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Dwarf Planet to Plutoid. Promotion or Demotion?

written by: S.L. Bradish•edited by: RC Davison•updated: 8/21/2009

Pluto, once considered a planet, then a dwarf planet has now been re-classified to a Plutoid. Is this a promotion or a demotion?

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    Pluto Reclassified

    Poor little Pluto, swinging through space in its orbit around Sol, just past Neptune. Since its discovery in 1930, Pluto has been considered a planet, just like the other eight planets in our solar system. It was distant but equal in the eyes of both astronomers and school children. It has moons which rotate in their own orbits. Its satellite (or moon) is called Charon and was discovered in 1978 by Jim Christy. Then, in 2005, two more moons were found thanks to the Hubble Telescope. Nix and Hydra, they were named, and they’re very small. Their diameters are between 60 and 200 Kilometers.

    Pluto’s orbit seems to cross that of Neptune, but it really doesn’t, although it has been closer to the sun than Neptune. Pluto’s rotation is backwards to that of most of the other planets (If on the planet you would see the sun rising in the west and setting in the east.) and it rotates synchronously with Charon - know as being "tidally locked". They are always facing each other!

    At one time, it was believed that Pluto was larger than it is, thanks to the images of Charon appearing so close through most telescopes. But the mighty Hubble has opened new windows into the galaxy and given us a new perspective on the universe as well as our own back yard, our solar system. Of course, nothing beats the “up close and personal" approach and the launch of “New Horizons" in 2006 will get us a very good look at Pluto in 2015.

    Pluto appears to be made of 70% rock and roughly 30% ice, molded onto a spherical shape by its own gravity. It was probably just passing through when Neptune's gravity pulled it into the orbit about the sun it now has. Charon, Nix and Hydra more than likely have the same reasons for being where they are, too. Prisoners of Pluto’s gravity they orbit, keeping their distance but never straying further.

    There has been a great deal of speculation as to what the Plutonian atmosphere is like. During most of its lengthy year, Pluto is probably covered with frozen methane, ethane, nitrogen and carbon monoxide. However during its “perihelion" (closest approach to the sun) the solidly frozen gasses may well melt and produce something like an atmosphere. It wouldn’t be breathable for humans, needless to say! The color bands that appear in telescopic photographs and computer imaging seem to show areas of differing composition in the atmosphere, but the New Horizons will be able to tell us more when it arrives. With any luck, the New Horizons will be able to see Pluto during the perihelion and learn more about the gaseous make-up of the little world.

    Planet or What?

    What is round, orbits Sol (our sun), and has its own satellites (moons)? A planet, of course! That was what everyone thought for all these years. But now the IAU scientists in Oslo, Norway have decided that Pluto isn’t a planet like the others. It doesn’t quite qualify under the strictest definition of “planet".

    After spending all of its known life as a planet, one among nine, as it were, Pluto was designated a "Dwarf Planet" by the IAU in 2006. In June, 2008 the IAU has changed its title again to “Plutoid" (Sort of like going from being the smallest guy on one team to being the captain of your own team!) And it has one other member, so far. Eris has been classified as a Plutoid, too! Now there are two Plutoids, and astronomers are scanning the solar system and beyond to find out if there are any more. Pluto could well wind up being the first in a long line of Plutoids!

    There was some early speculation that Ceres might be a Plutoid, but the scientists of IAU have decided that it doesn’t quite qualify. Ceres is a dwarf planet in the asteroid belt between Jupiter and Mars, and as far as they can tell, there are no others like it.

    Pluto, the first Plutoid has been promoted, not demoted! Instead of being just one among many, it is now officially the first of its kind. A true dwarf planet with its own separate designation and its own position in science. When New Horizons arrives, we will learn so much more about Pluto and what it means to be a true Plutoid!

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    Pluto and moons Charon, Nix and Hydra

    Pluto and its moons





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