – 25 times that of the Earth (making them more like Neptune than our own comfortable planet), and a sixth that was bigger yet, 65 times Earth’s mass (more like Saturn than Neptune). These planets orbit HD 10180 with periods of 5 – 2000 days. A seventh possible planet was detected, but the data weren’t strong enough to make a solid claim.

The new analysis looks at the old data in a different way, examining it using different statistical methods. Not only are the six planets seen in the new results, but the seventh is confirmed, as well as finding two additional planets in the data. If this result pans out, that means HD 10180 has nine planets, more than our solar system does!

The three additional planets have masses of 1.3, 1.9, and 5.1 times that of Earth, and orbit the star with periods (think of that as the planets’ years) of 1.2, 10, and 68 days, respectively.

Those first two are pretty firmly in the Earth-mass range, what astronomers call "super Earths". However, Earth-like they ain’t: they’d be cooked by the star. The first is only 3 million km (less than 2 million miles) from HD 10180, and the second barely any cooler at about 14 million km (8 million miles). This is much closer to the star than Mercury is to the Sun, and remember HD 10180 is very much like the Sun. If those planets are rocky, their surfaces are hot enough to melt tin, zinc, and on that inner planet, maybe even iron.

So yeah, not exactly a fun place to visit.


An added bonus is that the analysis looked at how stable the orbits are over time. Not all orbits are stable; if two planets occupy certain orbits then they can tug on each other enough over time to make the orbits unstable. It’s like pumping your legs on a swing; do it with the right timing and you can change your swing. In this case, the analysis showed the orbits are stable over time. That doesn’t prove the planets exist, but it does add confidence to the analysis.

And if this does all turn out to be correct, it’s amazing! We’ve been detecting planets around other stars for a while now, including those in multiple systems. But those generally have four planets or fewer; even finding six planets around HD 10180 would be a record. With three more, this would put HD 10180 firmly ahead of every other system detected.

Heck, it beats us. Mind you, no matter where you fall in the Pluto planetary club membership debate, these objects are all more massive even than Earth, so they are most assuredly planets.

Even though this system is very alien to ours, with far more massive planets packed more tightly around their star, most of them cooked to boiling, it’s still a very, very encouraging result. 15 years ago we didn’t know of any other planets orbiting other stars. Now we know of hundreds, with thousands more candidates. And many of these are parts of systems, planetary families a bit like our own. We used to wonder if our solar system was the only one like it in the Universe; unique among the stars.

And now we know the answer: No. And that’s a pretty cool thing to know.

Tip o’ the lens cap to Emily Lakdawalla. Image credit: ESO/L. Calçada; ESA, NASA, M. Kornmesser (ESA/Hubble) and STScI


Related Posts:

50 new worlds join the exoplanet list!
Another Kepler milestone: Astronomers find two Earth-sized planets orbiting the same star!
Nearby planetary system is seriously screwed up
A tiny wobble reveals a massive planet

CATEGORIZED UNDER: Astronomy, Cool stuff, Top Post
MORE ABOUT: exoplanets, HARPS, Hd 10180

Comments (104)

  1. Because of the orientation of HD10180, do astronomers think that the transit method would be effective at detecting even more planets too? I think it would be really interesting to set up some sort of detector to give that system a look with more than one method, since the doppler method tends to be biased towards heavier planets.

  2. Timothy from Boulder

    An interesting question when looking at systems like this is, of course, whether Earth-like planets exist in the habitable zone. It’s easier to find Earth-mass planets closer to the star, as in this case. With the number of postulated planets for HD 10180, a natural question to ask it whether an Earth-mass planet can have a stable orbit in the habitable zone. That is, *could* one exist, even though the current data doesn’t have the precision to be able to show it? I’ll have to read through the paper in detail to tease out all the ramifications of the proposed 9-planet solution.

    Phil – How was the Night of Total Destruction at S&EE on Wednesday? Are we going to get a report?

  3. Wow. Nine planets. I remember when we used to have nine planets.

  4. Mathias R.

    A question that has been nagging me for quite some time now.. it may be just bias caused by our detection methods, but it feels like a *LOT* of solar systems have planets in extremely close proximity to their host star.

    I mean, Mercury is positively close, but those other systems make its distance seem perfectly plutovian..

    so, just bias of the data, or is Sol the outlier here :D

  5. saphroneth

    #4 Yes, it is a bias of the Doppler method, as I understand it. A closer planet causes a larger shift in terms of magnitude over time, since the period of the orbit is smaller (it has to go around the common center in a shorter time, so we can observe a complete period more easily and the signal is larger).
    I am not aware of a corresponding bias in the transit method.

  6. Chris

    Is there a theoretical maximum number of planets per star?

  7. Jeremy

    The 8th planet has an orbital period of 600 days, which would put it closer than Mars and squarely in the habitable zone. Even if the planet itself is a gas giant inhospitable for life, there’s always the possibility of moons. I’m excited to see we’re finding so many new planets, I’m just wondering when we’ll find the next (Earth-species) habitable world, though finding one that hosts a completely different type of life would of course be amazing as well.

  8. andy

    Yes there are observational biases, but by now there is quite a bit of evidence that our solar system is at least moderately unusual in terms of orbital architecture.

    As the astronomer Greg Laughlin has pointed out, the super-Earth systems we’re discovering are more similar to scaled-up versions of the gas giant satellite systems, rather than scaled-up versions of the inner solar system.

  9. amphiox

    It occurs to me that it’s virtually guaranteed that many of these star systems will have more, smaller, planets in them than what we can detect, since many planets in the Venus-Mars-Mercury mass ranges will be too small for our instruments to pick up right now.

    This system, with 9 detected planets, very probably has more than 10….

  10. David

    Could this star be part of the same “litter’ that the Sun came from? Which is why its got so many similarities?

  11. amphiox

    I am not aware of a corresponding bias in the transit method.

    I’m pretty sure the transit method will be biased towards big (in diameter rather than mass) planets close in to the star.

    The closer the orbit, the greater the inclination of the orbit can be relative to earth and still be observed to transit, and the more transits we get to see in a given time period, letting us detect it earlier.

    The bigger the planet, the bigger the shadow it casts and the easier it will be for us to pick it up.

  12. “*LOT* of solar systems have planets in extremely close proximity to their host star. […]
    bias of the data, or is Sol the outlier here”

    The data are biased towards tighter systems, but I’ve seen analysis that says that such systems really are very common and possibly the majority.

    There’s a theory that such systems are produced in “gas rich” protoplanetary disks, whereas ours was “gas poor” (i.e. mostly dust).

    In a gas rich disk, the theory goes, gas giant planets form by hydrodynamic collapse farther out, then migrate inwards as they lose angular momentum to the gas. Probably some migrate right into the star. As the gas is either absorbed or dissipated, the migration stops and the planets are left where they were. If there’s a lot of gas, you get a lot of gas giants near the star. If there’s not much, you get a system more like ours.

    This theory is several years old, and I haven’t really kept up, so I’m not sure how accepted it remains.

    From a bioastronomical PoV, that might be bad news — it could be that only the rarer dust-rich disks form systems amenable to life. Or it could be irrelevant. It could be that most habitable worlds are actually moons.

    But that’s an old, old debate: “Does the fact that Earth is unique in some way necessarily mean that that uniqueness is necessary for life, or is it just a coincidence?”

    This has been used as an argument for the rarity of biospheres in the past (w.r.t. our peculiarly large Moon, for example — probably quite rare, especially if it really is the result of a massive collision between protoplanets, as was accepted theory last time I looked). I’ve seen rather convoluted proposed mechanisms for that — such as the idea that tidal forces limit our atmosphere, and keep it from becoming a Venusian hell, which might otherwise be the norm for Earth-sized planets.

    Personally, I don’t buy it — I think it’s more likely that it’s simply a matter of “give me enough random variables to look at and some of them will turn out to be extreme”. So we have a weird moon, and we live on a planet instead of a moon, big deal. Other life-bearing planets will be unique in other ways.

    We’re never going to know (about alien life) until we go look.

  13. @saphroneth (#5), yes, there is a very big bias in the transit method! Only about 1% of the planets in a habitable zone are even detectable by the transit method!

    A Kepler mission page expalins this, with pictures and everything. http://certificate.ulo.ucl.ac.uk/modules/year_one/NASA_Kepler/character.html Scroll down to the section that says “Geometric Probability” for the math.

  14. Timothy from Boulder

    @9 (amphiox) And according to the paper the gaps in the orbit parameter space include stable orbits for not-as-yet-detected planets in the habitable zone

    “Further, according to the rough dynamical considerations of the
    current work and the more extensive numerical integrations of
    Lovis et al. (2011), there are stable orbits for a low-mass companion
    in or around the habitable zone of the star. If such a companion
    exists, its minimum mass is unlikely to exceed 12.1 M according to our posterior samplings of the corresponding parameter space.”

  15. I’ve can’t help feel sorry for the guys who keep writing all the “Unique Earth” books. Every time one of the Drake Equation variables is found to be wildly common, they move to the next one to argue their “great filter.”

    Fermi’s question is getting harder to answer every day.

  16. Kelly Clowers

    We have 13 planets in our solar system, no? Or are dwarf mammoths no longer mammoths, and dwarf apple trees are no longer apple trees?

  17. @Jim Saul (#15) I would say that Fermi’s question was a very poorly phrased question. It’s really not at all hard to answer if you really understand the human-centric phrasing of the question, not to mention the incredibly vast distances involved when talking about stars. Not to slight Fermi at all, but he really goofed the question (or should I say quip).

  18. Tara Li

    I’m sorry – but the size of the body has *NOTHING* to do, really, with whether or not something is a planet, once it’s above the hydrostatic equilibrium point. I don’t believe Jupiter, put where some of the more distant KBOs found so far, would qualify as a planet. Of course, the IAU definition doesn’t give a clear definition of “cleared the neighborhood”, so one might even question Jupiter’s planethood in its current location, what with several hundred known asteroids sharing its orbit. That doesn’t sound like cleared to me!

    Bah! the IAU should be ashamed about the Pluto Debacle – the right answer would have been to promote Ceres to a planet.

  19. Paul

    What is the expected lifetime of that inner planet to orbital decay from the tides it raises on the star?

  20. J'Zargo

    J’zargo likes the idea of nearby planets. J’zargo wishes he would fly.

  21. T-storm

    excuse me sir. we have 9. they can have my pluto when they pry it from my really cold dead hands.

  22. andy

    I don’t believe Jupiter, put where some of the more distant KBOs found so far, would qualify as a planet.

    A Jupiter-mass planet would meet the planethood criterion out to distances much further than the Kuiper Belt. Earths are probably good out to a few hundred AU or so depending on which criterion you use to estimate the scattering efficiency.

  23. llewelly

    Notably, HARPS cannot possibly have detected any planets as far out as Jupiter.

    This star has 7 to 9 planets in its inner solar system. If it has a number of outer planets, we will not know for some years yet.

    That’s an important thing to keep in mind when looking at all this exoplanet research; pre exoplanet theories of solar system formation were strongly influenced by the outer planets – but, as present methods need about 4 orbits to confirm existence of a planet, we would need 48 years to confirm the existence of a planet at Jupiter’s distance.

    When it comes to the outer solar system, we still have a sample size of only 1, and that will continue to be the case for some time.

