Another 28 exoplanets found

By Stephen Withers
Tuesday, 29 May 2007 13:49

Science - Space

The California and Carnegie Planet Search team and the Anglo-Australian Planet Search team have jointly announced the discovery of another 28 planets outside the solar system, adding more than one-eighth to the previous tally.

The announcement was made at the meeting of the American Astronomical Society in Honolulu this week by University of California Berkeley (UCB) post-doctoral fellow Jason Wright and recent PhD graduate John Johnson.

According to Wright, the teams' ability to detect wobbles in stars caused by smaller or more distant planets has improved. "We're just now getting to the point where, if we were observing our own solar system from afar, we would be seeing Jupiter," he said.

The combined team has also found seven brown dwarfs (gaseous bodies that are too small to 'light up' as stars, yet far bigger than gas giants like Jupiter) and two objects on the borderline between gas giants and brown dwarfs.

In four cases, pairs of planets were found orbiting the same star, and another three solar systems probably contain a brown dwarf as well as a planet. According to Wright, at least 30 percent of all stars known to have planets have more than one.

Wright drew special attention to an explanet orbiting the star Gliese 436 discovered by the teams in 2004 and subsequently studied by Michael Gillon of Liege University. Gillon and his colleagues recently reported that the planet's mass is 22.4 time that of Earth, and that its radius and density are similar to Neptune's.

"From the density of two grams per cubic centimetre – twice that of water – it must be 50 percent rock and about 50 percent water, with perhaps small amounts of hydrogen and helium," said Geoffrey Marcy, professor of astronomy at UCB and co-leader of the California and Carnegie team. "So this planet has the interior structure of a hybrid super-Earth/Neptune, with a rocky core surrounded by a significant amount of water compressed into solid form at high pressures and temperatures."

According to Wright, the planet has an eccentric orbit close to the star, making it a "hot Neptune" and suggesting there may be another planet orbiting further out.