August 18, 2006
The IAU is getting ready to officially define what a planet is today (Thursday). Here's what they've come up with:
1) A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.
(2) We distinguish between the eight classical planets discovered before 1900, which move in nearly circular orbits close to the ecliptic plane, and other planetary objects in orbit around the Sun. All of these other objects are smaller than Mercury. We recognize that Ceres is a planet by the above scientific definition. For historical reasons, one may choose to distinguish Ceres from the classical planets by referring to it as a "dwarf planet."
(3) We recognize Pluto to be a planet by the above scientific definition, as are one or more recently discovered large Trans-Neptunian Objects. In contrast to the classical planets, these objects typically have highly inclined orbits with large eccentricities and orbital periods in excess of 200 years. We designate this category of planetary objects, of which Pluto is the prototype, as a new class that we call "plutons".
(4) All non-planet objects orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".
[...]
For two or more objects comprising a multiple object system, the primary object is designated a planet if it independently satisfies the conditions above. A secondary object satisfying these conditions is also designated a planet if the system barycentre resides outside the primary. Secondary objects not satisfying these criteria are "satellites". Under this definition, Pluto's companion Charon is a planet, making Pluto-Charon a double planet.
Which means that in addition to the nine planets we all learned about in school we have some newcomers.
The largest asteroid, Ceres, is a planet -- its own gravity smashed it into a ball, and it orbits the sun directly.
Pluto's ersatz moon, Charon, is now a planet in its own right, since it and Pluto orbit a common center of mass ("barycenter") that is outside of the body of Pluto. Thus Pluto-Charon is now a double planet.
(The Earth-Moon barycenter is 4670 km from the center of Earth, compared to Earth's radius of 6380 km. The Moon is still a moon, though in a few million years, as tidal forces slow Earth's rotation and move the Moon's orbit farther out, Earth-Luna will become a double planet as well.)
Also making the list is 2003 UB313, temporarily dubbed "Xena" by its discoverers. The IAU will some up with an official name at some point, until then it's officially named something that looks like a serial number.
So remember, kids, we have either eight planets now or twelve, but nine is no longer correct.
I haven't found any mention of Quaoar and Sedna, but it seems that they would also become planets by that definition. So... 14?
Um, check that. On the Wikipedia page about Quaoar, they also mention Orcus, 2003 EL61 and 2005 FY9, all of which appear to be round. Which takes the count up to 17.
I'm guessing that we'll stick with the eight major planets (possibly grandfathering in Pluto and maybe Charon) and teh only place the others will be called planets in in textbooks. I'm also guessing that by this time next year, very few people will know or care about the current planet count.
