2003 Ub 313: The Incredible Shrinking Planet?, No bigger than Pluto? Options

[Guest_BruceMoomaw_*]

(Thanks to Emily Lakdawalla):

http://sciencenow.sciencemag.org/cgi/content/full/2006/127/1

[Betelgeuze]

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?

[Jyril]

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.

[Stephen]

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.

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Stephen

[tedstryk]

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.

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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.

[Guest_BruceMoomaw_*]

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.)