  24. Tristan

    Like hell we are going to let them have more planets then us! If we need more, we’ll just have to weld two drawf planets together to make a 9th true planet!

  25. Might be a good place for a summer vacation. Get some tan, stare at the sun, that kind of thing…

    But no, joking aside, a very interesting discovery indeed. Makes me wonder how many fire-elementals are there in the universe, I could use one for my Dungeons & Dragons sessions.

  26. andy

    When it comes to the outer solar system, we still have a sample size of only 1, and that will continue to be the case for some time.

    Well there are a few exoplanet examples detected via imaging: HR 8799 is the poster child for this kind of thing (four giant planets with orbits ranging from 15 to 70 AU from the star). There’s the giant planet orbiting Beta Pictoris, which is in an orbit comparable to our own solar system’s gas giants. Also there is the case of Fomalhaut, but what is going on with the planet candidate is not entirely clear, it is something of an oddball – one of the leading theories at the moment is that it is a super-Earth surrounded by a dust cloud produced by collisions in an irregular satellite swarm.

    Finally there are a few other objects that are probably better regarded as very low mass brown dwarfs (i.e. non-fusing stars), e.g. 2M1207b.

  27. CB

    Of course, the IAU definition doesn’t give a clear definition of “cleared the neighborhood”, so one might even question Jupiter’s planethood in its current location, what with several hundred known asteroids sharing its orbit. That doesn’t sound like cleared to me!

    Those are nearly all Trojans, occupying the trailing and leading Lagrange points — they have been captured by Jupiter’s gravitational dominance of its orbit, much like its moons.

    Another way to look at this is to look at the Planetary Discriminants — this is the ratio of the object’s mass to the combined mass of all other objects in a similar orbit. All of the things currently defined as planets have a discriminant of over ten thousand, while all things currently defined as dwarf planets have a discriminant of less than one.

    So you see, the IAU did not have to give a precise definition of “cleared the orbit” because there’s a five order of magnitude difference between the least of objects that has cleared and the greatest of objects that has not.

    I’m ashamed by people who act like the IAU just wanted to draw an arbitrary line, like saying exactly where Europe ends and Asia begins, ignoring that it’s more like separating the Old and New Worlds — your line can be as wide as the Atlantic and you’re still okay, and it’s arguing that there is no difference that is silly.

  28. CB

    excuse me sir. we have 9. they can have my pluto when they pry it from my really cold dead hands.

    This, on the other hand, is hilarious. :)

  29. Magnum Serpentine

    I wish the IAU would hold a re-vote for what a Planet is. I believe the BBC or other news source said that only 400 decided this vote back when it was taken. I think all members should have a chance to decide so a Re-Vote would be very welcoming.
    And that’s my Opinion.

  30. RaginKagin

    Just wondering then, what would it be like if Jupiter was where Mercury was now in our solar system? Does large planets so close to the sun increase or decrease the odds of an earth-like planet, or would it hurt the chances of stable orbits?

  31. amphiox

    excuse me sir. we have 9. they can have my pluto when they pry it from my really cold dead hands.

    Well, if you actually have pluto in your hands, those hands would indeed be really cold, and really dead….

  32. amphiox

    Another way to look at this is to look at the Planetary Discriminants — this is the ratio of the object’s mass to the combined mass of all other objects in a similar orbit. All of the things currently defined as planets have a discriminant of over ten thousand, while all things currently defined as dwarf planets have a discriminant of less than one.

    IIRC, Pluto’s planetary discriminant is not only less than one, it is less than 0.01.

    But sometimes I think the IAU would have been better off you using the Planetary Discriminant itself instead of the vague language about “clearing the orbit”. Put it into hard, cold numbers, so every one can that the difference continues more than four zeros after the 1.

  33. amphiox

    I wish the IAU would hold a re-vote for what a Planet is. I believe the BBC or other news source said that only 400 decided this vote back when it was taken.

    It was my understanding that the first half of the debate was so acrimonious that lots of people ending up walking out in disgust.

    Either that, or the vote was the last thing done on the last day of the conference, and lots of people just left early to catch their flights home…..

  34. Dragonchild

    6. Chris Says:
    “Is there a theoretical maximum number of planets per star?”

    Snarky answer: Ask the IAU. >:D
    Serious answer. . . I haven’t heard of one. There isn’t any underlying science that comes to mind. Planets packed too closely together will de-stabilize each other’s orbits but we’ve found planets can be packed a LOT closer together than our system and yet can be in stable orbits out as far as one light-year! That’s a LOT of room to work with. This 9-planet system could very well have many more planets out in the range of Jupiter’s orbit and beyond, where it’d take decades to centuries to confirm them, if they can be detected at all. There’s nothing I’m aware of stopping a star from having a Mercury-sized planet orbit 6 billion km out, but we have literally no way of detecting that. It’s also theoretically possible for a protoplanetary disk to “run out of stuff” and I figure that’s the real limitation — there’s only so much heavy material, so even a metal-rich star like ours can have only so many planets. That said, at some point this is about chance so the upper limit could very well be in the hundreds or even thousands.

    23. T-storm Says:
    “excuse me sir. we have 9. they can have my pluto when they pry it from my really cold dead hands.”

    Honestly, “planet” isn’t even a technical term; the IAU just feels the need to make it one. But to avoid defying existing conventions they have to “back” the definition into our system’s context.

    I’m quite OK with dubbing Pluto an “honorary planet” because there’s no scientific gain in its promotion or demotion. The only real downside the IAU is trying to avoid is winding up with hundreds of planets with the majority of them being KBOs. But that just may be the reality — a reality early astronomers had no idea of when they first came up with the term, which just means “wandering star”.

  35. chief

    Would finding the same planets and orbits by the dopler method mean that the really close in bodies not have much of an effect on the movement of the parent star, and the further outer bodies orbiting to0 slowly to give a reasonable timely “tug” on the star, thus wouldn’t a planet with habitable zone give a larger spike in the stars movement more detectable.

  36. lepton

    I carefully read the wiki entry on IAU definition of planet:
    http://en.wikipedia.org/wiki/IAU_definition_of_planet

    I finally made peace with it. As amphiox said, if they just use Planetary Discriminant instead of “clearing the orbit”….

  37. Torbjörn Larsson, OM

    Well, I’m glad someone had the balls to go up against us!

    #3:

    I remember when we used to have nine planets.

    Your planet envy is showing. And it ain’t pretty. ;-)

    #6:

    No distribution (so a quantifiable characterization) as of yet, I think. These systems show that planet system formation is poorly understood.

    #8:

    by now there is quite a bit of evidence that our solar system is at least moderately unusual in terms of orbital architecture.

    Um, do you have any references to that quantification? As far as I know all systems look unusual (individual).

    I would be highly interested in any attempts to characterize that and how the solar system compares.

    # 12:

    I think you mean astrobiological, not bioastronomical.

    From a bioastronomical PoV, that might be bad news — it could be that only the rarer dust-rich disks form systems amenable to life. Or it could be irrelevant.

    Not irrelevant but advantageous. Planetary system formation mechanisms seems to have left a lot of habitables around the most numerous and long-lived stars (M stars).

  38. CB

    But sometimes I think the IAU would have been better off you using the Planetary Discriminant itself instead of the vague language about “clearing the orbit”. Put it into hard, cold numbers, so every one can that the difference continues more than four zeros after the 1.

    Well the beauty of using a qualitative notion is that it prevents them from actually having to pick an arbitrary number for the Planetary Discriminant to qualify. As it is, the absence of anything in the gigantic gap makes such a precise and arbitrary line unnecessary. I guess they could have said “high planetary discriminant” and “low planetary discriminant” to keep the qualitative nature — that would have been pretty good, I agree.

  39. Torbjörn Larsson, OM

    Egad! This shows how the “planet definition” nut pack is demolishing science in the cause of personal interest.

    Here we have a highly interesting result with ramifications for planet system formation and astrobiology that shows how ordinary (yet uniquely individual) our own system is. And they are busy loosing that science for a strawman of comparing planets and dwarfs with exoplanets!

    #29:

    Well said.

    There were several underlying factors for redefining planets, but a definition that distinguishes populations makes it potentially useful for research. Species is a similar hard to define but useful distinction, which has several conceptualizations. (At least 26 of them.) They too are tightly regulated when it comes to similar areas of taxonomy (I think).

    The planetary discriminant makes it clear that comparing planets with other populations is like comparing cellular life forms with viruses. It is a wide divide in most cases. (Never mind that some viruses like Mimiviruses seems to be parasitically simplified cellulars, and so are bound to be hard to qualify during the transition.)

    #31:

    Your description is an utter strawman: it was a democratic and well deliberated decision, never mind how few was interested in actually making it at the time.

    And yes, it can be redone, but you shouldn’t flip-flop unnecessarily like that. The historical precedent is Ceres, which was a planet for about as long as Pluto was. Yet no one reversed that decision nor discuss it today.

  40. amphiox

    Species is a similar hard to define but useful distinction, which has several conceptualizations. (At least 26 of them.)

    Over 140 at last count, as far as I know….

    The historical precedent is Ceres, which was a planet for about as long as Pluto was.

    I thought Ceres was a planet for only a decade or so, when a flood of discoveries of new asteroids resulted in the reclassification, as opposed to Pluto being a planet for almost a century. (Though that’s less than one log scale difference, so by some definitions, it is “about the same”!)

    It should also be noted that the IAU deliberately excluded exoplanets from its planet definition, so it actually isn’t really appropriate to talk about it with respect to exoplanets. It also means that the IAU definition is doomed (and the IAU must have known this and may even have intended it) to being revised again at some future point, when we know more about exoplanets….

  41. amphiox

    by now there is quite a bit of evidence that our solar system is at least moderately unusual in terms of orbital architecture.

    I think this statement is a stitch egocentric. All the known exosystems are quite different from our own, it’s true. But if you start comparing them to each other, putting yourself in the perspective of a hypothetical astronomer living in one of these other systems, they all look pretty unique from each other too, and ours is just another unique arrangement.

    And we really haven’t had a chance to look at all that many multiplanetary systems yet, so I’m not sure as to the precision of any statistics we might be able to derive from such an analysis.

  42. Yes its all so new and exciting, isn’t it? Except that out there somewhere a group of alien astronomers has also discovered how to find extraprimary planets and is gonna find US sooner or later. The more rapacious amongst their exploitationist political and business classes start doing whatever their equivalent of salivation is, then comes the death rays, bioweapons, pain, blood and other stuff such types like so much. For all we know, they are just now coming in past Pluto. Seriously, though, for the 1.3 mass innermost body, isn’t it likely to be tidally locked? Won’t it also be tidally interacting with further out bodies? So will there be a dark, cold face (if there is no atmosphere), as well as a possible molten core keeping things moving, so some atmosphere coming from within? If the atmosphere were thin enough, would it lose heat fast enough to offset the heat input from the star-side face? What about those further out, if also tidally locked?

  43. Messier Tidy Upper

    Great discovery – my congrats to the HARPS astronomers. :-)

    Although I’d hardly describe one hundred and thirty light years distant as “nearby” – all things may well be relative but still. ;-)

    Oh & Pluto, Eris, Ceres, Makemake, Haumea and other ice dwarfs count as planets in my book anyhow so we still have more as others have noted already.

