Star motions yield four more planets.

The hunt for planets outside our solar system continues to show results. The latest findings include a nearby, sunlike star that may have two companions: a planet and a heavier object, known as a brown dwarf. Studies also suggest that three other nearby stars have closely orbiting planets, bringing to 16 the number of extrasolar planets that astronomers have indirectly detected around sunlike stars. As recently as August, only 10 such planets had been identified (SN: 8/8/98, p. 88).

The main search strategy has stayed the same since the first extrasolar planet was discovered in 1992. By tracking the back-and-forth motion of nearby stars toward and away from Earth, astronomers infer the gravitational tug of planets too faint to be detected directly. This technique favors the detection of massive, closely orbiting planets, since these bodies induce the largest wobbles in their parent stars.

Researchers have identified two planets among a sample of 82 stars they had begun monitoring recently at Lick Observatory on Mt. Hamilton in California. The team had already been studying 107 stars at Lick for several years. One of the newly found planets, which orbits the sunlike star HD195019, is at least 3.51 times as massive as Jupiter and whips around the star in just 18.27 days.

The other planet found at Lick Observatory circles the sunlike star HD217107 once every 7.12 days and is at least 1.27 times as massive as Jupiter. Geoffrey W. Marcy of San Francisco State University and the University of California, Berkeley and his colleagues, including R. Paul Butler of the Anglo-Australian Observatory in Epping, Australia, will report both Lick findings in the January 1999 Publications of the Astronomical Society of the Pacific.

A third discovery, which Marcy announced Dec. 2, during a talk at Marymount College in Palos Verdes, Calif., concerns a star whose motion was tracked at the W.M. Keck Observatory atop Hawaii's Mauna Kea. The wobble of the star HD168443 suggests that it has a planet, which is at least 4.96 times as massive as Jupiter, in a highly elongated orbit.

The tug of a single object can't fully explain the star's motion, however. Marcy proposes that the star has another companion--either a tiny star or a brown dwarf, an object heavier than a planet but too lightweight to shine continuously as stars do.

The fourth find comes from a Swiss team working at the European Southern Observatory's La Silla Observatory in La Serena, Chile. Using a new telescope and spectrograph devoted to tracking stellar wobbles, the team found evidence of a planet circling Gliese 86, a dwarf star with a mass 0.79 times that of the sun. About 35 light-years from Earth, this is the second-closest star known to harbor a planet.

Gliese 86 has another distinction: It possesses an unseen stellar partner. The separation between the two stars is probably more than 100 times larger than the distance between the newly discovered planet and the star it orbits, the Swiss team reports. The planet circles the star once every 15.83 days and is at least 4.9 times as massive as Jupiter. It is separated from its parent by just over one-tenth the distance between the sun and Earth. Didier Queloz of the Geneva Observatory and NASA's Jet Propulsion Laboratory in Pasadena, Calif., and his colleagues announced the finding on Nov. 24.