The Solar System now has 12 planets (and counting!)

[Lorizael]

Originally posted by Az
Personally, I think that saying that the satellites of planets are not planets just because they're circling other planets is silly. Or why does it have to orbit a star? why is a star defined outside of a context of a solar system, while planets aren't?

Probably something to do with the fact that the star's presence is the reason there is a solar system. They are the context.

[Wernazuma III]

Originally posted by Oerdin
Bah! There are only 8 real planets.

[MichaeltheGreat]

Originally posted by reds4ever


Just checked, it maybe because the barycenter of the 2 bodies has its center of gravity outside the sphere of the primary member. This makes it a double planet and both members are named as planets. If the COG was within the sphere of the primary member then the secondary member would be defined as a satellite instead. Although by this rule our own moon will be a planet in about 40 million years

I would assume that the rule would be modified, or fleshed out in detail to avoid this result. There are a lot of differences in the two systems - earth/moon mass ratio is on the order of 100 to 1, whereas Pluto/Charon are likely closer to 10 to 1, but with some significant margin of error.. The moon is tidally locked to the earth and has a significant assymetry of mass with the COG displaced to its earth facing side from the physical center. Both of these conditions indicate a clear dominance of the the moon by the earth's gravity.

Pluto and Charon are mutually tidally locked, so that indicates a totally different story.

Another possible distinction is in the formation of the objects objects - given the distance and relative masses, it's unlikely in the extreme that Pluto could have captured Charon independently. Robin Canup published a paper in Science (IIRC) a year or so ago which modeled this, but it assumes a lot of unknowns about the masses and individual angular momenta of the two objects.

Another possibility, which is not the case with the earth-moon system, is that they were two independent but closely associated objects mutually captured by Neptune into a similar orbit. We don't know enough about the composition of the two, but there are significant differences in apparent surface composition and albedo features that make the formed by collision hypothesis much less certain than is the case with the earth and moon.

[Az]

Probably something to do with the fact that the star's presence is the reason there is a solar system. They are the context.

I am pretty sure there are plenty of celstial bodies that answer the definition of a planet, but don't circle a star, but drift in space. Why the hell aren't they planets?

[Lorizael]

An episode of Star Trek: Voyager deals with this exceedingly important phenomenon in great detail. Their answer: Planet

[Az]

Well, I guess it's sealed, then.

[MichaeltheGreat]

Part of the definition of planet is orbiting around a star.

There are likely billions of former planets in our galaxy which were gravitationally ejected from the stars they formed around, including a few from this solar system. Mercury and Mars aren't in completely stable orbital paths, so there is a very small chance they could be ejected in a little tug of war between the sun and Jupiter. Welcome to cold dark matter, or at least one of its constituents.

[reds4ever]

MtG,

The Earth/Moon thing was a throw away reference to the fact that the moon is moving away from the Earth (2.5" a year IIRC) and would qualify as a planet in the distant future.

[Az]

Part of the definition of planet is orbiting around a star.

yeah, but it's a stupid definition.

[Flubber]

I'll take the crotchety conservative approach and reject any change on this point since "that's the way it always was"

[St Leo]

Originally posted by Az

Probably something to do with the fact that the star's presence is the reason there is a solar system. They are the context.

I am pretty sure there are plenty of celstial bodies that answer the definition of a planet, but don't circle a star, but drift in space. Why the hell aren't they planets?

Because they are Planemos. And, if we let them in, we'll have a doozy of a time keeping hundreds of moons out. Planet refers to both being spherical and orbiting a star. Let's leave it at that.

[MichaeltheGreat]

Originally posted by reds4ever
MtG,

The Earth/Moon thing was a throw away reference to the fact that the moon is moving away from the Earth (2.5" a year IIRC) and would qualify as a planet in the distant future.

If barycenter outside the parent body is the sole criteria. Maybe the IAU figures we won't be around to worry about the definition then, but I would think there would be further evolution of the minumum standard at some point. I think the distinctions between earth/moon and Pluto/Charon are sufficient that the earth/moon system shouldn't ever rate double planet status - Pluto/Charon probably shouldn't either, although I have no problem with Ceres and 2003UB 313.

[MrFun]

It's stupid to redefine planets -- planets do not orbit around other planets.

[reds4ever]

We only have a small number of planets to go by, maybe binary are reasonably common outside our solar system?

[MrFun]

Well good luck on your exploratory mission, reds -- godspeed.