    The IAU, as most folks will probably agree – especially after thinking it over clearly for a bit – got it very badly wrong. :-(

    Dwarf planets are no less planets than dwarf plants and animals are still, well, plants and animals not minerals as #16. Kelly Clowers accurately pointed out. Being small and numerous doesn’t stop something being what it is.

  44. Messier Tidy Upper

    Mind you, no matter where you fall in the Pluto planetary club membership debate, these objects are all more massive even than Earth, so they are most assuredly planets.

    Er, BA don’t you mean unless they’re moons because by definition if one or more of these worlds is orbiting a larger exoplanet then it becoems a moon NOT a planet?

    Plus if they are in a double planet arrangement the IAU definition can’t handle that – or if they’re ona collision courseand thus lacking clear orbital paths they’re also not planets by IAu decree!

    Actually as a matter of fact,under the anti-Copernican, illogical, inconsistent and downright ludicrous (and also hopefully temporary) definition of the IAU planets are restricted to our Sun and our solar system *only*!

    No planet can orbit another star by IAU definition even if its an exact twin of Jupiter, Mars or Earth. In fact if we moved Erath or Jupiter or Mars into orbit around another star then by IAU definition they too would lose planetary status.

    You forgot about that particularly stupid clause of the IAU’s planet definition didn’t you? ;-)

    I quite understadn mind you. We all usually do – we ignore that clause and treat it with the contempt it deserves. We should be consistent and throw out the IAU definition altogether until they come up with a reasonable one or abandon the concept of defining planet altogether if they can’t.

  45. Messier Tidy Upper

    I think most of us know a planet when we see one.

    Most of us know that other stars can have planets too, even orbit starless and alone in the Black.

    Most of us have no problem saying that planets can come in double planet form -hypothetically at least in fiction so far and maybe unobserved as yet fact as well.

    Most of us know too that planets can collide or at least be on collision courses as our inner solar system may find in future aeons and have unclear orbits as ,well, every plante does because sun-grazing comets cross all planetary paths.

    Most of us – except for this breif time it seems the IAU. Which is their fault – not the planets or ours! ;-)

    Under the present, temporary IAU laws HD 10180 has exactly zero planets – depite the evidence of HARPS and despite the fact that you, I, and everyone but theIAU would call, heck is calling these planets.

    As so often the case, the common sense understanding and awareness of reality is far ahead of the bureacratic paperwork and the law is a .. synonymn for donkey! ;-)

    I fully expect and predict that the current ridiculous, illogical IAU definition will eventually be dropped and replaced as well it should be. It should never have been adopted in the first place. :-(

    Our Sun has way more than nine planets. Pluto is about average in planetary size bigger than Haumea, Sedna, Ceres and more. Pluto is one of the largest and first found examples of the most numerous ice dwarf class of planet joining the gas giants and rock dwarfs previously known.

    We’ve found many strange worlds, planets I call ‘em just like you do around other stars and HD 10180 has nine such worlds just confirmed by HARPS here making it one of the most numerous planetary systems known. Second only to our Sun assuming a sane understanding of what planet means is adopted.

  46. @33. amphiox :

    “excuse me sir. we have 9. they can have my pluto when they pry it from my really cold dead hands.” [ # 23. T-storm -ed.]
    Well, if you actually have pluto in your hands, those hands would indeed be really cold, and really dead….

    Unless T-storm were wearing a really great spacesuit – Oh & had also been hit with a really strong blast of an SF grow-ray and thereby been expanded to gianthood big enough to make Paul Bunyan look small! ;-)

    @31. Magnum Serpentine :

    I wish the IAU would hold a re-vote for what a Planet is. I believe the BBC or other news source said that only 400 decided this vote back when it was taken. I think all members should have a chance to decide so a Re-Vote would be very welcoming.
    And that’s my Opinion.

    It is also mine as well.

    As a matter of fact, of the 10,000 IAU members only 2,500 attended the 2006 Prague meeting that demoted Pluto and rejected the other planetary candidates, Eris, Charon and Ceres from planetary status. Furthermore, of those 2,500 only the merest handful – just 424 actually got to vote making therefore a very unrepresentative decision. Among those to excluded from voting and arguing their case in that last minute meeting were some highly relevant and articulate people – notably Pluto expert Alan S. Stern, head of the New Horizons mission. Stern’s later verdict on the IAU judgement was blunt :

    “ … idiotic. I have nothing but ridicule for this decision.” (Alan Stern, P.28, ‘Astronomy Now’, October, 2006.)

    Stern is, of course, a professional astronomer one of a number who strongly disagree with the current IAU definition.

    The IAU definition debate was – as the above clearly shows – far from fair, democratic and representative. Pluto’s demotion was a dubious call made by dubious unrepresentative people for dubious non-scientific reasons.

    @ 41. Torbjörn Larsson, OM :

    Egad! This shows how the “planet definition” nut pack is demolishing science in the cause of personal interest.

    No, we’re not. We’re really not – and calling us “nuts” is insulting and false. Disagreeing on this one specific issue for good, logical reasons does not make someone a “nut” thankyou. :-(

    Here we have a highly interesting result with ramifications for planet system formation and astrobiology that shows how ordinary (yet uniquely individual) our own system is. And they are busy loosing that science for a strawman of comparing planets and dwarfs with exoplanets!

    Well, we’re not *losing* the science – its still there and still wonderful but what is happening is that the planet count factor is again raising the issue of that divisive hetaed topic of the IAU’s erronoeus definition because falsely callingPluto a non-planet means our Sun appears to have less planets than HD10180 – providing you also ignore the part of the IAU planet definition which says exoplanets don’t count as proper planets either which everyone, inconsistently but reasonably does.

    The problem lies in that definition and its illogic and the understandable incredulity, disappointmentand even anger it has evoked in most people.

    There’s an easy way to resolve this problem – get the the IAU to see sense and alter the definition to a more reasonable alternative. My prefered choice there would be that a planet is an object that :

    I) Has never shone by core nuclear fusion so not a star or brown dwarf.
    II) Does not directly orbit another planet and thus is not a moon &
    III) Is round or if fast spinning ellipsoidal due toits own gravityand thus not an asteroid or comet.

    That’s simple, inclusive, clear and easily determined. It would include a lot of objects yes but so be it. Really what’s the problem with that definitionand how is taht not much better than the current confusing, illogical, mess which needs too much explaining and too many metaphorical “epicycles” added to really work at all?

    There are of course other better alternatives such as that suggested by astronomer and author Ken Croswell as well.

    Even the IAU giving up and just not defining planet at all would be better than what they’ve got, frankly.

    BTW. For more information and some excellent writing and discussion of the whole Pluto issue, I strongly recommend reading Laurel’s Pluto Blog which is linked to my name here.

  47. MaDeR

    Heeereee we gooo agaiiiin….

    “if they’re ona collision course and thus lacking clear orbital paths they’re also not planets by IAu decree!”
    You are BSing. If they are on collision course, they are very soon going to lose planetary status by any definition of this word anyway. In other words, if this is serious argument (its not, but lets pretend), then this is argument also against your definition of planet. Well done.

    “Under the present, temporary IAU laws HD 10180 has exactly zero planets”
    Yes, IAU folks deliberately excluded anything not orbiting our sun. It was deliberate decision, not some stupidity or mistake, as you imply. Temporarness also was deliberate. I guess there will be another run in few years – we should know a lot more about exoplanets by then. Unfortunately for you, I do not see Pluto coming back.

    “very unrepresentative decision”
    Since when it is IAU fault that those oh-so-supposedly-many proplutonists did not bother to show to vote?

    I noticed you did not mentioned antything about “clearing orbit”. Probably talking about it in same thread as “planetary discriminant” would be asking for trouble, heheheh. But I agree they sould be saying for example “gravitational dominance” instead of “clearing the orbit”.
    Not that it matters – with clearing orbits you behave like they are required to rid of every little speck on their orbit, I am sure you would find some equally crappy excuse if there would be different wording in IAU definition.

  48. Shandooga

    130 light years is “nearby”? Have we forgotten how many miles that is? How fast we *can’t* travel? How *short* a human life is? How *unlikely* that there would be anything to eat upon arrival? How *toxic* the atmosphere is likely to be on any unfrozen or non-molten planet that *might* be found? How *long* terra-forming would take *if* it were even possible with whatever materials we could bring on such an unlikely voyage? Why are you people even entertaining these pipe dreams? Oh, that’s right. Space travel is “heaven” for atheists. Well, enjoy your fantasy–until you die. On earth.

  49. Gary Ansorge

    50. Shandooga

    What part of planets, asteroids and comets being nothing more than raw materials don’t you understand? Hydrogen, oxygen, nitrogen, carbon and a few trace elements(and energy) are all we need to sustain life? Why would anyone travel 130 light years to live on a planet, when their space craft has done quite well supporting them for,,,generations? Unwrap your head from planets. They’re useless for a technological civilization. Space is the place to be. Space is the place to live.

    Space is not a “heaven”. It’s just a place to live. “Heaven” is only a place for the Dead…If you don’t like living, well, that’s YOUR choice. You can always leave…

    Gary 7

  50. SkyGazer

    Neil will save us from this menace.

  51. Digitalaxis

    @4. Mathias R

    I know this has already been said, but there are three effects here that bias toward small, tightly-packed solar systems.

    1.) Time coverage! Kepler has only been up for 3 years; they need to see AT LEAST three transit events to be certain they actually have a planet (think about it: One transit could be a fluke. Two might be two separate objects. A third transit at the same period as the second would imply all three observations are the same single object.)

    So: Kepler cannot POSSIBLY have any confirmed planets with orbital periods greater than 1 year. If the star is larger than the Sun, yes, they could find a planet with an orbit wider than 1 AU (they swing faster, mass*period^2=semimajor axis^3) but that’s it. There are other longer-running programs, but they’re still not very far. Remember, Neptune has completed only one orbit since it was discovered by humans.

    The Marcy/Butler/Fischer RV planetary search programs have been going since ~1989 (or 1980 if you include Campbell’s somewhat lower-precision work). With RV, you can chart the motions over time and you’ll only need one or two orbits to confirm the planet… which means we’re talking about maximum orbital periods of 11.5 years; that’s not quite sufficient to confirm Jupiter (11.8 years).

    2.) As periods increase, the amplitude of the radial velocity signal drops. Radial velocity is recording the SPEED at which the star is moving around the center of mass, not how far it moves. If memory serves, the Earth imparts motions of centimeters per second on the Sun, and Jupiter (despite its larger size) should be comparable if not smaller. That is HARD.

    3.) As periods increase, the time spent in transit decreases, and the requirements for line-of-sight become stricter. Something that might transit every three days is a less difficult observation to make than something that transits every eight years (you have to watch for EIGHT YEARS to see an eight-or-ten hour transit). And by the same token, the plane of that stellar system has to be EXACTLY bang-on for an outer planet to transit the star. For all we know, many known transiting systems HAVE outer planets that don’t transit: the inner ones, because they’re closer, pass over the top or bottom edge of the star, and the outer ones miss the star entirely.

  52. Digitalaxis

    @50. Shandooga

    We’re saying 130 light years is nearby in the context of a galaxy that’s at least 50,000 light years in radius, and in terms of a galaxy that is (mostly) hidden behind dust past a distance of 3,000 light years.

    Hell, if you want to drag the “can we go there” or “can we live there” questions into it, you should feel bad about MARS.

