Nine Planets Become 12 with Controversial New Definition

Posted by dormouse (16th Aug 2006, 17:58) -

Ceres, Pluto, Charon, and 2003 UB313 ('Xena') would be classified as dwarf planets or plutons. Pluto & Charon would be a double planet. "IAU members will vote on the proposal Thursday, Aug. 24. Its fate is far from clear."

The proposed definition: "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."

( BBC news has a lot on it as well.

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read biography for - John Kirk

 John Kirk ( 17th Aug 2006, 07:17, Rank: Patrician )  reply

I saw a bit about this on TV, and I tihnk the most valid point is that we need a definition which works for other solar systems; it's a big universe, after all. That said, I'd favour an alternate approach: keep the word "planet" for the existing nine (except Pluto), then have a new generic word that means "planet or pluton" (I'm less sure about dwarfs). I'm certainly in favour of using the orbiting source as a criterion rather than just the size, so I have no problem with Earth's moon not counting as a planet, even if it's bigger than some other new planets.

Oh, and "UB313" doesn't exactly roll off the tongue :) According to (, regarding "Xena" and other codenames, There is no chance whatsoever that these will become the permanent names of these objects!, so I think it makes sense to pick a proper name before the new classification.

read biography for - Sulkyblue

 Sulkyblue ( 17th Aug 2006, 12:30, Rank: Nazgul )  reply

"keep the word "planet" for the existing nine (except Pluto)" Sorry - you mean there'd be 8 planets and Pluto would not be?

I'm in the "pluto is a planet" camp on this one. I like the fact that it remains a planet (all be it a subclass of planet) and that other things get included to.

I'm not certain about Charon though, I assume there's a good reason it's not fallen into the "not a satellite of a planet" rule, but that seems a bit strange to me without losing my morning to reading info pages. Although that's very very tempting.

read biography for - dormouse

 dormouse ( 17th Aug 2006, 13:22, Rank: Jedi )  reply

( - "Pluto and Charon are being referred to as a "twin" planet because their common centre of gravity, or barycentre, is located in free space outside the surface of Pluto." But they also note that Pluto's other smaller moons are still moons, despite orbiting around a centre outside Pluto, because they are too small to be round.

read biography for - John Kirk

 John Kirk ( 17th Aug 2006, 14:31, Rank: Patrician )  reply

Yup, I'd say that there would be 8 planets and a bunch of plutons (including Pluto), all of which would be classed as "celestial bodies" (for want of a better generic term).

read biography for - dormouse

 dormouse ( 17th Aug 2006, 17:18, Rank: Jedi )  reply

I like 'pluton' :)

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 Dan ( 22nd Aug 2006, 14:29, Rank: Cylon Centurion )  reply

Pluton is a french, surface-to-surface tactical nuclear missile

read biography for - dormouse

 dormouse ( 22nd Aug 2006, 16:33, Rank: Jedi )  reply

also apparently an existing geographical term (

read biography for - John Kirk

 John Kirk ( 23rd Aug 2006, 00:57, Rank: Patrician )  reply

I have to agree that it is a fun word, though :)

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 Sulkyblue ( 24th Aug 2006, 14:41, Rank: Nazgul )  reply

( - failed. Pluto's been demoted.

read biography for - Sulkyblue

 Sulkyblue ( 25th Aug 2006, 11:24, Rank: Nazgul )  reply

( - war breaks out

read biography for - John Kirk

 John Kirk ( 25th Aug 2006, 12:26, Rank: Patrician )  reply

There's also a Joy of Tech strip along those lines: (

I'm not sure about all of the objections raised. For instance, voting over the internet makes sense, but I'd assume that they have some form of postal ballot rather than everyone having to be in the same room. Is it just that people attended the first 3 days of the conference, couldn't be bothered to sort out a proxy vote before they left, and are now whining because they lost?

read biography for - Sulkyblue

 Sulkyblue ( 25th Aug 2006, 14:05, Rank: Nazgul )  reply

Nope - votes like that, if you're not there, tough luck. Only 4% of 'astronomers' (whatever definition they're using to find that number) voted in this.

Sounds a bit screwy to me ;0)

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