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Emily LakdawallaSeptember 21, 2005

An official pronouncement may be coming about the "what is a planet?" debate

Since the discovery of 2003 UB313, larger than Pluto, there's been a lively debate going on in many places about what makes a planet. There's now an article in Nature talking about a proposal that would address the controversy. The theme of the proposal is that the word "planet" has come to identify too broad a class of objects. Mercury, Neptune, and the "hot Jupiter" orbiting OGLE-TR-56 have very few similarities to each other. The resolution that the IAU Working Group proposes is to declare the term "planet" meaningless unless it is accompanied by a modifying adjective that makes the term more specific.

Such terms are already in use today. We have "terrestiral planets," which means Mercury, Venus, Earth, and Mars (and sometimes used to name the Moon); we have gas giant planets (Jupiter and Saturn) and ice giants (Uranus and Neptune); and even "minor planets," a catch-all term for everything smaller than Pluto. The working group proposes "Trans-Neptunian planets" to name Pluto and 2003 UB313 and presumably several of the other big Kuiper Belt objects.

But there's still debate on what the proper definitions of all these classes should be. The "Trans-Neptunian planet" term describes objects based only on their location. Apparently the panel has proposed "cisjovian planet" to describe everything interior to Jupiter. But I think that planetary scientists agree that what distinguishes the different bodies in the solar system from each other is their composition, not their location. The two are very closely related; generally speaking, planetary bodies have more gas-rich compositions as you go out from the Sun because they formed at lower temperatures. But some bodies have migrated, and some have been tossed around through interactions with larger bodies, so location doesn't tell you everything. On the other hand, the composition of newly discovered bodies isn't always known. It's certainly not well known for most of the extrasolar planets. For those, location is all you have. But it's not necesssarily that informative.

For me, I find the whole debate tiresome. I don't care what the definition of "planet" is and whether Pluto is one or isn't. Pluto is cool and worth studying. It belongs to a class of objects that includes 2003 UB313, Quaoar, and probably Neptune's moon Triton, so studying one of those objects, or comparing two of them, tells you a lot about the other objects. Moreover, worrying too much about what the planets are makes you lose sight of some of the coolest places in the solar system, like Titan, Europa, Enceladus, and Miranda. They're all "worlds" to me -- places to go and visit, study, map, contemplate, imagine, paint or draw, or write about.

Read more: trans-neptunian objects, Pluto, Eris, explaining science, dwarf planets beyond Neptune

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Emily Lakdawalla

Senior Editor and Planetary Evangelist for The Planetary Society
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