  53. amphiox

    Oh & Pluto, Eris, Ceres, Makemake, Haumea and other ice dwarfs count as planets in my book anyhow so we still have more as others have noted already.

    But who knows how many dwarf planets any of these exosystems have? Other than the virtual certainty that they will have at least some?

    I always figured that one of the justifications for not including exoplanets in the IAU planet definition was the simple fact that things like dwarf planets simply cannot be detected yet around other stars, and the IAU didn’t want to commit to defining something that we can’t even observe yet.

  54. amphiox

    Most of us have no problem saying that planets can come in double planet form -hypothetically at least in fiction so far and maybe unobserved as yet fact as well.

    I’ve always wondered if the whole class of “moon” is the result of a historical artifact. In many ways it makes much more sense to classify the larger moons as planets, just as a very small star in a binary system with a very large star (assume the center of gravity is even inside the surface of the larger star) is still a star, a world like Ganymede which would be a planet if it were not in orbit around a bigger planet, should really still be a planet.

    And if earth had no moon, no satellite to have possessed the name of “moon” for thousands of years, I suspect that when we discovered the Galilean satellites, we would have called them all planets.

  55. amphiox

    There’s an easy way to resolve this problem – get the the IAU to see sense and alter the definition to a more reasonable alternative

    With some patience, that will happen for sure. This was guaranteed from the moment the IAU chose to exclude exoplanets from the definition. Perhaps the IAU intended it that way. The moment we get our technology to the point where we can identify Ceres to Pluto sized objects in exosystems, and discover a few of them, this planet definition is going down….

  56. amphiox

    130 light years is “nearby”? Have we forgotten how many miles that is? How fast we *can’t* travel? How *short* a human life is? How *unlikely* that there would be anything to eat upon arrival? How *toxic* the atmosphere is likely to be on any unfrozen or non-molten planet that *might* be found? How *long* terra-forming would take *if* it were even possible with whatever materials we could bring on such an unlikely voyage? Why are you people even entertaining these pipe dreams? Oh, that’s right. Space travel is “heaven” for atheists. Well, enjoy your fantasy–until you die. On earth.

    Dear, Shandooga, in case you weren’t aware, the title of this blog is “Bad Astronomy”. ASTRONOMY is the science that deals with OBSERVING objects in our universe, to learn about their properties, by LOOKING AT THEM, from GREAT DISTANCES. ASTRONOMY does NOT involve the technical challenge of traveling to these distant places. THAT would be astro-engineering, and while the blog master DOES post on this topic not infrequently, THIS thread, is not about that. THIS thread, is about ASTRONOMY.

    Getting there is irrelevant.

    So your concern is noted, but you are advised to take it elsewhere.

  57. Messier Tidy Upper

    @50. Shandooga :

    130 light years is “nearby”?

    Well, I made the same point in my comment # 45 so I know what you mean but, actually, astronomical distances are all relative and it depends what you mean by “nearby.”

    HD 10180 certainly isn’t one of our stellar next door neighbours like Alpha Centauri, Sirius or Gliese 581 but in the context of our Galaxy which spans a hundred *thousand* light years I guess you could describe a mere 130 light years as relatively nearby indeed.

    Have we forgotten how many miles that is?

    Not forgotten, I don’t think we usually calculate light years in miles – if you want to do so, then you better be prepared to write out an awful lot of zeroes! ;-)

    How fast we *can’t* travel?

    Yet. There are people working on and thinking about the prospects for improving our rate of travel.

    How *short* a human life is?

    Yeah, but again, our lifespans are growing and we’re working on extending them. It is a shame we’re so short-lived though I’ll agree.

    How *unlikely* that there would be anything to eat upon arrival? How *toxic* the atmosphere is likely to be on any unfrozen or non-molten planet that *might* be found? How *long* terra-forming would take *if* it were even possible with whatever materials we could bring on such an unlikely voyage? Why are you people even entertaining these pipe dreams?

    Well I’m not sure anyone has here actually suggested travelling there and living off the “land” assuming there is some on any of these worlds and terraforming these planets. Who exactly has been talking of these “pipedreams” for the worlds of HD 10180 do you think?

    What we *are* excited about is learning about such a remarkable exoplanteary system and that we can find it today with our present technology is something we would’ve said was impossible twenty years ago. This is a fascinating discovery in and of itself and wondering what it is like in real life is kinda a natural thing for us curious apes to do.

    Oh, that’s right. Space travel is “heaven” for atheists.

    It is, is it? Citation needed.

    FYI, I’m pretty sure most atheists don’t believe in any type of supernatural heaven at all and they don’t seek to replace an afterlife with anything other than acceptence of reality and trying to live their lives and make the world as good as they can while they’re alive.

    I’m agnostic personally, I don’t know if there’s any God / Goddess / Cycle of reincarnation or what to expect in any afterlife if there is one which I doubt. I have a lot of hopes and dreams but saying these are a substitute for heaven, well, I don’t think so.

    Well, enjoy your fantasy–until you die. On earth.

    Um, thanks, I guess I will.

    Of course I plan to enjoy a whole lot more things than just fantasies in my life and one day it’d be great to get off planet if we develop that capability in my lifetime. We’ll have to wait and see and work towards that.

  58. Messier Tidy Upper

    @55. amphiox :

    I always figured that one of the justifications for not including exoplanets in the IAU planet definition was the simple fact that things like dwarf planets simply cannot be detected yet around other stars, and the IAU didn’t want to commit to defining something that we can’t even observe yet.

    Hey, that didn’t stop astrophysicists when itcame to neutrinos did it? ;-)

    Or Black Holes, white dwrafs, browns dwrafs and more all of which were predicted and stdied via thoughtexperimentand claculation log befor ethey werre pohysically observed.

    @52. SkyGazer : Neil will save us from this menace.

    Neil who? Armstrong? ;-)

    Or do you mean that other Neil – DeGrasse Tyson? If he were to come and debate Pluto’s planetary status here with me that’s be great, I’d feel very honoured. I would also politely and respectfully tell Ndg. Tyson that I think he is wrong and explain why I think so.

    Because he *is* wrong on this topic – and I’d add that I find his comments and approach to this mean-spirited and nasty and reflect very badly upon him too. :-(

    Also “menace” – what menace?

    @49. MaDeR :

    Heeereee we gooo agaiiiin….

    If this issue doesn’t interest you, MaDeR then feel free to skip it and comment on other things that do interest you instead. :roll:

    This is an issue that I feel strongly about and you don’t get to tell me what I can and can’t comment on here. If the BA himself were to ass me to refrain from commenting on this issue I will reluctantly do so as its *his* blog. It is NOT your blog MaDeR and so you don’t get to make the rules – nor are you being forced to read and take part in discussions like this if you aren’t keen to do so.

  59. Ray

    With all this angst over the Pluto thing, does anyone else find it particularly stupid that the IAU “voted” on science? Since when do we vote on science?

    I don’t recall any vote on Relativity.

  60. amphiox

    With all this angst over the Pluto thing, does anyone else find it particularly stupid that the IAU “voted” on science? Since when do we vote on science?

    It wasn’t a vote on “science”. It was a vote on a definition. Definitions are arbitrary, and, ultimately, of only utilitarian concern. Pluto is still Pluto. Whatever its actual nature in reality is, that didn’t change.

    Of course, the primary utility of any definition is so that everyone can agree on using it such that everyone knows what everyone else is talking about when using shared terms. In that respect, the planet definition has obviously failed.

    Hey, that didn’t stop astrophysicists when it came to neutrinos did it?

    Ah, but the astrophysicists wanted to define the neutrino. I get the feeling that the IAU didn’t want to define exoplanets because it’s a convenient political out for them to revisit the question later. It really seemed to me that no one was really satisfied at the end of the day with what they came up with, but for whatever political reasons they decided they didn’t want to end their grand, hyped-up process of re-defining the planet with an admission of “well, we really couldn’t decide on anything, nothing to see here.”

    I mean, the first definition they discussed, and almost passed, had 13 planets in our solar system, with Pluto, Eris, Ceres, Charon and Sedna (I think?) included as planets, and this one almost passed in the morning. And then to go from that all the way to dropping all the dwarfs in the afternoon is a rather weird (you’d expect small adjustments/modifications of something that nearly everyone was close to agreeing to, not flipping over to the exact opposite…) and suggests a rather dysfunctional, unsatisfactory process.

  61. Mathias R.

    Thanks all the people for the explanations. Yes, I’m glad to see that it is as I suspected. Go, Science! :D

  62. chenille

    Ignoring definitions, within our solar system, there are:
    1. Four giant gas objects, all with relatively circularly orbits of low inclination and without much else in the same region.
    2. Four moderate-sized rocky objects with orbits of the same type, but near the sun.
    3. Lots of smaller objects scattered in the regions between and outside these, with varying and often overlapping orbits, some large enough to be round and others not.

    Pluto was regarded as a planet in no small part because this wasn’t well understood, and it originally thought to be closer to the more moderate-sized objects. Nobody can make you follow the IAU decision, but they did pick a real divide.

    And for those who don’t recognize this, the IAU did need to decide something. They keep track of the objects in our solar system, and that includes particular nomenclature for non-planets, which are given numbers or letters. That need isn’t yet there for exoplanets, as the IAU doesn’t regulate their names.

  63. mfumbesi

    “…it may have nine planets!”
    I see what you did there..

  64. SkyGazer

    @60. Messier Tidy Upper

    Any system with more planets than us is a menace period

  65. RUTH MC

    @70. SkyGazer “Any system with more planets than us is a menace period.”
    WHY?

  66. Messier Tidy Upper

    @49. MaDeR :

    “if they’re ona collision course and thus lacking clear orbital paths they’re also not planets by IAu decree!”
    You are BSing. If they are on collision course, they are very soon going to lose planetary status by any definition of this word anyway. In other words, if this is serious argument (its not, but lets pretend), then this is argument also against your definition of planet. Well done.

    Planets on collision courses are still planets until the moment of impact and even often afterwards too. The collision may not result in total destruction of both worlds – heck Earth went through a collision when it colided with a Mars sized planet resulting in our Moon and a similar event formed Charon and Pluto’s other moons.

    It is also possible that there would be a close encounter that ejects one or more of the planets leaving them intact or that there could be -with enough velocity a grazing collision allowing the planets to reform post-impact. Imagination, MaDeR, you lack it – as well as civility and a reasonable argument there.

    “Under the present, temporary IAU laws HD 10180 has exactly zero planets” [- MTU -ed.] Yes, IAU folks deliberately excluded anything not orbiting our sun. It was deliberate decision, not some stupidity or mistake, as you imply.

    No, if it was deliberate then it was just a deliberately stupid mistake.

    Claiming that exoplanets aren’t planets just because they orbit another star is ridiculously stupid and fails the Copernican principle aka principle of mediocrity making it an unscientific error. To have deliberately chosen such an error just makes the IAU look even sillier here.

    Temporarness also was deliberate. I guess there will be another run in few years – we should know a lot more about exoplanets by then. Unfortunately for you, I do not see Pluto coming back.

    We’re already learning a lot more about exoplanets that makes it wise to have a very broad and inclusive definition. We already know of exoplanets in orbital relationships analogous to Pluto’s with Neptune (HD 45364) and of colliding exoplanets and that planets are extremely diverse in nature. You don’t see Pluto returning do you? Well I and many others do.

