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.

Re:Now every geek's question is...

[DoktorTomoe]

Basically, in no way at all, as the manga states that "every celestial object has its Sailor Senshi" (japanese edition, volume 18.). For reference, see the later senshi-to-be Amazon quartet (based on "Dwarf Planets" in the Kuiper Belt) appearing in TV SuperS, and the Senshi that apear in "BSSSM Stars", who are based on extrasolar planets.

Just again realized what a crazy otaku I really am...

Re:Astrologers panic!

[smooth wombat]

NPR had an astrologer on last night and he said the same thing. Their definition of a planet is different than what the 'experts' say is a planet.

Then again, when you're dealing with flimflam you can pretty much say whatever you want.

The fact that NPR had this segment only served to legitimize this nonsense and continued to give hope to the gullible that astrology is valid.

Mary?

[gafferted]

Most volcanoes erupt mulberry jam sandwiches under normal pressure

Andrew

Re:A new one

[Anonymous Coward]

More viagra enhances my Johnson's sexual usefulness, nightly!

Re:my take on it:

[Adhemar]

I wouldn't call it a screw-up.

The draft proposal was:

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.

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:

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.
A dwarf 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, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
Pluto is a dwarf planet by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.
All other objects orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

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.

Still arbitary

[Anonymous Coward]

In truth there are three classes of planets. There are rocky planets like the Earth and Mars then there are gas giants like Jupiter and Saturn and finally icy planets like Pluto and other round Kupier Belt objects. None of the planets have perfectly round orbits so orbit is a poor watermark for planet status. Round shape and orbits the Sun are the most important features since roughly round shape denotes a certain mass. The number should increase not decrease there should simply be three classes of planets. If Pluto is to be considered little more than a large comet than the gas giants should be classed as failed stars. The definition for brown dwarf is fairly arbitary like the planet definition. It's arbitary if some compositions are called planets and others not since the remaining planets vary greatly in composition.

Re:So why does Neptune qualify?

[shma]

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 [nasa.gov] shows how far it really is at the cross (chart is in AU =~ 149 billion meters).

Re:Anyone? Anyone?

[Nemosoft Unv.]

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 [wikipedia.org]; 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.

Re:So why does Neptune qualify?

[phlegmofdiscontent]

"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.

Re:Anyone? Anyone?

[noretsa]

1) No. First off, while at one point Pluto is closer to the Sun than Neptune and all 2D maps show the orbits crossing, in 3D the orbits do not intersect. Secondly Pluto is in a resonance with Neptune (I think 2:3), so any orbital deviations will be "corrected" by Neptune resulting in a stable solution for billions of years. Eventually they will lose enough energy through gravitation radiation to start migrating inward towards the sun's dead corpse but who knows what the solar system will look like by then.

2) Pretty cool. Probably cooler than the Shoemaker-Levy impact on Jupiter (and that was very impressive).

3) No... Pluto is basically the size of China and Neptune is considerably bigger. Neptune will come out of it not significantly changed.

4) Colliding worlds might have been relatively common in the early system. You might be interested to know that the most popular current theory of the origin of the moon involves a trojan mars-sized planet striking an early earth with the debris collecting into the moon. This is called the Giant impact [wikipedia.org] hypothesis.

Re:my take on it:

[GrayCalx]

I think they added that to specifically exclude all moons, not just Charon.

Re:Still arbitary

[phlegmofdiscontent]

The definition of brown dwarf is pretty well... defined. There was no debate over the upper size limit for a planet because the dividing line is the ability to fuse deuterium (heavy hydrogen). This is theorized to be about 13 Jupiter masses. The upper limit for deuterium fusion is about 83 Jupiter masses (8% the mass of the sun), at which point the object can fuse hydrogen and is considered a normal star. So, really, the definition of brown dwarf is not arbitrary at all (being an object between 13 and 83 Jupiter masses).

Re:So why does Neptune qualify?

[ArbitraryConstant]

If you added up the mass of Pluto and all the other similar objects that cross closer to the Sun than Neptune, you only get a tiny fraction of Neptune's mass. Neptune completely dominates the mass at that distance from the Sun.

Re:So why does Neptune qualify?

[frankie]

Actually, Pluto's orbit isn't erratic, and Neptune dominates its orbit quite nicely. Pluto is 3:2 resonance locked, which makes it a pseudo-moon of Neptune (not entirely dissimilar to Cruithne's relationship with Earth).

http://www.google.com/search?q=pluto+neptune+reson ance [google.com]
http://en.wikipedia.org/wiki/3753_Cruithne [wikipedia.org]

Re:my take on it:

[Chysn]

> 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.

Re:.. but given enough time...

[shellbeach]

Yes, in a few billion years. If you're still around then, you get to say "nyah nyah told you so".

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 ... at that stage the IAU might need to come up with a new definition of what forms a planet, or accept that we're living on a dwarf planet. At least they've got a while to think about it ...