Four planets?

[redbird]

I don't care much whether Pluto is called a planet, though it's nice seeing people actually talk about the Solar System.

But any definition of a planet that excludes Earth and Jupiter is worthless. "Sweeps out its orbit" is a kludgy phrasing to deal with the Neptune:Pluto 3:2 resonance, but no matter how you phrase it, Pluto is still there, crossing Neptune's orbit, and half the planets in the system have associated asteroids.

Comments

[purplecthulhu.livejournal.com]

'Sweeps out its orbit' may be a simplified description of a more technical concept. I've not seen the details of the resolution though, but its meant to have been influenced by dynamicists who would be a bit more detailed than this description.

Pluto

[minnehaha.livejournal.com]

http://www.worth1000.com/emailthis.asp?entry=312604

B

[beginning.livejournal.com]

Our IT guy studied astronomy and is one of the people circulating the "re-planet Pluto" petition. This is all we talked about at work the past few days.

[barberio.livejournal.com]

"Sweeps it's orbit clear" is actualy a mistaken rephrasing of the requirement.

The requirment is "has cleared the neighbourhood around its orbit."

And further to this, 'cleared the neighbourhood', has a more specific meaning in IAU terminology. It refers to objects that share the *same* orbit as each other, not cross each others orbit.

[redbird]

It still leaves Trojan asteroids as a problem, I suspect.

[barberio.livejournal.com]

No. Because the Trojans are not orbiting the Sun in the same path as Jupiter, but orbiting the two lagrangian points (L4 and L5) in Jupiter's orbit.

[drplokta]

No, because objects that are gravitationally bound to the primary don't count. Earth's moon is bound to the Earth, the Trojan asteroids are bound to Jupiter and Pluto is bound to Neptune, so none of them stop Earth, Jupiter and Neptune from being planets.

[timill]

Pluto?

[lysana.livejournal.com]

Pluto is not bound to Neptune. It has an eccentric solar orbit that crosses Neptune's path, but that is not the same thing.

[drplokta]

It's in a 3:2 orbital resonance with Neptune, as are most of the other Neptune-crossing plutinos. Pluto's orbit is absolutely controlled by Neptune's gravitation. It's a similar relationship to that between Cruithne and Earth.