    As I noted earlier MaDeR you seem to be lacking imagination and lack of understanding the implications of what you are saying here. (Shrug.) Exoplanets have orbital relationships like Pluto and like it and the other ice dwarfs can be found in very “uncleared” orbits – maybe even sharing orbits. Including exoplanets means that for the sake of consistency, Pluto – plus Eris, Ceres, Sedna etc .. need to be counted as full planets just as exoplanets do.

    “very unrepresentative decision”
    Since when it is IAU fault that those oh-so-supposedly-many proplutonists did not bother to show to vote?

    Since the meeting was called at the last minute and they weren’t given enough of an opportunity that’s when. It seemed in fact that previous meeting had just concluded that Pluto – and Eris, Ceres and others were indeed to be counted then there was a surprise change of plan and ambush by the Pluto-hating faction. The way that Prague IAU meeting wa s conducted was to say the least extremely dubious.

    As I’ve pointed out already in comment #48 the decision was highly unrepresentative and not democratic even amongst the IAU members.

  67. Messier Tidy Upper

    Part II : (NB. Sorry about the delay inresonding here – struggling to find time at present)

    Again @MaDeR :

    I noticed you did not mentioned antything [sic] about “clearing orbit”. Probably talking about it in same thread as “planetary discriminant” would be asking for trouble, heheheh.

    Not at all. “Planetary discriminant” eh? That’s the latest metaphorical epicycle added to justify and rationalise the IAU’s blunder replacing the old Soter “unterplanet and uberplanet” nonsense? I don’t buy it at all.

    Yes there are huge planets like Jupiter, medium-sized planets like Earth and small planets like Pluto. Just as there are huge stars, red supergiants so large that could engulf our solar system out to the orbit of saturn, hundred mass stellar behemoths that shine five million times as bright as our Sun and then also there are neutron stars so small they are the size of a city and white dwarfs with half the mass of our Sun packed into the siz eof our globe and reddwarfs with a tenth of the solar mass shining with a hundreth of our Sun’s brightness. Stars and planets and galaxies cover some huge ranges of sizes , shapes and other things.

    Astronomical objects (like biological ones) are very diverse and cover a very sweeping continuum. To begin with you need broad catgeories that cover basic types of object at the animal-mineral-vegetable level hence the broad star-planet- asteroid classes.

    If you were doing a multiple choice question and had to put Pluto into one of those three main categories for astronomical bodies MaDeR which would you put Pluto into?

    The answer clearly isn’t star and equally clearly isn’t asteroid. Pluto is a planet.

    Next step from there is saying what kind of planet and on that, sure its a dwarf planet made of ice and orbiting far in the outskirts of our solar system. The division into dwarf planet, gas giant and rock dwarf comes at the next step of the categorising process.

    That seems very clear and obvious surely doesn’t it?

  68. Messier Tidy Upper

    Question for MaDeR & the other Pluto-haters :

    If the Royal Zoological Institute decalred that a mouse no longer counted as an animal because it is sosmall compared to blue whales and humans and theer aresomany of them would you be defending and agreeing with that decision?

    Its a fairly clear and matching analogy I think.

    Blue whale = Jupiter and the even larger exoplanets that have been found around other stars and mouse =Pluto and the other even smaller ice dwarfs such as Haumea, Sedna, Varuna, et al.

    Indeed animals range in size from the blue whale down to unicellar zooplankton and plants from the largest giant sequoias down to the tiniest single-celled phytoplankton and algal slime. Why should nature in the sky be defined more strictly and less diversely than biology?

    Another thought experiment : Put Pluto where Mars is and it would clearly be a planet, put Earth where Pluto is and Earth wouldn’t be. Does that actually make any kind of reasonable sense?

    A planet logically should be a planet wherever it orbits – whether close into its star or fra outside or even roaming space star-lessly. Unless its orbiting another much larger planet in which case its a moon but that’s a separate distinction again & very much another story.

  69. Messier Tidy Upper

    Hypothetical question to consider again for everyone out there :

    If Pluto was orbiting as the only planet of a star – say we put Pluto into orbit around an otherwise planet-less red dwarf or at vast distance from an O-type stellar giant that had driven away its panetary forming disk and had nothing but vast swathes of empty space -and Pluto orbiting around it far enough out to survive – then we’d call Pluto a planet would’nt we, right?

    Sure we would! ;-)

    We’d even do that with Ceres I bet!

    Really, when it’s all boiled down to its essence, the problem is that in our solar system, Pluto is one of a number of similar small planets that orbit in roughly the same zone of thesolar system -justas tehgas giants occupy their far smaller, closer n zone and tehinner rocky worlds from Earth out to Ceres do the same a step further and closer in.

    The logic, the wording and classification needs to be consistentand reasonable. The IAU definition just isn’t.

    As the following basic excercise demonstrates :

    Qu. A dwarf tree is still a _________? A. correct answer – tree.
    Qu. A dwarf species of animal is still an ____ ? A. Correct answer – animal.
    Qu. A dwarf human being is still a _____ ? A. Correct answer – human being.

    So then we get to the question : A dwarf planet is a ____ ?

    And the IAU answer is : “No, it isn’t a planet?”
    Bzzzt. Wrong. Just wrong.

    If Pluto isn’t a planet then by basic logic and basic use of English, a dwarf plant isn’t a plant, a dwarf animal isn’t an animal and, yes, a dwarf person would lose their human rights and cease to count as being human.

    Would MaDeR and the other Pluto haters enter a room full of dwarf people – a convention of jockeys say – and shout out or even think to themselves that because they are all small and because there seems to be a lot of them gathered in the one place they don’t count as being properly human? Really?

  70. Nigel Depledge

    Tara Li (19) said:

    I don’t believe Jupiter, put where some of the more distant KBOs found so far, would qualify as a planet. Of course, the IAU definition doesn’t give a clear definition of “cleared the neighborhood”, so one might even question Jupiter’s planethood in its current location, what with several hundred known asteroids sharing its orbit. That doesn’t sound like cleared to me!

    Hah! And Jupiter is not even round. Although, of course, no-one has ever defined how round something has to be to count as being in hydrostatic equilibrium.

    Or does the pragmatism extended to one criterion not deserve to be extended to a second criterion, in your view?

  71. Nigel Depledge

    T-storm (23) said:

    excuse me sir. we have 9. they can have my pluto when they pry it from my really cold dead hands

    Wow, what a persuasive argument. I bet the IAU never thought of that.

  72. Nigel Depledge

    Amphiox (42) said:

    It should also be noted that the IAU deliberately excluded exoplanets from its planet definition, so it actually isn’t really appropriate to talk about it with respect to exoplanets. It also means that the IAU definition is doomed (and the IAU must have known this and may even have intended it) to being revised again at some future point, when we know more about exoplanets….

    Sounds very plausible.

    It would indeed be crazy to try to come up with a definition that included exoplantes when we (currently) know so little about them.

  73. Nigel Depledge

    MTU (45) said:

    Oh & Pluto, Eris, Ceres, Makemake, Haumea and other ice dwarfs count as planets in my book anyhow so we still have more as others have noted already.

    And as has been pointed out in previous threads:

    1. The IAU definition is intended for use in technical discussions, and I’m pretty sure they don’t much care how the term is used in casual conversation.
    2. I expect that the IAU does not give two figs for your opinion, on account of your not being a professional astronomer.

    The IAU, as most folks will probably agree – especially after thinking it over clearly for a bit – got it very badly wrong.

    You have repeatedly claimed this, and repeatedly failed to demonstrate it.

    Stop dissing the IAU about their terminology, unless you are going to come up with something actually convincing.

    Dwarf planets are no less planets than dwarf plants and animals are still, well, plants and animals not minerals as #16. Kelly Clowers accurately pointed out. Being small and numerous doesn’t stop something being what it is.

    Except that a dwarf planet is not like a dwarf variety of a plant or animal. Dwarf plants and animals are – by and large – descended from (or share common descent with) their normal-sized relatives. Dwarf planets have not evolved – instead they f0rmed from what is likely to have been a diferent process than planets.

    Sure, maybe the term “dwarf planet” is a bit confusing to people who don’t work with this stuff, but I daresay the IAU had their reasons. You have yet to show that your questioning of their definition has any validity.

  74. Nigel Depledge

    @ all the “Pluto is a planet” whining brigade:

    1. Comparing planets from our solar system with exoplanets is a journey into the unknown, which is why the IAU restricted their definition to our solar system only. Until such time as we know enough about exoplanets to have a go at a universal definition.

    1a. Thus, any “if Earth orbited star XYZ . . .” arguments fail, by virtue of the fact that Earth does not orbit star XYZ.

    1b. A corollary to this is that any “If exoplanet ABC orbited Sol . . .” arguments fail, by virtue of the fact that exoplanet ABC does not orbit Sol.

    2. The “orbital clearance” criterion, despite requiring a measure of pragmatism to apply it, makes perfect sense, because it recognises the very obvious and natural discontinuity that exists in our solar system, as pointed out by CB above (#29). And since the IAU definition applies only to our solar system, that matters.

    3. Any complaint that the “orbital clearance” criterion is illogical (i.e. that there is no definition of “clear”) fails to acknowledge 2. above, and fails to recognise that we apply the same level of pragmatism to the hydrostatic equilibrium criterion, that everyone here seems to accept unquestioningly.

    3a. IOW, if you are going to demand an answer to the question “how clear is clear?”, you must also, for the sake of logical consistency, demand an answer to the question “how round is round?”. To do otherwise is to be hypocritical.

    4. Any arguments on the basis of moving a planet to a different part of the solar system (such as “if Earth were in the Kuiper Belt it wouldn’t count as a planet”) are purely hypothetical and fail because (for example) Earth is not in the Kuiper Belt. The definition is a practical one, on the basis of what is, not on the basis of what might have been.

    4a. And neither is the Kuiper Belt anywhere near Earth’s orbit.

    4b. And neither is the main asteroid belt anywhere near Mercury’s orbit.

    5. Arguments from authority (along the lines of “professional astronomer X has said that the IAU definition is crazy”) do not count either. If a majority of IAU members disagreed with the definition, then they would have it changed. Since the definition has stood for 6 years or so, it is reasonable to conclude that the majority of IAU members are satisfied with the definition.

    6. Since – to the best of current knowledge – Pluto, Eris and so on more closely resemble KBOs than they do any of the eight planets, it makes sense to group those objects with the remainder of KBOs.

    6a. To group the largest KBO, or the largest main-belt asteroid, with planets would be akin – to steal MTU’s biological taxonomy analogy – to grouping the largest insect as a bird.

    6b. It seems likely that Pluto formed through a process and under conditions much like those that formed the other KBOs, and unlikely that Pluto formed through a mechanism (and conditions) akin to that of the formation of Earth.

    7. I have seen it argued that Pluto should count as a planet because it has moons and weather and an atmosphere, but these features are not strictly planetary characteristics.

    7a. Pluto’s atmosphere is so tenuous that it more closely resembles hard vacuum than it does an atmosphere such as Earth’s.

    7b. Titan has a thick atmosphere, yet is quite obviously a moon.

    7c. Mercury has no atmosphere to speak of either (with apologies to scientists studying Mercury’s tenuous atmosphere).

    7d. Titan also has weather, while Mercury has pretty much no weather to speak of.

    7e. At least two planets have no moons, and it has been noted that some asteroids have moons. Moons are not a defining planetary characteristic.

