David A. Wheeler's Blog

Mon, 21 Aug 2006

What’s a planet? Why I’m glad there’s an argument

The International Astronomical Union (IAU) has a proposed definition of the term “planet” that is currently being vigorously debated. I like the definition, and I’m even more delighted that there’s a vigorous discussion about the proposed definition.

We’ve used the term “planet” for thousands of years, without trouble. But now that we’re learning more about the heavens, we now know about many objects orbiting other stars, and about many more objects orbiting our own Sun. As a result, our simple intuitions aren’t enough. Obviously objects will orbit other objects no matter what we call them, but since we want humans to be able to communicate with each other, it’s very important that we have definitions that help rather than hinder communication.

The latest proposed definition is actually very sensible. Basically (and I’m paraphrasing here), it defines a planet as an object that (1) orbits a star, and (2) has enough mass that its gravity can make itself “round”. The real definition has some clever nuances that make this workable. For example, Saturn rotates so fast that it bulges, but since it has enough mass to make it round, it’s clearly a planet by this definition. Also, if objects orbit each other and their center of gravity isn’t inside any of them, then they’re all planets - so both Pluto and Charon become planets under this definition. The object “2003 UB313” (unofficially called Xena) would be recognized as a planet as well, as would Ceres.

I think this definition is very sensible, because it’s based on observable basic physical properties, which are at least somewhat less arbitrary. One previous proposal was “orbits a star and is at least as big as Mercury” - which makes the “Pluto isn’t a planet” group happy, but is incredibly arbitrary. Another approach, which I happened to prefer before this proposal surfaced, is “orbits a star and is at least as big as Pluto” - which makes the “Pluto is a planet” group (like me) happy, and causes fewer changes to textbooks, but it’s still really arbitrary. All such definitions are a little arbitrary, but this new proposed definition is at least somewhat less arbitrary - this definition emphasizes a fundamental physical characteristic of the object. Namely, it has enough gravity to force a change in its own shape.

Some astronomers have complained that this proposed definition doesn’t account for “how the planets were formed”; I think this is nonsense. Humans weren’t around to watch the planets form; our current theories about planet formation may be grossly mistaken. In fact, I think it’s almost certain we’re wrong, because we can’t observe much about planets of other stars to verify or debunk our current theories. And what’s worse, since we can’t really observe much about objects orbiting other stars, we can’t really know if they’re planets based on some “formation” definition - because we can’t get enough data to figure out their ancient history. And here’s a funny thought experiment - someday we may be able to create planets ourselves. Yes, that’s not exactly likely to be soon, but it’s a great thought experiment. If we create a planet, is it a planet? It should be. Any definition like “planet” should be based on its obvious observable properties, not on best guesses (likely wrong!) about formation events of long ago. A definition that uses only unimpeachable data is far more useful, and when observing farway objects we get very little information.

No one wants to claim that every pebble orbiting a star is a planet — our intuition says that there’s something fundamentally “big” about a planet that makes it a planet. I asked an 11-year-old what made a planet, a planet. Her first answer, before hearing about this argument, was “it’s round” (!). While that’s not a scientific survey, it does suggest that the creators of this definition really are on to something - kids can intuitively understand (and even guess at!) this definition.

Are there weird things about this definition? Sure! Charon becomes a planet too, as I noted above. Since our own moon is slowly moving away, in a few billion years our moon might become a planet (assuming the Earth and Moon don’t get destroyed by the Sun first). I guess you could “fix” the definition for those two cases by saying that the “most massive” object of a group was the planet, but I don’t see the need for this “fix”. In fact, you get an interesting insight if your definition forces you to note that their centroid isn’t inside any of them. The object Ceres, now considered an unusually large asteriod, becomes a planet. That’s okay, Ceres was originally considered a planet when it was discovered - it even has a planetary symbol. It’ll make people rewrite the textbooks, but we’ve learned so much recently that they need rewriting anyway. It’ll make it easy to see which books are obsolete - they’re the ones which say we have only 9 planets.

What’s really great about this debate is that it’s making people think about the heavens. Any definition of “planet” is in some ways arbitrary, frankly. I think this essential arbitrariness makes such definitions the hardest things to agree on in science, because there’s no way that more observations can prove or disprove a theory. This definition is much less arbitrary than others people have come up with, because it focuses on an important “change in state” of the object. And that’s a pretty good reason to endorse this definition.

In any case, this debate has caused many people to discuss and think about what’s happening in the larger universe, instead of focusing on the transient. And that is probably the most positive result of all.

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