2003 Ub 313: The Incredible Shrinking Planet?, No bigger than Pluto? |
2003 Ub 313: The Incredible Shrinking Planet?, No bigger than Pluto? |
Guest_BruceMoomaw_* |
Jan 31 2006, 09:20 PM
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Apr 12 2006, 05:15 AM
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Junior Member Group: Members Posts: 37 Joined: 21-December 05 Member No.: 614 |
QUOTE The problem is that it's easy to say that now with just one solar system and nine (or eight or ten, depending on who's counting) planets. What happens when we start finding solar systems where the equivalent of Mercury is the size of Europa or Triton? Is the response of astronomers to be: Sorry, you have to be "this big" to qualify as a planet. For some reason the exoplanets in the PSR 1257+12 system never get a lot of attention, yet they are as small or even smaller as Pluto. Did we discover small planets or are they the first asteroids/minor planets we found around another star? |
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Apr 12 2006, 05:45 AM
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Member Group: Members Posts: 249 Joined: 11-June 05 From: Finland (62°14′N 25°44′E) Member No.: 408 |
PSR 1257+12 B and C are a few times more massive than the Earth, A is about the size of the Moon. There's probably yet another body, but not even the discoverers call it a planet as it is has only one fifth the mass of Pluto.
-------------------- The universe is not only stranger than we imagine, it is stranger than we can imagine.
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Apr 12 2006, 06:05 AM
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Member Group: Members Posts: 307 Joined: 16-March 05 Member No.: 198 |
PSR 1257+12 B and C are a few times more massive than the Earth, A is about the size of the Moon. There's probably yet another body, but not even the discoverers call it a planet as it is has only one fifth the mass of Pluto. Interesting. Yet again it seems to be only the term "planet" which attracts this debate. Consider neutron stars. They are not even as big as many asteroids or Kuiper belt objects, much less Pluto, yet no astronomer seems to be suggesting that their diminutive size means we should stop calling them "neutron stars" and dub them (say) "neutron objects" or "neutron dwarfs" instead. ====== Stephen |
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Apr 14 2006, 04:28 AM
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Interplanetary Dumpster Diver Group: Admin Posts: 4405 Joined: 17-February 04 From: Powell, TN Member No.: 33 |
Interesting. Yet again it seems to be only the term "planet" which attracts this debate. Consider neutron stars. They are not even as big as many asteroids or Kuiper belt objects, much less Pluto, yet no astronomer seems to be suggesting that their diminutive size means we should stop calling them "neutron stars" and dub them (say) "neutron objects" or "neutron dwarfs" instead. ====== Stephen Stars have a clear definition, in that they are capable or were capable of sustained nuclear fusion. So there are clear criteria here. The problem is that, despite what Britt thinks, the term planet is still largely pre-scientific, without clear criteria behind it. The IAU & everyone else has to face up to one fundamental fact: the objects in the Solar System exist along a continuum of sizes, from individual hydrogen atoms to Jupiter. Defining what is and is not a planet will always be a purely arbitrary convention by any objective standard, except for the apparent distinction that a planet has to independently orbit the Sun. Maybe it's time to throw out the concept entirely...? Well, short of that heresy, maybe we just need to distinguish between "major" and "minor" planets. If that definition were adopted, I'd say that Mercury becomes the standard minimum body, and we have eight major planets. (Let's face it: it's embarrassing that Pluto is only half the size of the Moon!) That works until we find a KBO larger than Mercury. -------------------- |
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Guest_BruceMoomaw_* |
Apr 14 2006, 07:21 PM