    I have yet to see any cogent argument explaining any of the following:
    a. Why we should classify Pluto as a planet rather than as a KBO, when its characteristics seem to resemble other KBOs more than the eight planets.
    b. Why we should ignore the possible logical issues with the “how round is round?” question, while focussing on possible logical issues with “how clear is clear?”.
    c. Why we should ignore the obvious and natural discontinuity that exists in our solar system (whereby Mercury, Venus, Earth, Mars, Jupiter, Saturn, Ouranos and Neptune all stand out as distinct objects whereas Pluto and Ceres seem to be only the largest objects in classes of many similar objects).
    d. Why we should have a definition that – despite our current ignorance of exoplanets – is universal, rather than a definition that applies specifically to our solar system.
    e. Why the IAU should care about the opinion of people who are not working in a relevant field.
    f. Why anyone feels that the IAU is trying to impose anything on people who are not professional astronomers.

    In other words – stop whining and live with it.

  75. Nigel Depledge

    MTU (75) said:

    Hypothetical question to consider again for everyone out there :

    Translation – “irrelevant question to consider again for everyone out there ”

    But what the hey . . .

    If Pluto was orbiting as the only planet of a star – say we put Pluto into orbit around an otherwise planet-less red dwarf or at vast distance from an O-type stellar giant that had driven away its panetary forming disk and had nothing but vast swathes of empty space -and Pluto orbiting around it far enough out to survive – then we’d call Pluto a planet would’nt we, right?

    Nope, it’d be an exoplanet.

    Sure we would!

    Well, obviously, you would.

    We’d even do that with Ceres I bet!

    Ditto.

    Here’s an analogy that shows the irrelevance of your hypothetical questioning:

    If a hedgehog had scales instead of fur, and laid eggs instead of giving birth to live young, and did not lactate, and had a double-hinged jawbone instead of a single-hinged jawbone, it would be a reptile instead of a mammal.

    Well, sure it would, but so what? The hedgehog doesn’t have those characteristics. It has mammalian characteristics, which is how come it classifies as a mammal.

    Similarly, Pluto isn’t orbiting on its own around a red dwarf star. It orbits Sol, along with a whole bunch of other stuff in the same orbital region, and Pluto does not dominate that region.

    All of your hypothetical arguments suffer the same flaw – the IAU definition is a practical pragmatic one that deals with what is. Your attempts to find hypothetical scenarios in which it breaks down have been numerous and facile, and all of them are irrelevant.

    Really, when it’s all boiled down to its essence, the problem is that in our solar system, Pluto is one of a number of similar small planets that orbit in roughly the same zone of thesolar system -justas tehgas giants occupy their far smaller, closer n zone and tehinner rocky worlds from Earth out to Ceres do the same a step further and closer in.

    Why is this a problem? It just is, that’s all.

    But it makes far more sense to classify Pluto along with those objects it most closely resembles, rather than to try and force it into a category in which it would stand as the odd one out. Just because we’ve called Pluto a planet for 70-odd years does not mean it ever made sense to do so.

    In fact, I think it did once make sense to classify Pluto as a planet, but as we started discovering other large KBOs, it made more sense to change the way we classified Pluto. Or do you think we should call Pluto a planet even if we were eventually to discover another billion KBOs ranging unbroken in size from Pluto-size down to ring-particle-size?

    The logic, the wording and classification needs to be consistentand reasonable. The IAU definition just isn’t.

    This is a claim that you repeatedly fail to support.

    The logic is as good as it can be, given the current state of our knowledge. The wording is clear enough, and the classification is the only one that makes sense, in light of our current knowledge.

    Ironically, it is your illogical and puerile arguments against the IAU that have convinced me that the IAU did a fair job. If you hadn’t made IAU-bashing your new hobby 6 years ago, I’d probably still be on the fence.

  76. Clyde

    Ah… it has the same number of planets we do… 9

  77. Nigel Depledge

    @ Clyde (82) –
    No.

    Unless you have a counter for the arguments made above, just no.

  78. Messier Tidy Upper

    @ ^ Nigel Depledge : there are plenty of counters that have already been made. Your refusal to listen to and accept them doesn’t thereby make them false.

    @79. Nigel Depledge :

    @MTU (45) And as has been pointed out in previous threads:
    1. The IAU definition is intended for use in technical discussions, and I’m pretty sure they don’t much care how the term is used in casual conversation.
    2. I expect that the IAU does not give two figs for your opinion, on account of your not being a professional astronomer.

    I may not be a professional astronomer but many professional astronomers – Alan Stern among others – share my views.

    As for “casual conversation” versus technical jargon does itmake sense to you to alienate people from astronomy by adopting technical jargon needlesslyand confusing people by saying that things the public recognise as planets – exoplanets and ice dwarfs are not going to be called planets based on some needless technicalitity of very dubious value?

    The demotion of Pluto and refsual to say that dwarf planets are planets brings astronomy into disrepute. It doesn’t help the necessary relationship between professional astronomers and the public or aid in communicating astronomy to the public. It has negative consequences for the whole field of astronomy because it was a bad decision chosen on criteria that do not make reasonable sense and this reflects badly on astronomy and astronomers.

    You have repeatedly claimed this,(“The IAU, as most folks will probably agree – especially after thinking it over clearly for a bit – got it very badly wrong.”) and repeatedly failed to demonstrate it.

    Oh for pity’s sake! :roll:

    The numberof times I’ve demonstrated that proposition on this blog in past debates on this issue is probably into the hundreds by now! Just because you aren’t personally convinced by something doesn’t mean that the person convincingly demonstrating it for you hasn’t done so if you haven’t got a mind that’s open to being changed based on valid arguments.

    What demonstration, what facts would it take to convince you Pluto and the other ice dwarfs are planets?

    Will you only change your mind after the IAU changes theirs?

    Except that a dwarf planet is not like a dwarf variety of a plant or animal. Dwarf plants and animals are – by and large – descended from (or share common descent with) their normal-sized relatives. Dwarf planets have not evolved – instead they f0rmed from what is likely to have been a diferent process than planets.

    That’s an extraordinary claim indeed – especially given that dwarf planets are indeed planets! Your extraordinary evidence to back up the non-evolution of dwarf planets and their utterly different method of formation claim would be *what* exactly?

  79. Nigel Depledge

    MTU (84) said:

    @ ^ Nigel Depledge : there are plenty of counters that have already been made. Your refusal to listen to and accept them doesn’t thereby make them false.

    This is true.

    They have always been false for other reasons. Most often because you have claimed something is so but either failed to show that or have failed to demonstrate the relevence of your contention (such as, for example, the “need” you perceive for a definition of “planet” to be universal, or any relevance at all to your many hypothetical scenarios that break the IAU definition by placing planets out of their known context, or by changing the context of a planet).

  80. Nigel Depledge

    MTU (84) said:

    I may not be a professional astronomer but many professional astronomers – Alan Stern among others – share my views.

    But, as I point out above, they must be in a minority among the IAU or the definition would have been changed already.

    As for “casual conversation” versus technical jargon does itmake sense to you to alienate people from astronomy by adopting technical jargon needlesslyand confusing people by saying that things the public recognise as planets – exoplanets and ice dwarfs are not going to be called planets based on some needless technicalitity of very dubious value?

    This paragraph is typical of your rhetoric. You make one point explicitly, but frame it with a handful of unjustified contentions.

    I think the alienation of people from astronomy is beside the point. First, this distinction is not all that difficult, except for people who continue to cling to the concept of Pluto being a full-on planet. Second, every field of science has its necessary technical jargon. For instance, if I were to say to you that I expressed the HSV-1 dUTPase by transforming a plasmid vector containing the cloned gene into E.coli, then purified the recombinant protein from cell lysate by cation exchange chromatography, does that alienate you from biochemistry? Well, perhaps, but there isn’t any simpler way to say what I just said unless I use at least ten times as many words. Postulating the potential difficulty of technical terminology is not an argument against the need for a precise and meaningful definition of “planet”.

    The demotion of Pluto

    This is hollow rhetoric. Pluto has been reclassified, not “demoted”.

    and refsual to say that dwarf planets are planets brings astronomy into disrepute.

    Maybe. But in this case, the fault lies not with the definition of “planet”, but with the term “dwarf planet”. Call it something else, and move on. How about “gizmoid”?

    What matters most is not the term in and of itself but the relevance of the classification. Pluto, Ceres, Eris, Sedna, Haumea, Makemake and so on are not like Mercury, Venus, Mars, Earth, Jupiter, Saturn, Ouranos and Neptune, except in a few superficial ways.

    It doesn’t help the necessary relationship between professional astronomers and the public or aid in communicating astronomy to the public.

    Does it?

    I’ve not seen it have any impact on the BA’s communication ability. What is so difficult about discussing “exoplanets” (for example), as opposed to “planets around other stars”?

    It has negative consequences for the whole field of astronomy

    Evidence needed. This is merely your opinion, which on this topic I know well.

    because it was a bad decision

    Again with the opinion.

    Maybe it was a bad decision, but it was still better than or as good as any alternative I have seen proposed, including your own favoured candidate (which, incidentally, would have every hunk of rock or ice that isn’t a moon and happens to be big enough to be roundish counting as a planet). And it were far better to have some technical definition than none.

    chosen on criteria that do not make reasonable sense

    Again, this is only your opinion.

    As I have explained, the criteria make very reasonable sense.

    What would make no sense would be to classify Pluto, Eris, Ceres etc. in the same category as Mercury, Venus, Mars, Earth, Jupiter, Saturn, Ouranos and Neptune. Pluto and Ceres are but the largest examples of large classes of objects (KBOs and asteroids, respectively).

    and this reflects badly on astronomy and astronomers.

    Again, this is your opinion, not an argument to support your contention that the definition alienates the amateur.

    You have repeatedly claimed this,(“The IAU, as most folks will probably agree – especially after thinking it over clearly for a bit – got it very badly wrong.”) and repeatedly failed to demonstrate it.

    Oh for pity’s sake!

    Every argument you have made has been pretty easy to demolish. What I said was true.

    The numberof times I’ve demonstrated that proposition on this blog in past debates on this issue is probably into the hundreds by now!

    No, because all of your attempts have failed.

    Your arguments boil down into five main categories:

    1. Hypothetical scenarios that break the IAU definition. Yet you fail even to attempt to show why these scenarios are relevant to a definition tailored specifically to our solar system as it exists now.
    2. Attacks on the orbital clearance criterion – or the “how clear is clear?” question – while ignoring both (a) the obvious and natural discontinuity between Mercury, Venus, Mars, Earth, Jupiter, Saturn, Ouranos and Neptune and the other solar-system objects, and (b) the same necessary pragmatism required to apply the gravitational roundness criterion.
    3. Attempts to justify Pluto as a planet according to its characteristics such as possessing moons, despite the fact that you have found no characteristic that is possessed only by the eight IAU planets and not by any other solar system body apart from Pluto. In fact the eight IAU planets have only one characteristic that is unique to planets – they gravitationally dominate their orbital region.
    4. Emotional appeals based on unjustified criticism of the IAU (to whit, see your employment of rhetoric above).
    5. “Dwarf planets are still planets, aren’t they?” – this is a flaw with the term “dwarf planet”, not a flaw in the definition of “planet”.

    Just because you aren’t personally convinced by something doesn’t mean that the person convincingly demonstrating it for you hasn’t done so if you haven’t got a mind that’s open to being changed based on valid arguments.

