On the Scales

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The census of known planets beyond our own solar system is at more than 300 and climbing.

But some of the details of these systems are still fuzzy. The technique that astronomers have used to discover most of the planets is good at finding them, but it leaves some questions. It doesn't tell us how heavy the planets are, for example. It reveals a minimum mass, but the true number could be a lot bigger.

Consider a system known as HD 33636. Astronomers discovered a possible planet in the system several years ago, and found that it was at least nine times as massive as Jupiter, the giant of our own solar system.

A couple of years ago, though, a team led by Texas astronomer Fritz Benedict looked at the system with a different technique. The team used the tracking system aboard Hubble Space Telescope to plot tiny motions in the star's position on the sky. Those motions are caused by the gravitational pull of its companion.

When combined with observations from telescopes on the ground, the technique revealed that the companion is more than 140 times as massive as Jupiter. An object that heavy isn't a planet at all -- it's a star. So instead of a star with a big planet, HD 33636 is a binary star system.

The astronomers are using Hubble to study several other stars with suspected planets. Their work will fill in the details of some of the worlds beyond our own solar system.

Script by Damond Benningfield, Copyright 2009

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