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That works until we find a KBO larger than Mercury. Actually, it doesn't work right now. The trouble is that some of those super-Mercurian iceballs may be in the Oort Cloud, and if so we'll NEVER know whether they exist there or not -- they're too far away for any conceivable type of direct observation. (The remarkable orbit of Sedna, which is pretty big itself, is suggestive.) Alan Stern suggested long ago that there's a real chance of some icy planetesimals out there bigger than Earth (indeed, we can't quite rule them out in the Kuiper Belt at this point). Alan HAS suggested an alternative "scientific" definition for the minimum size of a planet -- namely, objects big enough to round themselves gravitationally. The trouble is that (1) This definition, too, has seriously fuzzy edges -- consider Iapetus' distinct but not overwhelming departure from the spherical, and the fact that Proteus looks like a marshamallow. The last straw may have been the discovery of the remarkable rapid spin rate and resulting high elongation of 2003 EL61, which approaches Pluto's diameter on its long axis -- but is only half that on its short axis. Unlike the upper-size scientific definition of a planet -- an object too small to ignite deuterium fusion in its interior, as brown dwarfs do -- the "roundness" definition seems to me just too seriously shaky. (2) If we do accept it, then there's also a veritable swarm of new objects which will have to be called planets, including at least four asteroids and a very big collection of KBOs. This may be acceptable to scientists, but the public will raise hell. Combine these two factors, and I think that in the end we have to accept that the definition of "planet" will be unavoidably highly arbitrary. (2003 EL61 produces enough problems by itself; its long diameter is about 2000 km, which would fit my own proposal for defining a "planet" -- but its short axes don't.) |
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Apr 14 2006, 08:18 PM
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Member Group: Members Posts: 809 Joined: 11-March 04 Member No.: 56 |
Alan HAS suggested an alternative "scientific" definition for the minimum size of a planet -- namely, objects big enough to round themselves gravitationally. The trouble is that (1) This definition, too, has seriously fuzzy edges -- consider Iapetus' distinct but not overwhelming departure from the spherical, and the fact that Proteus looks like a marshamallow. The last straw may have been the discovery of the remarkable rapid spin rate and resulting high elongation of 2003 EL61, which approaches Pluto's diameter on its long axis -- but is only half that on its short axis. Unlike the upper-size scientific definition of a planet -- an object too small to ignite deuterium fusion in its interior, as brown dwarfs do -- the "roundness" definition seems to me just too seriously shaky. But if you turn the definition on its head, and say that a minor planet is something that is small enough that it can stably retain a non-round shape, then you almost have a workable boundary, one that for "major" planets would exclude objects smaller than Iapetus or 2003 EL61 (and possibly even larger objects). And instead of adding in "a swarm" of small objects to the "major" planet category, the only marginal additions (at this date) would be Pluto, 2003 UB313, and possibly 2003 FY9. It would also place the boundary just around the 2000 km diameter measurement that has been proposed as well. Of course, we might settle all this, and then when New Horizons arrives at Pluto, discover that it's actually shaped like a top hat with the crown pointing toward us. |
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Guest_BruceMoomaw_* |
Apr 17 2006, 06:45 PM
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But if you turn the definition on its head, and say that a minor planet is something that is small enough that it can stably retain a non-round shape, then you almost have a workable boundary... No, you've still got a seriously fuzzy one, given the question of what constitutes "round". (Iapetus? Proteus?) Ultimately, you HAVE to set some kind of arbitrary figures for the different dimensions of such worlds. (2003 EL61 presents a problem already, since it's almost as wide as Pluto on its long axis but only half as wide on its short axes. I suppose you'll have to make my "2000 km diameter" definition an object's minimum diameter along any axis to call it a planet.) |
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Apr 17 2006, 07:17 PM
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Member Group: Members Posts: 809 Joined: 11-March 04 Member No.: 56 |
No, you've still got a seriously fuzzy one, given the question of what constitutes "round". (Iapetus? Proteus?) Nope and nope. In any case, 2003 EL61 is considerably larger than both, and is clearly "unround", which would put both objects well below the seriously fuzzy boundary. QUOTE Ultimately, you HAVE to set some kind of arbitrary figures for the different dimensions of such worlds. Don't expect any argument from me on that point. |
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