Andrew Gray (shimgray) wrote,
Andrew Gray

That Pluto Proposal

Summary: define and broaden category "planets", and give that category new subcategory "plutons". [Press release] They're voting on it next week, it seems; here's a summary of the draft resolution:
Point one: They state a planet has enough mass for gravitation to put it into hydrostatic equilibrium (ie: it's round), and it "is in orbit around a star, and is neither a star nor a satellite of a planet". There will be a process to evaluate boundary candidates. They also define a distinction between satellites and binary planets, which makes Pluto-Charon a double planet (at least under current data - this one may change, I suspect).

Point two: They distinguish between the eight classical planets (pre-1900) and other planetary objects, the latter being far away and moving weirdly. They also recognise that "Ceres is a planet by the above definition", and suggest that for historic reasons we might want to say "dwarf planet". They also accept that if Pallas, Vesta, or Hygeia are found to be in hydrostatic equilibrium, they're planets too.

Point three: Pluto is explicitly recognised as a planet, "as are one or more recently discovered large Trans-Neptunian Objects". They note these objects are orbitally very different from the classical planets, and create a new category of "plutons" to encompass them (ie, pluton = trans-neptunian planet).

Point four: "Minor planet" as a classification is abolished; everything orbiting the sun is now either a planet or a "Small Solar System Body", including comets, asteroids, TNOs.
(Also see the IAU's FAQ list)

So what does this mean? Under this proposal, we will have twelve planets - Mercury, Venus, Earth, Ceres, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon, 2003 UB313 - with an option on twelve identified possible other candidates, three in the asteroid belt, with more of the latter waiting to be discovered. It's going to be interesting. (It also means the race is on to get a name approved for UB313, fast - will "Xena" stick?)

(Current "possibles", presumably awaiting further study of size - 2003 EL61, 2005 FY9, 90377 Sedna, 90482 Orcus, 50000 Quaoar, 20000 Varuna, 55636 2002 TX300, 28978 Ixion, 55565 2002 AW197, 4 Vesta, 2 Pallas, 10 Hygiea)

Will it pass? I strongly suspect so. The BBC's said that all the IAU divisional chairs backed it on sight; it looks likely that their colleagues will think the same way.
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