Pluto on the Bubble

I thought a sphere was a three-dimensional surface, all points of which are equidistant from a fixed point.
 
I think any definition of "planet" will have to be arbitrary in some way: arbitrary restrictions on size, shape, distance from the sun, eccentricity of its orbit, etc. So I have a feeling the definition we eventually come up with will place the arbitrary dividing lines such that Pluto is a planet and the other similar Kuiper belt objects aren't.
 
Starthinker said:
I thought a sphere was a three-dimensional surface, all points of which are equidistant from a fixed point.
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Technically the surface is two-dimensional, the rgion enclosed by the surface is three dimensional. This can also be used as a definition of a sphere, but the definition of a spheroid by geni is more general, a sphere being the special case where the ellipse being rotated has equal major and minor axes (ie. a circle).
 
Should the new one simply be called "Planet X", instead of "Xena"??? Let's start a letter-writing campaign!!

Still, I'd hate to have to come up with a planet mnemonic that has an "X" in it.
 
geni said:
An object that isn't a star that orbits at least one star that has a gravitational field large enough to overcome its own material strength to the degree that is becomes spheroid but does not share it's orbit with other objects with a total mass greater than it's own.
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Well, let's see if the IAU agrees with you :rolleyes: .

Besides, that means including Xena in the club, potentially more...

The Terrestrial, Giant and Dwarf planets thing works for me.
 
and the winner is..................

drumroll.......


Pluto!

oh, and Charon, Ceres, and 2003 UB313......



Science textbooks will have to be ripped up - the solar system is about to get a bunch of new planets.
Astronomers want to redefine our home in the Milky Way as a place with 12 - rather than nine - planets orbiting the sun.

The proposal comes from a two-year project by the International Astronomy Union (IAU) to create the first ever scientific definition for the term planet.

It will also mean that Pluto keeps its status as a planet, despite calls from many astronomers, revealed in the Guardian on Monday, that it should be demoted because of its diminutive size.

If the ideas are approved at the general meeting of the IAU in Prague next week, schoolchildren will, in future, have to learn that the solar system has 12 planets: eight classical ones that dominate the system - Mercury, Venus, Earth, Mars, Jupiter, Saturn, Neptune and Uranus - and four in a new category called plutons.

These are Pluto, its moon Charon, a spherical asteroid that sits between Mars and Jupiter called Ceres, and an object called 2003 UB313 but nicknamed Xena by American astronomers who found it.
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http://www.guardian.co.uk/science/story/0,,1851323,00.html
 
Jimbo07 said:
Well, let's see if the IAU agrees with you :rolleyes: .
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They didn't

Besides, that means including Xena in the club, potentially more...
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No it didn't. That runs into the "but does not share it's orbit with other objects with a total mass greater than it's own."
 
Are we going to have to renumber all the astroids since number 1 appears not to be astoroid any more?
 
Astronomer A: Any round body that orbits the sun is a planet!

Astronomer B: Then my ass is a planet!

Astronomer A: You said it, fat boy, not me!
 
geni said:
What about sedna? Or 2005 FY9?

We may be about to have rather a lot of planets.
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According to the cited article, there are about 12 "candidate planets" that are being monitored to see if they qualify. Is Sedna known to be spherical?
 
I fail to see how Charon can be a planet if all other moons larger than this aren't. I could cope with 11 planets, although calling an asteroid in the middle of a pile of similar objects a planet is a bit of a stretch, but including Charon is just plain stupidity. I very much doubt this will be the final descision.
 
geni said:
ok if you want the full version

An object that isn't a star that orbits at least one star that has a gravitational field large enough to overcome its own material strength to the degree that is becomes spheroid but does not share it's orbit with other objects with a total mass greater than it's own.
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Excuse my ignorance, but which are the bodies larger than Pluto, with which it shares its orbit? (Apart from the fact that if you wanna do the math you can argue that any object in the solar system is in some way orbiting any other.)

Or isn't Pluto spheroid enough?

Hans
 
Cuddles said:
I fail to see how Charon can be a planet if all other moons larger than this aren't. I could cope with 11 planets, although calling an asteroid in the middle of a pile of similar objects a planet is a bit of a stretch, but including Charon is just plain stupidity. I very much doubt this will be the final descision.
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It is part of a binarly planet
 
Cuddles said:
I fail to see how Charon can be a planet if all other moons larger than this aren't.
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Charon isn't a moon, but arguably part of a double-planet system. Ganymede technically orbits Jupiter -- the center of mass of the Jupiter/Ganymede system is somewhere inside Jupiter, and at many points in its orbital path, Ganymede actually moves backwards (retrograde) relative to the Sun.

Charon does not -- both it and Pluto orbit a common center of mass external to both bodies, and they both are always moving forward w.r.t. the Sun.
 
I would imagine that astrologers are resisting the change with much fervor. Otherwise they'd have to change all their charts.
 
Cuddles said:
I fail to see how Charon can be a planet if all other moons larger than this aren't. I could cope with 11 planets, although calling an asteroid in the middle of a pile of similar objects a planet is a bit of a stretch, but including Charon is just plain stupidity. I very much doubt this will be the final descision.
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Well, the Pluto/Charon situation is a bit unique. I believe (correct me if I'm mistaken) that Pluto and Charon are the only pair of orbiting bodies in our solar system where the center of mass of the system doesn't lie inside one of the bodies, which means that they really do orbit around each other, rather than there being a clear primary and satellite.

Or, to put it in a different perspective, suppose there were two roughly Earth-sized bodies orbiting each other. Would you deny one of them planethood just because it was somewhat smaller than the other? Or would you consider it a dual-planet system?

If you don't believe Charon qualifies as a planet on the basis of size, then I'd definitely agree with you. But I don't think the fact that Charon is gravitationally bound with Pluto should necessarily disqualify it as a planet.
 
drkitten said:
Charon isn't a moon, but arguably part of a double-planet system. Ganymede technically orbits Jupiter -- the center of mass of the Jupiter/Ganymede system is somewhere inside Jupiter, and at many points in its orbital path, Ganymede actually moves backwards (retrograde) relative to the Sun.

Charon does not -- both it and Pluto orbit a common center of mass external to both bodies, and they both are always moving forward w.r.t. the Sun.
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Fair point I suppose. But that doesn't mean I have to like it. :p

alfaniner said:
I would imagine that astrologers are resisting the change with much fervor. Otherwise they'd have to change all their charts.
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They seem to have managed fine before. There were originally thought to be 5 planets. Any more changes will just make them claim that this is why they could have been wrong before. I believe some astrologers already use other objects like Ceres and Halley's comet.
 
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