For decades, one of the leading theories of how the Moon was born said that a rapidly spinning Earth flung a large chunk of itself into space. Today, that idea's been dropped -- but only for Earth. It's possible that other moons formed that way -- including one in the deep freeze of the outer solar system.
Orcus is a big chunk of rock and ice beyond the orbit of Neptune, the most distant planet. Orcus's orbit is similar to that of Pluto. The two orbits are synched in such a way that when one body is closest to the Sun, the other is farthest. Because of that, Orcus is sometimes called the anti-Pluto.
Orcus is probably about half as big as Pluto. Astronomers can't yet determine its shape, though. If it's round, then it'll be classified as a dwarf planet, just as Pluto is.
Astronomers do know that Orcus has a moon. Its size is uncertain, but it appears to be about one-third the diameter of Orcus. Known as Vanth, it orbits Orcus once every nine and a half days. And Orcus spins rapidly -- about once every 10 hours.
A recent study found that those numbers are consistent with the idea that Orcus was spinning so fast that it flung a hunk of its own body out into space, forming the moon. That doesn't mean the case is settled -- only that there's a good possibility that Vanth was born from Orcus itself.
We'll talk about some other ice worlds in the outer solar system tomorrow.
Script by Damond Benningfield, Copyright 2010
For more skywatching tips, astronomy news, and much more, read StarDate magazine.