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A friend of mine who's a planetologist points out that the IAU's definition is really not a good one from a planetologist's point of view. It's centered around

A friend of mine who's a planetologist points out that the IAU's definition is really not a good one from a planetologist's point of view. It's centered around how the planet moves (which is what an astronomer can see) instead of around how it's composed (for which we need to send a probe). It's not even that good from the astronomy point of view. A planet has to have cleared its orbit of other bodies—so what about Jupiter's Trojan asteroids? Are they just handwaved out of the way, or is Jupiter no longer a planet?

hugh3 16y ago
It's centered around how the planet moves (which is what an astronomer can see) instead of around how it's composed (for which we need to send a probe). What does its composition matter? That won't help you distinguish anything. Indeed, to first approximation there are only three types of stuff in the solar system: "gas" (meaning hydrogen+helium), "ice" (meaning water, methane and ammonia) and "rock" (meaning... well, rock and metals). And there's planets made out of each; Jupiter and Saturn are gas, Neptune and Uranus are ice, and the inner planets are rock. As for the "cleared the orbit" bit, check out wikipedia's article on "Dwarf planet"; talk of clearing the orbit is just shorthand for a genuine mathematically-defined criterion which Jupiter fits and Pluto doesn't.
- metageek 16y ago
  >What does its composition matter? I said she was a planetologist. To planetologists, composition is everything. Composition is what the planet is, not where it is. >Indeed, to first approximation there are only three types of stuff in the solar system: Yeah, but planetologists don't stop at the first approximation. >As for the "cleared the orbit" bit, check out wikipedia's article on "Dwarf planet"; Let's see...ah. OK, here's the clause that I didn't know about: "and there are no other bodies of comparable size other than its own satellites or those otherwise under its gravitational influence" (emphasis added). The Trojans are clearly under the gravitational influence of Jupiter. Thanks.

hugh3 16y ago

It's centered around how the planet moves (which is what an astronomer can see) instead of around how it's composed (for which we need to send a probe). What does its composition matter? That won't help you distinguish anything. Indeed, to first approximation there are only three types of stuff in the solar system: "gas" (meaning hydrogen+helium), "ice" (meaning water, methane and ammonia) and "rock" (meaning... well, rock and metals). And there's planets made out of each; Jupiter and Saturn are gas, Neptune and Uranus are ice, and the inner planets are rock. As for the "cleared the orbit" bit, check out wikipedia's article on "Dwarf planet"; talk of clearing the orbit is just shorthand for a genuine mathematically-defined criterion which Jupiter fits and Pluto doesn't.

metageek 16y ago
>What does its composition matter? I said she was a planetologist. To planetologists, composition is everything. Composition is what the planet is, not where it is. >Indeed, to first approximation there are only three types of stuff in the solar system: Yeah, but planetologists don't stop at the first approximation. >As for the "cleared the orbit" bit, check out wikipedia's article on "Dwarf planet"; Let's see...ah. OK, here's the clause that I didn't know about: "and there are no other bodies of comparable size other than its own satellites or those otherwise under its gravitational influence" (emphasis added). The Trojans are clearly under the gravitational influence of Jupiter. Thanks.