Tuesday, November 01, 2005

Pluto a Planet again? Is anything a planet?

So, if Pluto really has three moons does it increase its stature in the "is it really a planet?" game?

My sense is "yes", but then I am in the liberal "of course Pluto is a planet" camp... but then I'm in good company on that one. Phbt.

See here and here

More seriously, it is clear that the IAU will be re-visiting the definition of planet soon.
And for mostly impure motives (hint: changing the definition so that a planet is only something orbiting a main-sequence star is not going to help anyone convince the Academy that they should get the Nobel prize. Chasing the wrong variable, as I understand how the process works).

This is going to be a bad and nasty process. On the other hand, the community does need to decide whether to go with a traditional, and possibly inconsistent, phenomenological definition; or to go with a consistent physical definition that may be undecidable for any given object.
It is somewhat aesthetically objectionable that under the current definition, a perfectly nice planet, like the Earth, would cease to be a planet if we were to be inadvertently ejected into interstellar space. The Earth as a sub-substellar object just doesn't sound right just because there is some accident of orbital mechanics.


WGESP IAU definition

The Berkeley definition

Note the "stellar remnant" variant. Under one definition, when the Sun exits the AGB phase, the planets cease to be planets, even though they were planets for 9-10 Gyrs. But if a planet is ejected through dynamical instability at any point, it ceases to be a planet under either definition.

On the other hand, no one seems to want a low mass object formed in isolation to be a planet; even if it quacks like a planet. And phenomenologicall it'd be impossible for the foreseeable future to untangle the dynamical history of free floating planet-like objects.

Double bah.


Blogger Adam Solomon said...

Bah, what a debate. I think me, Kelle, and a couple of her undergrads had this debate/discussion at some point over the summer. I wonder if it's best to simply consider Pluto a planet for the cultural significance, leave it as the exception to the rule simply because it is Pluto, and then say that no KBOs (unless we find something really strange, in the future) are planets?

The problem is that, at least as I see it, you can't make Pluto a planet without either introducing arbitrary, Pluto-specific borders, or without significantly increasing the number of planets in our solar system, and even looking beyond just science, I think we have to examine the cultural significance of what a planet is. Who was it that said that the term planet no longer belongs to astronomers?

Way back in the beginning of the 19th century, when Piazzi first discovered Ceres, it was considered a planet, and then we found Pallas, and Juno, and added them to the list, and by the time we found Vesta in 1807 (correct me if I'm getting my asteroid order wrong, BTW!), we realized all these "planets" in the exact same orbital region of the solar system might best be classified as something else. Now imagine if Pallas wasn't discovered one year after Ceres, but seventy. And suddenly we had an influx of objects in a similar region. When our knowledge of the asteroid belt was beginning to cement, none of the new "planets" had achieved any cultural significance and as such it wasn't too tough to just demote them all.

Now we try this with Pluto, and what happens? Simply view the Kuiper Belt as a modified and further-out version of the asteroid belt. The methods we devise to keep Pluto as the only (or one of only a couple) KBO planet seem far too arbitrary. Setting some sort of radius, or saying it must have at least this many moons/companions, etc. If Pluto is a planet, then if all or most of the rest of them aren't, I don't think that's very sound science, unless you make Pluto some kind of explicit exception.

BTW... Was wondering if you saw this, and what you thought? Professional opinion...:)

9:30 PM  
Blogger Adam Solomon said...

By the way, as for the other bit you mention, the upper limit of planetary mass and the "accident" of orbital mechanics, whether something's a planet or a tiny brown dwarf, is a pretty interesting debate (coming from the brown dwarf guy here :) ). Is there really a formative difference between the two, and if so, is it at all visible observationally? The impression I was under was that planets formed from circumstellar disks and that stars/BDs formed from gravitational collapse, which should give a fundamental difference between the two, and eliminate any dependency on orbital mechanics for the definition of a planet--make a free-floating planet a planet nonetheless.

I mean, at a point you get to extremes. How is Mercury supposed to be considered a tiny brown dwarf? There's a definite distinction between the two, and I'd imagine it would have to do with the formation, but maybe there are problems with that idea...I don't know...

9:35 PM  

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