Our solar system is just packed with planets. We have eight of them -- from Mercury, which is close to the Sun, to Neptune, which is about three billion miles away. Within that range, the planets are packed just about as tight as they can be.
The same thing seems to apply to many other planetary systems as well -- the planets are packed as close together as nature will allow. And that presents a tool for finding new worlds: If you find a gap in the sequence of planets, and it's possible for a planet to inhabit the gap, then there's a good chance that one is there.
In fact, last year a team of astronomers used just this technique to find a planet in a star system known as HD 74156.
Earlier observations had revealed two planets in the system. But there was a big gap between them. From the orbits and masses of the known planets, astronomers predicted that another world would inhabit the gap. They predicted a specific orbit for the planet, and a specific mass.
University of Texas astronomers searched for the planet, and they found it -- just where it was supposed to be. What's more, the mass was on target, too -- a little heavier than Saturn. It was the first discovery of a planet based on predictions of its position in a century and a half -- since the discovery of Neptune.
The same astronomers are applying the technique to other planetary systems, trying to find worlds that should be packed in tight.
More about extrasolar planets tomorrow.
Script by Damond Benningfield, Copyright 2008
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