    True, but not relevant here.

    As I said further up this thread, your attempts at arguing for the planethood of Pluto have actually convinced me that the IAU did the right thing. Initially, I was of the mindset that, “well, the IAU can call it what they like but I’ll still think of Pluto as a planet”, until I started seeing the flaws in your arguments.

    What demonstration, what facts would it take to convince you Pluto and the other ice dwarfs are planets?

    Simple.

    Do they share any characteristic that is uniquely a planetary characteristic?

    Or, to put it another way, what makes Pluto, Eris, Ceres etc. more like Mercury, Venus, Mars, Earth, Jupiter, Saturn, Ouranos and Neptune than they are like the other objects with which they orbit? Gravitational roundness alone is not enough, because it seems likely that we’ll discover dozens or hundreds of round (or round-ish) hunks of ice out in the far Kuiper Belt, and objects like these are most likely to be merely one end of a continuous spectrum of sizes of KBO, in the same way that Ceres (as far as we can tell) is simply the largest main-belt asteroid.

    To rephrase in a third way – what distinguishes Pluto from other KBOs apart from its size? Any class of objects has to have a largest object, so why should the largest object of a class be classified as a different kind of object?

    Will you only change your mind after the IAU changes theirs?

    Not necessarily, but if they make a decision with which I disagree, I shall give in gracefully, not carry on whining about it for 6 years.

    Except that a dwarf planet is not like a dwarf variety of a plant or animal. Dwarf plants and animals are – by and large – descended from (or share common descent with) their normal-sized relatives. Dwarf planets have not evolved – instead they f0rmed from what is likely to have been a diferent process than planets.

    That’s an extraordinary claim indeed – especially given that dwarf planets are indeed planets! Your extraordinary evidence to back up the non-evolution of dwarf planets and their utterly different method of formation claim would be *what* exactly?

    OK, I was a bit unclear here. I meant that they formed under a different set of conditions than the eight planets, which is as true as we can be sure of. But they certainly have not evolved in the way that plants and animals have evolved. Or are you trying to claim that our present day solar-system objects breed and undergo selection?

    My point was that dwarf varieties of plants and animals share common descent with their normal-sized relatives, whereas such processes do not occur with planetary objects. The planets and dwarf planets and other solar-system objects formed from the same protoplanetary disc, perhaps at around the same time, but with different sets of conditions pertaining in different parts of the disc. And while planetary objects do indeed change over time (e.g. the current front-runner theory for the formation of Earth’s moon), there is no selection process that parallels the natural selection of biological evolution, and there is no heredity that parallels biological heredity, and thus planets etc. have no process that is analogous to biological descent with modification. Thus, planets do not evolve in the way plants and animals have evolved.

  81. Prentice

    In response to, “A nearby star may have more planets than we do”. So what? You’ll never see them. As far the general public knows, they don’t exist.Do we have to believe everything that comes out of the mouths of astronomers as truth? They have all these instruments that can detect this and that and we are supposed to believe the instruments can tell us what’s out there. Not so. Don’t believe anything you hear and half of what you see.

  82. Pippa

    Sorry to ruin a great ongoing debate, but does it really matter what we call, or do not call, planets anyway? To believe so is really human – centric. I too was initially upset that Pluto was redefined, but it has not made much of a difference in my personal or scientific life. There will always be layman-term planets and scientific, what-ever-we care- to call-them bodies circling stars. So, lets call Pluto whatever we want, write papers using a common scientific language, – and congratulate HARPS on their work. Hopefully one of many ongoing discoveries that become more and more sophisticated. That’s what we need if we are ever to find human life friendly planets.
    That being said, I appreciate T Storms passionate defence of Pluto’s planetary designation! To many of us, she will always be a planet, emotionally speaking.

  83. Prentice (87): Yes, so we should believe you, Random Internet Person.

  84. Messier Tidy Upper

    Part II :

    Continued @ 79. Nigel Depledge :

    Sure, maybe the term “dwarf planet” is a bit confusing to people who don’t work with this stuff, but I daresay the IAU had their reasons.

    And those hypothetical IAU reasons would be what exactly too? Any suggestions that actually make sense?

    You wouldn’t accept “God must have Her /His /Its reasons for X” why settle for that when its the IAU?

    Like many Pluto defenders itseems to me you are ultimately replying on the argument from Authority Fallacy ie. the IAU said it and that makes it so. As though they’ve never made a mistake and incapable of doing so.

    As I askedMaDeR :

    If the Royal Zoological Society declared that mice were too small and too numerous to count as animals – would you accept & defend that too?

    Or would you, like me, say, “Hang on that’s just silly! Your saying so, doesn’t make it so?!

    I see that as a very closely matching analogy – mice are animals, Pluto is a planet – they may be small and one of a numerous class but it doesnt make them any less animals /planets than blue whales / people / Jupiter / Earth are. They’re different sort of animals / planets but still very much planets and pretending otherwise is just wrong and silly. However you try to justify with convoluted arguments to the contrary.

  85. Messier Tidy Upper

    Part III

    @86. Nigel Depledge :

    Every argument you have made has been pretty easy to demolish. What I said was true.

    Or so you* think, Nigel Depledge. :roll:

    As you may have noticed, I totally disagree with your own assessment of your and my arguing here.

    As for “casual conversation” versus technical jargon does itmake sense to you to alienate people from astronomy by adopting technical jargon needlesslyand confusing people by saying that things the public recognise as planets – exoplanets and ice dwarfs are not going to be called planets based on some needless technicalitity of very dubious value?” -MTU
    This paragraph is typical of your rhetoric. You make one point explicitly, but frame it with a handful of unjustified contentions.

    Not unjustified at all. Entirely justified and I keep providing source sand supportingevidence to back my views up.

    I meant that they formed under a different set of conditions than the eight planets, which is as true as we can be sure of.

    Well every object ahs its own individual history of formation but Pluto and the other ice dwarfs formed in the same way that other planets did – via accretion.

    Pluto’s history even paralllels tehformationof earth’s in several respects with the Big Splah forming Earth’s Moon and a similar collission forming Charon, nix, Hydra, P4 and perhaps a Plutonian ring system.

    Or are you trying to claim that our present day solar-system objects breed and undergo selection?

    You do know what an analogy is right? :roll:

  86. Messier Tidy Upper

    @80. Nigel Depledge :

    @ all the “Pluto is a planet” whining brigade:
    1. Comparing planets from our solar system with exoplanets is a journey into the unknown, which is why the IAU restricted their definition to our solar system only. Until such time as we know enough about exoplanets to have a go at a universal definition.

    Hey, do you notice how I’m *not* resorting to insulting you & your side of this issue?

    I’d perfer a civil argument & debate here if you please.

    Also y’know I think we know enough about exoplanets now to call it. We know they’re stranger than we think; we know they can collide and be in orbits analogous to Neptune & Pluto and we know they keep surprising us with what (exo)planets can be & do.

    So best to make our definitions as broad and inclusive as possible, no?

    1a. Thus, any “if Earth orbited star XYZ . . .” arguments fail, by virtue of the fact that Earth does not orbit star XYZ.

    Yet other planets identical to Earth in all other respects may very do so. Doesn’t thatmake you stop and think just a little?

    Science is also about wondering “what if” and imagining potential possibilities that we may well observe one day y’know.

    1b. A corollary to this is that any “If exoplanet ABC orbited Sol . . .” arguments fail, by virtue of the fact that exoplanet ABC does not orbit Sol.

    The failure there is in your (poor?) imagination and ability to say “well ok, then what if it did.”

    Einstein himself has apparently noted that “imagination is more important than knowledge.”

    2. The “orbital clearance” criterion, despite requiring a measure of pragmatism to apply it, makes perfect sense, because it recognises the very obvious and natural discontinuity that exists in our solar system, as pointed out by CB above (#29). And since the IAU definition applies only to our solar system, that matters.

    The application of the term “planet” solely to planets in our solar system is think especially stupid and ludicrous.

    There are several natural & very obvious discontinuities in our solar system. The most obvious is that between Jupiter and all other planets, the second most obvious between the gas giants and all other planets – are you now going to argue that Earth and all the other worlds such as Venus and Mars and, oh yeah, Pluto too are not proper planets?

    3. Any complaint that the “orbital clearance” criterion is illogical (i.e. that there is no definition of “clear”) fails to acknowledge 2. above, ..

    I acknowledge 2. above and I also acknowledge that it is manifestly stupid and unscientific & illogical and rubbish violating the Copernican “mediocrity principle” and so reject it simultaneously ..

    ..and fails to recognise that we apply the same level of pragmatism to the hydrostatic equilibrium criterion, that everyone here seems to accept unquestioningly.

    Your case for that would be what then? We do?

    Now, I’m prepared to allow some leeway for worlds such as Haumea and Jupiter and maybe even Vesta that aren’t perfectly spherical but I’m also keen to point out that the dividing line between asterpodis and planets is betweebn worlds that are gravitationally visually rounded (elipsoidal or oblate spheroidal for fast spinners) and now you are saying this hurts my case, well, how exactly?

    I’ve always argued that the definition of planet should be broader & more inclusive rather than narrower and less inclusive because, heck, the universe is a weird place and planets are clearly a diverse bunch.

  87. Messier Tidy Upper

    Continued :

    @80. Nigel Depledge :

    3a. IOW, if you are going to demand an answer to the question “how clear is clear?”, you must also, for the sake of logical consistency, demand an answer to the question “how round is round?”. To do otherwise is to be hypocritical.

    The answer is – round enough to know that the planet is roud or would be if it was spinning slowly enough. So I ask you how clear is clear or clear enough?

    Note that following the KISS principle it is very easy to tell at a glance whether a planet is round or round~ish.

    It isn’t easy or aywhere near so clear to tell whether a planet has a clear orbit or not. Simplicity in science is a virtue – eg. Occams razor.

    The simplest definition – with the best ability to tell at a glance and without referring to the planet’s broader environment – is going to be beter. The more superflous questions and the more convoluted the definition (eg. “clear” except for trojan asteroids, comets and other bodies, excluding sun-razing comets, Near Earth asteroids, all those sorta things) then theworst it is. Elegance and simplicity recall are scientific ideals – the IAU definition is neither elegant nor simple and raises a hell of a lot of questions.

    4. Any arguments on the basis of moving a planet to a different part of the solar system (such as “if Earth were in the Kuiper Belt it wouldn’t count as a planet”) are purely hypothetical and fail because (for example) Earth is not in the Kuiper Belt. The definition is a practical one, on the basis of what is, not on the basis of what might have been.
    4a. And neither is the Kuiper Belt anywhere near Earth’s orbit.
    4b. And neither is the main asteroid belt anywhere near Mercury’s orbit.

    So tell me again, what part of “hypothetical” and “thought experiment” and “imagine” exactly don’t you get? :roll:

    Also you do realise our solar system turns out to be rather atypical judging from those we’ve found so far right?

    5. Arguments from authority (along the lines of “professional astronomer X has said that the IAU definition is crazy”) do not count either. If a majority of IAU members disagreed with the definition, then they would have it changed. Since the definition has stood for 6 years or so, it is reasonable to conclude that the majority of IAU members are satisfied with the definition.

    Not necessarily. Anyhow, that’s again argument from authority fallacy. Just because the IAU are one possible source of authority it does NOT mean that they are correct or infallible.

