I am in mourning for the loss of our ninth planet, which is no longer a planet, Pluto. Astronomers have decided it isn't "a celestial body that is (a) 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" it's not a planet anymore.
it's a "dwarf planet." asteroids in the asteroid belt orbit the sun, but they're not planets either. there are a few other dwarf planets as well, such as Xena and Ceres.
I thought Charon and Pluto would be known as 'twin planets'? I guess they still aren't /real/ planets. Charon and Pluto are similar to Earth and the moon, because they technically orbit each other (unlike Jupiter which couldn't possibly orbit its moons), however Pluto and Charon's centre of mass isn't inside Pluto or Charon, so they aren't planets. Whereas the Earth and Moon's centre of mass is somewhere inside the Earth.
Astronomers have decided it isn't "a celestial body that is (a) 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" it's not a planet anymore.
