Pluto demoted

Being demoted is not a dignified experience. But being downshifted from being an actual planet to being an icy ball of rock spinning in space has got to be pretty embarassing. Poor Pluto. After many years of wrangling, the International Astronomical Union's (IAU) general assembly has officially decided that Pluto is not a planet but a 'dwarf planet'. It's much smaller than the rest of the planets, and is really far away - in fact, it is dwarfed by many moons in the solar system (including our own). But it's not these flaws that bar it from being a 'classical planet'. Pluto's mistake is in its orbit. The orbit of a 'classical planet' must have cleared the neighbourhood around its orbit. Pluto's orbit crosses that of massive Neptune, thus disqualifying it.

The general assembly was presented with a proposal to officially grant it the status of a planet, and to boost the planetary status of three other celestial bodies at the same time, but instead Pluto was cut down to size. Today's decision means that the ex-planet will have to be removed from an awful lot of textbooks. At time of writing, the Wikipedia entry has already been altered, but changing all them books wont be quite so easy...

More from BBC News here. And at the Nasa Worldbook page here.

(PICTURE: ADROCKER). ONE PLUTO THAT IS DEFINITELY NOT A PLANET.

Comments

[olivier blanchard]

I just know that's probably going to screw up my horoscope somehow.

[Elohite]

I have to admit I'm not that surprised, my totally unscientific gut feeling has always been that Pluto was a moon of Neptune that was knocked out of orbit by a collision with Charon (which my guess would have been a Kuiper belt object nudged into a lower orbit) and they now orbit around each other on a common focus. It's certainly more exciting than they just being formed from the solar nebula. It's a fringe theory but hey, the current theory for the origin of Luna is a collision between Earth and a Mars sized object so I feel a little justified in thinking the early Sol system was a wild and crazy place.

New Horizons will give us a lot better look at this little family than we're likely to get for a while. With any luck we'll be getting video feeds right into our brains by 2015! Ok maybe not.

[Steffen]

I am not quite sure that Neptune is the argument for Pluto not being a plant. Because then Neptune also would not be one, as it 'did not clean its neighbourhood'. It is more all those other Kuiper Belt Objects which circle around out there. This definition is marvelously fuzzy :-)

[Peter]

What bugs me is that they decided this after the discoverer Clyde Tombaugh is dead.

I have met Clyde and his friend/famous astronomer and biographer David Levy - both insisted it was a planet.

I think its in poor taste to demote the planet Clyde worked so hard to find especially after he has passed on.

[Jesse M.]

Steffen wrote:
I am not quite sure that Neptune is the argument for Pluto not being a plant. Because then Neptune also would not be one, as it 'did not clean its neighbourhood'.

Steffen, see the paper at http://xxx.lanl.gov/pdf/astro-ph/0608359 by Steven Soter of the American Museum of Natural History--"clearing the neighborhood" means that the planet's own mass is greater than the mass of everything else in the area of its orbit by some critical threshold (Soter chooses it to be a factor of 100), and Neptune is about 8600 times more massive than Pluto so this shouldn't be a problem. I don't know if any planet has cleared its orbit 100%, there are certainly some asteroids whose orbits cross the earth's.

[Steffen]

Jesse M., thanks a lot for the link, that paper slipped my through my daily arXiv-scanning. However, even though my naive assumption why Neptune is not the argument for Pluto not being a planet is not quite right, Neptune still isn't the culprit.

Soters writes: "Pluto crosses the orbit of Neptune, but its 3:2 mean motion resonance with the planet shields it from a collision."

That in turn means that Pluto and Neptune do not share an orbital zone (according to his definition 5).

The reason why Pluto is not a planet is that there are a lot of other bodies sharing an orbital zone with Pluto and then Pluto has not enough mass to fulfill the cleaning criterion.