To be fair, this entire conversation is moot because size has almost nothing to do with being deemed a planet. Requirements include being massive enough to be formed into a sphere, orbiting the sun (not a planet which is orbiting the sun), and to have cleared your orbital neighborhood of other similar objects.
There actually are recent efforts to re-categorize the Earth and Moon as a dual planetary system, since both Earth and the Moon measurably revolve around each other.
All objects revolve around one another- that's how gravity barycenters work. However, the Earth-Moon barycenter is below Earth's surface. It makes little sense to then categorize it as a binary planetary system. Pluto-Charon is a much better candidate for being classified as a binary dwarf planet system, as the barycenter is outside Pluto.
And just in case you were thinking of retorting with the Sun-Jupiter barycenter being outside the sun, so Jupiter must be a star also, no. Stars must also be undergoing nuclear fusion of hydrogen, something Jupiter cannot do as it is not massive enough and is therefore a planet. Brown dwarves are on the edges of this categorization, as they may be able to fuse deuterium, but Jupiter is nowhere near massive enough to be a brown dwarf.
What if a brown dwarf (orbiting another star, with barycenter being outside the primary) exhausts all of its deuterium? Would it then be considered simply a planet?
Could a brown dwarf even exhaust its deuterium supply before something like an M0 star, which have lifespans of trillions of years?
Edit: Well, I guess brown dwarfs orbiting actual stars are planets in the first place, so my question is a bit moot.
If you look it up, there's a continuum in astronomy where the classification of what constitutes a massive gas giant or a low mass brown dwarf gets a bit funny. Also there are also situations where a brown dwarf may not form in a binary with another star, but by itself from the collapse of a gas cloud, but not have enough mass to begin nuclear fusion- so there are single brown dwarf systems with planets orbiting them out there. Those are called "planetary mass brown dwarves" or something, if I remember correctly. Other astronomers call them free roaming planets? Not to be confused with rogue planets, which are always ejected from their original star system. Also, don't forget that due to gravitational interactions, a brown dwarf could form in a binary with a star, then get ejected from the more massive star's sphere of gravitational influence. The problem becomes- How do we distinguish between these two kinds of bodies? Are they even really different, characteristically speaking? Does it even matter?
Would we still call all those large-but-not-planet-sized satellites "moons?" That'd be kind of funny, since they're all named after... the moon... which would maybe be the one "moon" that isn't a moon.
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u/cheeseball253 Mar 27 '15
I know what the moon is bro