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2003 Ub 313: The Incredible Shrinking Planet?, No bigger than Pluto?
dvandorn
post Apr 17 2006, 08:47 PM
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Well -- for scientific purposes, it would be far more useful to catelogue Solar System bodies using several different criteria, such as size, mass, primary/secondary composition, current location and presumed location of origin. That's a six-criteria statement, which isn't all that hard to represent in some form of classification system.

But that's categorization for scientific purposes. As has been recognized here, there are also cultural purposes for such categorizations. And those criteria are far different. That's a point I've been trying to make for quite some time.

-the other Doug


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“The trouble ain't that there is too many fools, but that the lightning ain't distributed right.” -Mark Twain
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Guest_BruceMoomaw_*
post Apr 17 2006, 09:59 PM
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Size makes more sense than discovery date, by any standard (scientific or popular). After all, two planets were discovered after we started discovering asteroids.
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Guest_JamesFox_*
post Apr 18 2006, 01:56 AM
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When it comes to 2003 EL61, it's odd shape is due to a super-high rotation rate. Since many planets in the Solar System are oblate spheroids, one could perhaps say that objects count, if they were spherical if not rotating.

Irregardless of what is decided, I'll state that I'll be happy if at least small, spherical worldlets (like Ceres, Quaoar, and Sedna), are given a status distinct from regular small asteroids, KBO's, and comets. Even if they are not as impressive as the like of Mercury or Mars, they are still much more than 'lumps of rock', and deserve names, symbols, and sailor senshi biggrin.gif .
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Stephen
post Apr 18 2006, 04:15 AM
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QUOTE (BruceMoomaw @ Apr 17 2006, 09:59 PM) *
Size makes more sense than discovery date, by any standard (scientific or popular). After all, two planets were discovered after we started discovering asteroids.

IMHO neither make much sense.

But let's suppose for argument's sake some kind of line did get drawn on the basis of size. What happens if KBOs bigger than the agreed line start being found? One or two might well be accommodated, but suppose a dozen or more were found? Are they all to be honoured with the label "planet" or do the goalposts get moved again? (And if the newly found KBOs are allowed the honour why not Pluto?)

This line of argument can be taken further. What happens if KBOs start being found out there which are bigger than Mercury? Do we start calling them planets or does Mercury also get demoted from the "planet" club when the line gets moved yet again to weed out the unwanted?

If there has to be a line of some kind which defines what is a planet and what is not then no matter where you draw it, be it to exclude Pluto or to include Pluto, there surely ought to be some kind of rational scientific justification for putting that line in a particular place. If the position is simply some arbitrary value then what exactly has been achieved by having a line at all, let alone at that particular point?

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Guest_BruceMoomaw_*
post Apr 18 2006, 06:40 AM
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Well, you've got to set the goalposts at SOME point. And the case for setting them at just a bit smaller than Pluto is pretty good, precisely because Pluto has been thought of as a planet for so long -- given the rate at which KBOs are being discovered now, it's unlikely that there will ever again be a gap so long between the discovery of KBOs more than 2000 km wide.

Of course, we should also very definitely be teaching kids that at this point the word "planet" doesn't have any scientific significance, that all it means is "object bigger than such-and-such a diameter", and that at this point we have no idea how many "planets" the Sun has even by that arbitrary criterion (and, for that matter, we never WILL know, thanks to the possibility of big Oort Cloud objects). If we're not going to admit that "planet" is strictly an arbitrary standard, we should stop using the word at all -- but I very much doubt that's going to happen. All we can do is make sure that people know what it actually signifies at this point.
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AndyG
post Apr 18 2006, 09:08 AM
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QUOTE (BruceMoomaw @ Apr 18 2006, 07:40 AM) *
Well, you've got to set the goalposts at SOME point.

Why not set goalposts using the highest surface escape velocity? This neatly captures elements of diameter and mass, and avoids issues of how to define 'rotundity'.

Set limits something like:

asteroid < 750m/s
> 750m/s small planet < 2500m/s
> 2500 m/s medium planet <5000m/s
> 5000m/s large planet

Ceres has an escape velocity of ~500m/s. It's an asteroid, albeit a large one.

Pluto is ~1070m/s. Small planet.

Mercury & Mars are medium planets. Earth & Venus are large.

If the body orbits a planet, it's a moon. Change "planet" to "moon" and set the size qualifier one higher. "Asteroid" changes to "moonlet". So Titan and Ganymede (~2770m/s) become "large moons" and Europa (2040m/s), as with our Moon, become "medium moons". Miranda is a moonlet. And quite right too.

