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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