This is an amazing story -- it's being reported everywhere, but there's a particularly good piece at The Boston Globe. According to the International Astronomical Union, there are now 12 planets in the solar system, three more than familiar nine.Pluto, once a candidate for demotion, is still a planet. The three new planets are:
- Ceres, once called a planet but now considered a large asteroid located in the dense asteroid belt between Mars and Jupiter;
- Charon, Pluto's "moon"; and
- Xena, a body located in the Kuiper Belt past Pluto that has yet to be officially named.
If accepted by the IAU's general assembly on Aug. 24, there will be a new, formal definition of what a planet is: "The proposal defines a planet as an object that circles the sun and is massive enough that its own gravitational forces compress it into a roughly spherical shape. Depending on its composition, a planet would have to be at least roughly 250 to 500 miles in diameter to qualify. It designates a new subcategory of planet, the 'pluton,' a Pluto-like planet that takes at least 200 years to circle the sun. Pluto, Charon, and Xena are all plutons, and scientists expect many more to be discovered. Under the proposal, Ceres is an ordinary planet."
Possible additional planets are Vesta, Pallas, and Hygiea in the asteroid belt and Sedna, Orcus, and Quaoar in the Kuiper Belt. There are known to be "53 objects that meet the [new] criteria" -- and there are likely many, many more.