Andy G
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ugordan
post Apr 18 2006, 09:21 AM
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QUOTE (AndyG @ Apr 18 2006, 10:08 AM) *
"Asteroid" changes to "moonlet". So Titan and Ganymede (~2770m/s) become "large moons" and Europa (2040m/s), as with our Moon, become "medium moons". Miranda is a moonlet. And quite right too.

Somehow calling a 500 km diameter object (such as Enceladus) a "moonlet" is a bit of a stretch for me. These moons are indeed small by standards of our own Moon, but they still merit being called regular moons. After all, they are round, not irregular. Are you also suggesting we put Enceladus in the same category as, for example, Daphnis, a recently discovered moonlet around Saturn, which is a couple of km, tops?


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edstrick
post Apr 18 2006, 10:23 AM
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Also... at all 4 giant planets, there is a clear division of moons with relatively few objects that are really transitional.

There is "outer gravel", of which Phoebe is a large example.

There are large and small "major moons". Large are Triton size and bigger, small are Iapetus/Rhea/Titania/Oberon size and smaller down to the smallest round moons

There are "inner gravel" moons and "ice chips", all clearly substantially smaller than the smallest round moons with no as yet proven overlap.

And there are Intra-ring moonlets and ring rubble.

All 4 systems except Neptune's have these features fully developed, though scaling between gas giant and ice giant systems confuses things a bit.

At Neptune, the capture of Triton into an elliptical retrograde orbit apparently took out all the pre-existing major moons, but the outer and inner gravel and rings are still there.

Clearly there is a pattern shared with variations between all 4 outer planet systems and we need to pick "natural" divisions to define a nomenclature.
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David
post Apr 19 2006, 08:31 PM
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QUOTE (AndyG @ Apr 18 2006, 09:08 AM) *
Why not set goalposts using the highest surface escape velocity? This neatly captures elements of diameter and mass, and avoids issues of how to define 'rotundity'.


Yes, it does capture "elements of diameter", but it does so in a distinctly non-intuitive way -- keep the mass of an object the same, but increase its diameter, and its surface escape velocity will decrease. It's only because larger objects tend to have considerably more mass than smaller ones that a list of objects by escape velocities looks anything at all like a list of objects by diameter or mass. (Surface gravity is even worse; going by gs, Neptune outranks Saturn, Earth outranks Uranus, and Mars and Mercury are almost the same). The information you get from a ranking by escape velocity you might get just as well, and less ambiguously, using a ranking by mass.

The question of where to draw the line remains in any case. Let's say we use escape velocity. Should we put the division at one quarter the Ve of Earth's, admitting only Mercury-size and above? Or, let's say, one-tenth the Ve of Earth, letting in Pluto? And we're going to have to wait a while to make a judgment on 2003 UB313, because we're still unsure about both its mass and diameter.

In short, it's no worse a proposal than others, but doesn't seem to be any better, either.
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edstrick
post Apr 20 2006, 09:36 AM
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The arm-waving idea I've posted here before is that a planetesimal becomes a planet when, during accretion, its gravity field is great enough so that it's accretional capture diameter is significantly, perhaps 2 times, greater than it's physical diameter.

During accretion, at any given time and any given distance from the sun, there is a mean encounter velocity between planetisimals. It's probably bigger near resonances and near accreting giant planets. The mass of an accreting body and the average encounter velocity between it and candidate stuff to be accreted determines the "capture diameter".

Something 1 km across will have a capture diameter of something like 1.0001 km. Something 10,000 km across might have a capture diameter of 25,000 km. That's what lets accreting planets start to grow faster than their smaller competitors and end up eating the competition. That really is a "physics of origins" related distinction between planet and planetesimal
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ljk4-1
post Jul 21 2006, 02:40 PM
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Though it is not online, the July 24, 2006 issue of The New Yorker magazine
has an article on the tenth "planet" and its discoverer, Michael E. Brown.


--------------------
"After having some business dealings with men, I am occasionally chagrined,
and feel as if I had done some wrong, and it is hard to forget the ugly circumstance.
I see that such intercourse long continued would make one thoroughly prosaic, hard,
and coarse. But the longest intercourse with Nature, though in her rudest moods, does
not thus harden and make coarse. A hard, sensible man whom we liken to a rock is
indeed much harder than a rock. From hard, coarse, insensible men with whom I have
no sympathy, I go to commune with the rocks, whose hearts are comparatively soft."

- Henry David Thoreau, November 15, 1853

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