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.