Planet hunters find new worlds

Artist's view of a planet around 79 Ceti (Greg Bacon)

By BBC News Online science editor Dr David Whitehouse

Planet hunters have crossed an important threshold in their search for other Earth-like worlds in space.

They have detected two planets circling distant stars that may be smaller than Saturn.

Of the 30 or so extrasolar planets so far discovered, all are larger than Jupiter, the largest world in our Solar System.

From their understanding of how planets form, astronomers expect that there will be many more smaller planets than larger ones. Until now, however, their techniques have not been sensitive enough to detect anything that has the mass of Jupiter and below.

"It's like looking at a beach from a distance," says Geoff Marcy of the University of California. "Previously we only saw the large boulders, which were Jupiter-sized planets or larger. Now we are seeing the rocks."

Largest telescope

As yet, though, astronomers do not have the ability to detect Earth-sized planets around other stars.

Using the world's largest telescope at the Keck Observatory in Hawaii, Geoff Marcy and Paul Butler of the Carnigie Institute, and Steve Vogt of the University of California, found a world less massive than Saturn orbiting the star HD46375, which is 109 light years from Earth.

Another sub-Saturn planet was discovered circling the star 79 Ceti, which is 117 light years away.

These planets are in very close orbits around their parent stars and have short orbits of three and 75 days.

To detect the planets, the astronomers looked for a wobble in their parent star's motion, detectable as a shift in the star's spectra. A tell-tale oscillation in the frequency of certain bright lines in the star's spectra reveals the presence of an unseen companion.

Life conditions

The Saturn-mass planets are probably gas-giant worlds. Being close to their stars, they will be hot and it is therefore very unlikely they have the conditions in which life could develop.

However, the researchers point out that some of the giant planets discovered could have Earth-sized moons or that Earth-sized worlds could also be present elsewhere in these planetary systems.

It is believed that the two new Saturn-sized worlds formed much further out from their stars and then migrated to closer orbits.

As they did this, it is possible that they could have scattered the orbits of terrestrial-sized worlds. In all the planetary systems so far discovered, this seems to be the rule rather than the exception.

This announcement is a logical step following the discovery of planets larger than Jupiter circling other stars. It remains to be seen if the present technique of planet detection can be made more sensitive so that it can detect smaller worlds.

Marcy and Butler told a Nasa news conference that they had six to 10 other planets which they hoped to confirm in the next year. However, none of these suspected planets is thought to be as small as the Earth.