    6. Since – to the best of current knowledge – Pluto, Eris and so on more closely resemble KBOs than they do any of the eight planets, it makes sense to group those objects with the remainder of KBOs.

    Except for allthose majpor difffrences that the ice dwarf lanets have fromthe other cometary cnuclei present inthe Edgewoerth -Kuiper cometary belt.

    Y’know, being round, geologically differeniated, having atmospheres, moons and rings and much, much more.

    Almost like they’re (gasp!) actually planets intheir own right rather than justasterodis or cometary nuclei!

    (NB. Will continmue againwhen I next have time.)

  88. Messier Tidy Upper

    Continuing :

    @80. Nigel Depledge :

    7. I have seen it argued that Pluto should count as a planet because it has moons and weather and an atmosphere, but these features are not strictly planetary characteristics.

    Maybe so, but they are indicative of planets. If something has moons and rings and an atmosphere – and seasons and a solid geologically differentiated surface and so forth, (as Pluto has) it is what you’d expect a planet to have and thus a sign that it is more likely than not to be a planet all things being equal.

    7a. Pluto’s atmosphere is so tenuous that it more closely resembles hard vacuum than it does an atmosphere such as Earth’s.

    What kinda like Mars then? ;-)

    Pluto’s atmosphere, however tenuous, is there and is a key feature of that planet – Pluto has seasonal snowfall and is detectable all the way from Earth so, yeah, it may be thin but I think the Plutonian atmosphere is still pretty remarkable and significant.

    7b. Titan has a thick atmosphere, yet is quite obviously a moon.

    If Pluto were orbiting Saturn then I’d happily call it a moon too.

    It isn’t. Next. :-)

    7c. Mercury has no atmosphere to speak of either (with apologies to scientists studying Mercury’s tenuous atmosphere).

    Which contradicts your earlier statement and helps your case *how* exactly?

    Yes, Pluto is more of a planet than Mercury ticking more typical planetary traits than Mercury does. You are happy to call Mercury a planet so therefore you should equally be happy – happier even – to call Pluto a planet too. :-)

  89. Messier Tidy Upper

    Part II – @80. Nigel Depledge :

    7d. Titan also has weather, while Mercury has pretty much no weather to speak of.

    Pluto also has weather. Titan is a remarkable moon – moon because it orbits Saturn. If Mercury or Pluto orbited Saturn -or another larger planet then they’d be moons. They don’t therefore they ain’t.

    You clearly accept Mercury as a “proper” planet (as do I) why then will you not accept Pluto as one given Pluto boasts more typically planetary features than Mercury does?

    7e. At least two planets have no moons, and it has been noted that some asteroids have moons. Moons are not a defining planetary characteristic.

    I never said they were a *defining* characteristic but I do think they are an *indicative* or IOW *suggestive* characteristic of something being a planet – if it is round and not orbiting another planet or ever shone by core nuclear fusion.

    I have yet to see any cogent argument explaining any of the following:
    a. Why we should classify Pluto as a planet rather than as a KBO, when its characteristics seem to resemble other KBOs more than the eight planets.

    Pluto unlike other Edgeworth-Kuiper Cometary Belt Objects (EKCBOs) is round, is relatively massive, has seasons and much more that usual non ice dwarf EKCBOs lack.

    Pluto is NOT just an asteroid or cometary nuclei but something that is very obviously different and more planet-like than them.

    b. Why we should ignore the possible logical issues with the “how round is round?” question, while focussing on possible logical issues with “how clear is clear?”

    Because gravitational self-roundness is a much better and clearer and mor eeasilydetrmined and more useful criteria than orbital clearence is.

    You can see at a glance of something is round and it isan intrinsic quality not one dependent on toomany other more complex factors like having a clear orbit is.

    A planet is round where-ever it is in a planetary system but whetheror not it has a clear orbit depends on what its surrounding environment is like, how far from its star it is, how stable or chaotic the orbits of other planets in its system are, etc ..

    That’s why.

  90. Messier Tidy Upper

    Part III for today :

    Once more unto the comment #80 of Nigel Depledge –

    c. Why we should ignore the obvious and natural discontinuity that exists in our solar system (whereby Mercury, Venus, Earth, Mars, Jupiter, Saturn, Ouranos and Neptune all stand out as distinct objects whereas Pluto and Ceres seem to be only the largest objects in classes of many similar objects).

    Jupiter and Saturn, Earth and Venus are the largest examples of their classes of similar objects too. Does that disqualify them from planethod? No. Why then should we disqualify Pluto and Eris from planethood?

    How about the natural discontinuity between supermassive Jupiter which emits more radiation than it recieves from our Sun Vs all the other planets? In many respects, Jupiter is far less “worldly” than Pluto is and far more different to what most people think of as planets.

    If we have to say that some planets are so alien, such different things from what we usually think of by the word “planet” then the gas giants are that and more deserving of separate non-planetary status than Pluto and the ice dwarfs are, methinks. In many respects Pluto resembles Earth more than any world in our solar system except perhaps for Mars and maybe Titan.

    d. Why we should have a definition that – despite our current ignorance of exoplanets – is universal, rather than a definition that applies specifically to our solar system.

    Yeesh! You really seriously mean that question? :roll:

    Because – d’uh! – astronomy applies to the whole universe, the entire cosmos about us not just our minuscule corner of it, that’s why!

    e. Why the IAU should care about the opinion of people who are not working in a relevant field.

    Because they are part of the wider human culture and society and planet. Because amateur astronomers & science communicators like myself are worjking invery relevantfieklds. ‘k?

    f. Why anyone feels that the IAU is trying to impose anything on people who are not professional astronomers.

    What? That;s an awfullybadly phrased question /assertion there Nigel. Maybe because that’s exactly what the IAU *are* doing in telling everyone in the world that they think Pluto doesn’t count as a planet justas inmy fictional example of the Riyal Zoological Society bizarrely and absurdly and wrongly claiming that they no longer accept that mice are animals!

    Mind you, I’m sure the biological equivalent of the IAU have far more sense than to make any such ridiculous statements. I expected much better from the IAU too.

  91. Messier Tidy Upper

    To conclude for tonight – @ the last line of 80. :

    In other words – stop whining and live with it.

    No. What the IAU decided was wrong, W-R-O-N-G, wrong. It is bleedingly obvious they got it wrong and this isn’t something that I think should be ignored but instead needs correction. ASAP.

    If, say, the US Congress had decreed that pi = 3 or 1+ 1 = 5 would you be happy to let that stand because the Congress had so ruled and they’re the boss or would your view on that be the same as mine regarding Pluto’s planethood?

    I don’t think you can call Pluto a non-planet any more than you can say pi =3 or 1 +1 = 5.

    @85. Nigel Depledge : Very clearly we disagree on that and on this issue.

    @86. Nigel Depledge :

    Your arguments boil down into five main categories:
    1. Hypothetical scenarios that break the IAU definition. Yet you fail even to attempt to show why these scenarios are relevant to a definition tailored specifically to our solar system as it exists now.

    Our solar system as it exists now is NOT how it was before or will be in the future and is NOT typcial of most of the exoplanetary systems we have disciovered since.

    Therefore defining planets narrowly in view of our current solar system is misleading and false and excessively exclusive just as it would be to say that because bats fly all mammals fly. It overlooks the reality that most planets are not like those in our currrent solar system – eg. Hot Jupiters, pulsar planets, Super-Venuses, etc ..

    Are you really rejecting the Copernican “mediocrity” principle of science as much as you appear tobe doing here? I am baffled and frustrated and exasperated by just how much you are NOT seeming to get this & the other points I keep raising. :-(

    I thought you were smarter than that Mr Depledge. :-(

    <blockquote.2. Attacks on the orbital clearance criterion – or the “how clear is clear?” question – while ignoring both (a) the obvious and natural discontinuity between Mercury, Venus, Mars, Earth, Jupiter, Saturn, Ouranos and Neptune and the other solar-system objects, and (b) the same necessary pragmatism required to apply the gravitational roundness criterion.

    See comnmet #96. (MTU – April 15th, 2012 at 6:34 am.) Yes, ice dwarfs are quite different sorts of worlds from gas giants and rock dwarfs just as those planetary types are equally different from each other. That doesn’t make them non-planets.

    This is hollow rhetoric. Pluto has been reclassified, not “demoted”.

    Reclassifying Pluto as an asteroid – effectively – a non-planet that doesn’t count as a planet is very obviously a demotion and a reclassification can certainly also be a demotion. You & the IAU you seeminglyworship just can’t get away from this reality.

  92. Okay, can’t resist one last post on this tonight – @86. Nigel Depledge :

    3. Attempts to justify Pluto as a planet according to its characteristics such as possessing moons, despite the fact that you have found no characteristic that is possessed only by the eight IAU planets and not by any other solar system body apart from Pluto. In fact the eight IAU planets have only one characteristic that is unique to planets – they gravitationally dominate their orbital region.

    Er, doesn’t our Sun do that? ;-)

    Also that gravitational dominence is :

    I) Relative and dependent on other factors such as the closeness of other large planets.

    II) Temporary – because in the past there were other planets that collided & otherwise interacted with the current ones eg. the gas giant we think was ejected from our early solar system and the mars-sized planet that hit the Earth creating our Moon – plus in future Mercury’s orbit may get pulled into a more elliptical state leading to chaos and possibly collisions in the inner solar system.

    III) Not sufficient basis to determine what aplanet is because of the points above and because we know of exoplanets that are clearly planets but not so gravitationally dominant eg. HD 45364 which is analogous to the Neptune-Pluto orbital relationship but with two gas giants. (Click on my name for source & info.)

    4. Emotional appeals based on unjustified criticism of the IAU (to whit, see your employment of rhetoric above).

    My criticism of the IAU on this is completely justified and indeed has been justified repeatedly in almost every post I’ve made here on this. Your refusal to accept this reality doesn’t make it go away.

    5. “Dwarf planets are still planets, aren’t they?” – this is a flaw with the term “dwarf planet”, not a flaw in the definition of “planet”.

    A definition of “planet” that excludes dwarf planets is as flawed and as blatantly erronous as a definition of “star” that excludes dwarf stars or a definition of dwarf animals, plants or people that excludes the dwarf varieties of those would be!

    How can you really, *really* NOT get that, Nigel Depledge!?! *Headdesk.* :-(

  93. Nigel Depledge

    Pippa (88) said:

    Sorry to ruin a great ongoing debate, but does it really matter what we call, or do not call, planets anyway? To believe so is really human – centric. I too was initially upset that Pluto was redefined, but it has not made much of a difference in my personal or scientific life. There will always be layman-term planets and scientific, what-ever-we care- to call-them bodies circling stars. So, lets call Pluto whatever we want, write papers using a common scientific language, – and congratulate HARPS on their work.

    I agree.

    Any chance you could persuade MTU to stop bashing the IAU?

  94. Nigel Depledge

    @ MTU (lotsa posts) –
    Time forbids today. Will return later.

  95. SteveC

    Could we use the study of HD 10180 to aid in finding a possible twin to our sun?
    I mean from the standpoint of the orbits of the planets in HD 10180 (resonances in particular) and corrolate(sp?) those to our own system? Could HD 10180 have a twin star as well, since most stars in the galaxy are part of binary systems?
    Just some food for thought I guess.
    Thanks.
    Steve C.

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