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IAU Demotes Pluto to 'Dwarf Planet' Status
davidwr writes "It's official. Pluto's been demoted. It's now one of several 'dwarf planets.' I guess we can drop the 'Period' from 'Mary's violet eyes make John stay up nights.'" (Of course, no one says you have to privately agree with the International Astronomical Union.) Several readers have contributed links to the BBC's coverage of the downgrade, as well as the usefully illustrated story at MSNBC.
I wouldn't call it a screw-up.
The draft proposal was:
Pluto would continue to be a planet, and Ceres, Charon and 2003 UB313 would become planets. However, this criterium is reached by hundreds, even thousands of other celestial bodies in our solar system. Under that proposal, all could gain planet status.
The final text is:
This definition does not define the terms "nearly round", nor "neighbourhood". But having a definition, rather than just an enumeration, is in my opinion a big leap forward. Demoting Pluto is a small price to pay.
I quite like the additional criterion of dominance of a body in its neighbourhood. It's not as arbitrary as simply requiring a minimum mass or size.
On the other hand, I do not like the fact that a planet should orbit to Sun to be called a planet. On this point, I preferred the original proposal in orbit around a star. I don't see why our solar system should be any different, why planet-like celestial bodies orbitting other stars are not called planets.
Actually, while Pluto comes closer to the Sun than Neptune, they are never that close. Pluto's erratic orbit ensures that it is well above the solar system equator when it does cross. The chart here shows how far it really is at the cross (chart is in AU =~ 149 billion meters).
I came here for a good argument
Is it possible for Pluto and Neptune to one day (like within the next couple billion years) collide?
Nope. Their orbits are in 3:2 orbital resonance; basicly this means they constantly miss each other (a bit like your average commuter bus and train schedule :P). Also, due to the declination of the Pluto orbit it doesn't even touch the Neptune orbit. When seen straight from above, the orbits overlap, but if you go off-angle to just the right spot the Pluto orbit can be seen to be completely separated from Neptune.
"Fix it? It has been disintegrated, by definition it cannot be fixed!" - Gru in Despicable Me.
"Clearing" basically means that all other bodies within an orbital range are much smaller. There are many asteroids that cross Earth's orbit, but none are larger than a few tens of km. All 8 planets have cleared their orbital zones. The remainder of objects in those oribtal zones are assorted junk (comets, Trojans, NEOs, Centaurs, Atens, etc). Pluto and Ceres do not qualify because there are objects of comparable size in their respective orbital zones.
> On the other hand, I do not like the fact that a planet should orbit to
> Sun to be called a planet. On this point, I preferred the original
> proposal in orbit around a star. I don't see why our solar system
> should be any different, why planet-like celestial bodies orbitting
> other stars are not called planets.
Because they're only defining what a Solar planet is, not the general meaning of the word "planet":
The IAU therefore resolves that planets
and other bodies in our Solar System be
defined into three distinct categories
in the following way...
So the new definition doesn't apply to extrasolar planets. Why didn't they broaden their scope? Maybe the whole point of the exercise was just to deal "once and for all" with the Pluto problem. It's not going to affect the current work of people looking for extrasolar planets.
--I'm so big, my sig has its own sig.
-- See?
Actually (if I did the math correctly) in about 3,529,037,195 years. That's still within the projected lifetime of the solar system, so yes