zomg dirty dozen!
The executive committee of the International Astronomical Union has added Ceres, Charon, and Xena to the list of official planets. Opening the door to chaos and anarchy, I say! ("There are 53 objects that meet the panel's criteria and probably many more to be discovered.")
For those of you worried that Charon's new status means that Ganymede and Titan will become planets: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.
Moons are excluded from planetary status, using a criterion that depends on the relative mass of two bodies that are gravitationally tied. If one body is much smaller than the other, then it is considered a moon. Pluto and Charon are closer in mass, and so they are dubbed a double planet. The Earth's moon is round and much larger than Pluto, but it is so much smaller than Earth that it is considered a moon, not a planet.
For those of you worried that Charon's new status means that Ganymede and Titan will become planets: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.
Moons are excluded from planetary status, using a criterion that depends on the relative mass of two bodies that are gravitationally tied. If one body is much smaller than the other, then it is considered a moon. Pluto and Charon are closer in mass, and so they are dubbed a double planet. The Earth's moon is round and much larger than Pluto, but it is so much smaller than Earth that it is considered a moon, not a planet.