Note: this marks my 100th blog entry. I'm not one for artificial goalposts, but it surprised me that I've written this many. Maybe when I get to 1000 I'll make more of a note of it. :-) Also (added October 13, 2005), this entry was featured in the 19th Skeptics Circle at Time to Lean.
If something has a moon, does that make it a planet?
The quick answer is no. Actually, so is the longer answer. But the really long answer would entail the definition of a planet, and I'm not gonna get into that here. One of these days I'll write the 5000 word essay bumping around in my head about that topic, but not now.
Suffice to say that there is no real definition of what a planet is; the list of planets is decided in a back (presumably cigar-smoke-filled) room of the International Astronomical Union (IAU). Currently, they have said we have 9 planets in the solar system, including Pluto.
However, the discovery of large bodies out past Pluto has some planet-definers alarmed. One is smaller than Pluto, but the other is definitely larger (I discuss them in more detail here and here). The larger one, with a catalog number of 2003 UB313 and the unofficial name of Xena, is currently nearly three times as far from the Sun as Pluto is. It's an unresolved dot in even the biggest telescopes, but (as I describe in that first link above) astronomers know it is at least as big as Pluto, and possibly substantially bigger.
Realistically, it's still kinda dinky though. A good guess to its size is 2700 kilometers across, which is smaller than our Moon (at 3400 km across). So we're not even talking Mercury-size, here, and Mercury is the smallest planet no one argues about.
So is it a planet? That IAU group has been in discussion about this for a while now. Rumors have been around for weeks that they might add adjectives to the planets now, so Earth would be a major planet, say, and UB 313 would be a minor planet. And as I write this, minutes ago a news release says they've done just that:
Using this [new IAU] method, Earth and Venus would be known as terrestrial planets, Saturn and Jupiter as gas giants and Pluto as a Trans-Neptunian planet.
Personally, I think that's a silly solution. No one is going to use adjectives when talking about Jupiter, so this won't help at all.
Interestingly, though, the news was just released that UB313 has a moon (unofficially called Gabrielle, of course). Its size is unknown at present, but once it is observed enough to get an orbit, that will tell astronomers the mass of the parent body, Xena. So it's a useful discovery.
But one thing that some people argue about planets is that they have moons. I've heard this from a few people (mostly non-astronomers), that something with a moon can be more properly called a real planet. This is specious at best; many small asteroids have moons (the other new object past Pluto, EL61, also has one), and two "real" planets-- Mercury and Venus--- don't have moons. So the presence or lack thereof is only cause for further confusion.
I'll note that the discoverer is calling it a planet, and though he dismisses the idea that having a moon makes it more of a planet, the way he mentions it makes it clear he doesn't mind if some people think that having a moon helps.
As for me, do I think it's a planet? I think the question is meaningless, because, again, our definition of planet is arbitrary. If those guys from the IAU had come out of their room and decided that the Sun is a planet, or a tree is a planet, or Scarlett Johansson is a planet, then that would be the official word. As you can imagine, I certainly have an opinion about that-- what they did say wasn't much better. But that's an essay (and a long one) for another day.