What Makes A Planet A Planet?

What makes a planet a planet? It's been a big topic of debate in certain brainiac circles, and astronomers are set to put it to a vote next week in Prague. NPR's got the scoop, and David Kestenbaum writes (and thanks to him for teaching me almost all I know about Pluto) that the astronomy types are prepping a definition of planethood, so that we can all know, once and for all (until the next big debate), whether the solar system we all grew up with is going to be the one we take further into the new millennium. You know the solar system I mean: the sun in the middle and nine planets circling it for all they're worth. Actually the list of planets in our solar system is pretty likely to change, either getting smaller or larger, but not staying static. Since the last time we checked in with the order of things, somewhere around high school physics, there have been some new discoveries that call the familiar model into question, specifically at least one planet-like object, out past baby Pluto, that's bigger than Pluto is. It's called Xena, and has forced the astronomy debaters to debate this particular debatable issue. You see, they have to decide whether Xena's a planet, and that depends partly on whether Pluto's a planet.

So, what questions are the science gods considering? Taking it as a given that planets orbit suns (thanks to alert reader Su for pointing that out in the comments)--What defines a planet? Is it how big it is? Is it having a moon or two? Is it having enough gravity to make it round? (That is one of the criteria being debated. See, I knew major astronomical bodies were supposed to be round.) Is it having a relatively round orbital path in its journey around the big old ball of fire in the daytime sky? The answers to these questions will define whether our solar system models get bigger, or smaller in the coming years. You see, Pluto gets different status depending on which questions take precedence. While having a couple of moons of its own, Pluto is also tiny, smaller than the Earth's Moon. It's round, but has an orbit that's rather catawampous. If the debate tilts the wrong way for Pluto, the number of planets pops down to eight, but if it goes the other way, there might be a whole new category that bumps the count upward, since there are other items out in the Kuiper Belt (the gathering of hundreds of objects out at the fringe of our solar system of which Pluto is a part) that could qualify for planet status along with the undersized and familiar one we all know and love.

One of the proposals is to define a new class of planets. A panel of astronomy experts met in June to hammer out some possible definitions:

Several panel members have favored dividing planets into categories: terrestrial planets (Mercury, Venus, Earth, Mars), giant planets (Jupiter, Saturn, Uranus, Neptune) and a third class that would include Pluto.

"We'll call them dwarf planets or something," says Iwan Williams, an astronomer at the University of London who favors the idea and also served on the panel.

Sources say the panel's new definition for planets would, in fact, create a third category embracing Pluto.

The new category would also embrace Xena, since if Pluto qualifies, so does the Warrior Princess. A few others could take their place on the list, too, and then we would all get to argue about our favorite ancient Greek and Roman names to call them. Time to start watching Hercules reruns. Of course, first we have to decide whether they get their own category in the first place, and what to call that. Dwarf planets sound cute, don't you think? Let's vote for that! Oh yeah, only real astronomy buffs, with degrees and everything actually get to vote in Prague, and even with the voters kept to that reasonable minimum they aren't sure what the outcome will be:

It's unclear what astronomers will make of the new definition or how they will vote on it. Observers say the definition will have to be concise and unambiguous. What is too small to be a dwarf planet? Do moons count? What about round comets?

Oh goody, more questions. I love questions. I really do. All the best answers come from people asking questions. Since there aren't really answers yet, because nobody's actually voted yet, I have to leave you hanging on this one, but I'm sure I'll keep an eye out and pass the information along when it comes my way. Until then, try to think up those Greek-sounding names, okay? How about Podiatry, or Hermeneutics? Too esoteric? Oh well, see if you can do better.

Hat tip: Futurismic