Alpha Centauri

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So far, we don't know any Earth-like planets outside our own solar system -- small, rocky worlds in cozy orbits around their parent stars. But a good place to look for neighboring worlds is the closest neighboring star system: Alpha Centauri, in the southern constellation Centaurus, the centaur.

The system is a little more than four light-years away, and it consists of three stars. One of them is small and faint, and a long way from the other two; we'll have more about Proxima Centauri tomorrow.

But the other two stars are more intriguing, because both of them are similar to the Sun. One of the stars is a little bit bigger, heavier, and brighter than the Sun, while the other is a little toward the other end of the scale.

On average, the two stars are a couple of billion miles apart. That's a long way by human standards, but practically nothing on the astronomical distance scale. So as they orbit each other, their gravity stirs things up, so there's no way to have planets that are far out from the stars.

But it is possible to have small, rocky planets in close orbits -- planets like Earth. Each star could have several of these planets. And it's even possible that planets could orbit both stars.

No one's discovered any planets in the Alpha Centauri system so far. But detecting Earth-like worlds in such a system is tough. So astronomers are continuing the hunt for neighboring worlds around these neighboring stars.

Script by Damond Benningfield, Copyright 2008

For more skywatching tips, astronomy news, and much more, read StarDate magazine.