12 Planets

[den]

Congratulations Pluto for being confirmed into the Planet club! And congrats to Charon, UB313 and Ceres on your promotions!

HOORAY!

Comments

[lostwanderfound.livejournal.com]

My thinking is, anything large enough to have other objects orbiting it is more then just another chunck of rock.

It's not really significant from a definitional point of view. Quite a lot of comets and other small thingies have bodies orbiting them (to at least the same degree that Charon "orbits" Pluto; ie, barycenter above the surface), and there's no real reason why we can't have a planet without any satellites.

The definition they appear to be going with is that anything massive enough to be pulled into a globular form (at least roughly) by its own gravity gets called a planet, unless it's big enough to drive fusion, in which case it's a star.

(this makes small Brown Dwarfs the remaining categorisation problem: big planets, small stars, or a category all their own?)

Incidentally, it seems likely that one of the major factors in not kicking Pluto completely out of the planet category was political, not scientific: it's easier to get research funding to investigate a planet than it is to investigate a "Trans-Neptunian Object".

[nekura-ca.livejournal.com]

I was just thinking that as a sufficient condition, rather then a necessary condition. It doesn't rule out moonless planets. I was thinking if an object has a strong enough gravity field to hold a stable moon within it's Hill Sphere, it's more then a small asteroid.

Cometary debris clouds are not stable, and are either blown off, creating the tail, or fall back to the surface as the comet heads back to the outer solar system.

The definition also includes the condition that it's orbiting a star, so brown dwarfs are still "up in the air" as it were ^_^

That, and it's a lot easier to change the definition, then to admit that they've be wrongly calling it a planet for the past 75 years.

[nekura-ca.livejournal.com]

Ok, after finding this list of asteroids with satellites (http://www.johnstonsarchive.net/astro/asteroidmoons.html), I guess it's a little more common then I thought. Interesting stuff. Obviously it's not sufficient ^_^

[lostwanderfound.livejournal.com]

Oh, and also (obviously), has to not be a moon, which thanks to Charon, now appears to mean: globular object orbiting a planet with a barycenter below the surface.