A planet in a four-star system

[cim]

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A planet is a celestial body that 
    (a) is in orbit around the Sun, 

Well, that's just invalidated the original topic, then.

[Commander McLane]

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A planet is a celestial body that 
    (a) is in orbit around the Sun, 

Well, that's just invalidated the original topic, then.

Especially if it's indeed "the Sun" that's to be orbited, because that'd exclude anything outside our own solar system anyway.

[Selezen]

That's why any planets not in orbit of our own sun are referred to as "exoplanets".

[PhantorGorth]

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A planet is a celestial body that 
    (a) is in orbit around the Sun, 

Well, that's just invalidated the original topic, then.

Especially if it's indeed "the Sun" that's to be orbited, because that'd exclude anything outside our own solar system anyway.

Yeah I noticed that. I assume they use the term Sun to be the sun of the system. After all "our sun" is called Sol. (As well as the fact that term "our sun" that makes perfect sense implies there must be more than one. The term "the sun" is therefore just relative to you.)

[DaddyHoggy]

Sorry, wrong DH. It's a "Dwarf Planet" and it is large enough to pull it into a sphere. The criteria it fails to be a full blown planet is "(c) has cleared the neighbourhood around its orbit" as there are a lot more object in similar orbits nearby.

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A planet is a celestial body that 
    (a) is in orbit around the Sun, 
    (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and 
    (c) has cleared the neighbourhood around its orbit 

The definition of the Dwarf Planet is:

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A celestial body that 
    (a) is in orbit around the Sun, 
    (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, 
    (c) has not cleared the neighbourhood around its orbit, and 
    (d) is not a satellite.

You're quite right I'm wrong - weird, that I was sure I'd read somewhere that Pluto wasn't sufficiently spherical to be a planet, but on Googling I could find no evidence that this was the case and plenty to say that it was!

I find the comments on this (old) article fascinating - especially that the decision was taken by so few members and is still ignored by many respected Astronomers: http://www.universetoday.com/13573/why- ... -a-planet/