Planet hunt strikes rock; hot kin of Earth orbits nearby star.

Moving closer to the goal of finding a planet just like home, astronomers this week announced their discovery of the closest known cousin to Earth--a solid world just 15 light-years beyond the solar system.

With its surface temperature hot enough to roast a chicken, the newfound orb can't sustain life as we know it. But given the recent advances in planet detection, the discovery bodes well for finding habitable planets, comments theorist Doug Lin of the University of California, Santa Cruz.

The new find, which is about 7.5 times as heavy as Earth, is the least massive of the 156 extrasolar planets detected to date and is the most likely to have a rocky composition. Geoff Marcy of the University of California, Berkeley and Paul Butler of the Carnegie Institution of Washington (D.C.) led the team that presented the finding at a National Science Foundation press briefing in Arlington, Va.

"This could be the first [known] rocky planet around any normal star other than the sun," says team member Jack Lissauer of NASA2s Ames Research Center in Mountain View, Calif.

Because the planet was detected indirectly, researchers can't be sure that it's rocky. However, its low mass precludes it from having packed on substantial amounts of gas around a solid core, which is the way Jupiter, Saturn, and similarly massive extrasolar planets seem to have formed. The planet whips around the M dwarf star Gliese 876 in just 1.9 days at one-fiftieth Earth's distance from the sun.

As with the discovery of nearly all other extrasolar planets, the Marcy-Buffer team inferred the orb's presence from the wobble it induces in the motion of its parent star. The planet joins two other previously known, much more massive planets that orbit Gliese 876 at greater distances (SN: 1/13/01, p. 22). All three were found with a sensitive spectrograph at the Keck Observatory atop Hawaii's Manna Kea.

The three-planet system around Gliese 876 resembles two other multiplanet systems found last year. In each, a relatively small inner planet--about 15 times Earth's mass--has much bigger, Jupiterlike siblings (SN: 9/4/04, p. 147). Those outer planets have smaller orbits than Jupiter does.

All three systems "look like sealed-down versions of the solar system," says Alan Boss of the Carnegie Institution. He suggests that the miniaturization came about because the planets migrated much closer to their parent stars than did the orbs in the solar system.

Migration is the key to understanding the existence of hot, Earthlike planets, asserts Lin. In a theory first detailed in Science News (SN: 3/26/05, p. 203), he maintains that such a planet may reside within the orbit of every hot, closely orbiting Jupiter.

In his scenario, a Jupiterlike planet coalesces from the outer part of a flattened disk of gas, dust, and ice that swaddles newborn stars. As the disk gradually spirals inward, it drags the Jupiterlike planet toward the star. During this forced march, the planet plows together debris ranging from pebbles to moonsize bodies that lie in its path. This consolidated material forms rocky, Earthlike planets.

Alternatively, notes Lissauer, small, closely orbiting planets might have begun as giant Jupiters and then had their gaseous envelopes stripped away during their starward migration. In either case, because 70 percent of the stars in the Milky Way are M dwarfs, detecting a hot Earth around an M dwarf "is good news for ultimately finding [bona fide] Earths," says Boss.