Rocky road: planet hunting gets closer to Earth.

Astronomers have discovered the three lightest planets known outside the solar system, moving researchers closer to the goal of finding extrasolar planets that resemble Earth. One of the new planets joins three others orbiting the same star, forming the first-known quadruple-planet system.

Each new orb weighs between 14 and 25 times the mass of Earth, or roughly the mass of Neptune. Although the compositions of the newfound bodies aren't known, their relatively low masses suggest that they could be the discovered extrasolar planets with solid surfaces. Nearly all of the other 135 or so planets detected beyond the solar system are about as heavy as Jupiter--about 300 times the mass of Earth--and are assumed to be mostly gas.

"Up until now, the technology has limited planet detection to those in the Jupiter--and Saturn-mass range," says R. Paul Butler of the Carnegie Institution of Washington (D.C.), a codiscoverer of two of the new planets. Finding lower-mass planets bodes well for finding Earthlike planets sooner rather than later, he adds. The trio of newfound planets is probably too hot to support life.

"We're in a new realm of planetary detection," comments theorist Alan P. Boss of Carnegie. The newfound planets represent either the big brothers of rocky, terrestrial planets or ice giants similar to Neptune, he says.

Too faint to be imaged, the planets revealed themselves by causing tiny wobbles in their parent stars. These wobbles reflect the gravity of a planet pulling its star to and fro.

Two of the findings were announced this week at a NASA press briefing in Washington, D.C., and will appear in the Astrophysical Journal Letters. A team led by Butler and Geoffrey W. Marcy of the University of California, Berkeley monitored the red dwarf star Gliese 436 at the Keck Observatory atop Hawaii's Mauna Kea. Red dwarfs are the most common type of star in the Milky Way.

Butler and Marcy report that Gliese 436 harbors a planet at least 21 times, but probably no more than 25 times, the mass of Earth. The planet races around Gliese 436 every 2.64 days.

Also at the briefing, Barbara E. McArthur of the University of Texas at Austin reported a planet around the sunlike star 55 Cancri. Other researchers previously detected three Jupiter-size planets orbiting the star. Using the 9.2-meter Hobberly-Eberly Telescope near Fort Davis, Texas, McArthur's team found an additional tiny wobble, presumably due to a small planet whipping around the star every 2.81 days. Hubble Space Telescope observations suggest the planet's mass is 18 times that of Earth.

In yet a third planetary revelation, Nuno C. Santos of the University of Lisbon in Portugal and his collaborators have found a planet with a minimum mass of 14 Earths. The team used a sensitive new spectrograph on the European Southern Observatory's 3.6-m telescope in La Silla, Chile, to monitor the nearby star mu Arae. Santos reported the findings last week at the EuroScienee Open Forum meeting in Stockholm.

Scientists propose that planets coalesce from disks of gas, dust, and ice that swaddle young stars. Particles of ice or rock condense to form a solid core, which then attracts vast amounts of gas, leading to Jupiter-size bodies. The small masses of the newfound planets; suggest they're mostly solid, however. They may be too small to have ever captured much gas, Butler speculates.

Planets that now reside close to their parent stars may have formed farther out in space and migrated in. It's unclear whether the newfound planets were once gas giants that got whittled down to a solid core during such a migration. The newly discovered planet orbiting mu Arae is the most likely to be rocky because it probably formed so close to its hot parent star that its raw ingredients couldn't have been primarily ice, Boss says.