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Extrasolar planets emerge from the dark.

Last month, two landmark discoveries drove home the message that some of the stars closest to our sun may harbor planets. One group of astronomers inferred the presence of a large companion to the sunlike star 51 Pegasi, which lies 42 light-years from Earth. A second team went beyond inference, directly imaging a faint object-either a heavyweight planet or a failed star-adjacent to an even closer neighbor, the low-mass star GL229 (SN: 10/21/95, p.260).

Now, follow-up analyses as well as new observations have begun to shed more light on both findings. Two research teams report hints that a second planet may orbit 51 Pegasi. Another team has used the Hubble Space Telescope to make a sharper image of the massive body circling GL229.

In the Nov. 23 Nature, Michel Mayor and Didier Queloz of Geneva Observatory in Sauverny, Switzerland, provide further details about their indirect detection of a planet orbiting 51 Pegasi. In tracking the star's velocity along the line of sight to Earth, the researchers detected a periodic wobble. They ascribe this back-and-forth motion to the tug of an unseen, Jupiter-mass companion. Whipping around the star every 4.2 days, the body ventures 100 times closer to 51 Pegasi than Jupiter does to the sun.

Mayor and Queloz suggest that the object is either a planet or, less likely, a brown dwarf that 51 Pegasi has stripped of 95 percent of its mass. Brown dwarfs form as stars do but lack the mass to sustain nuclear burning. In either case, astronomers have calculated that, despite its proximity to 51 Pegasi, the companion will not burn up.

The companion's origin has proven more difficult to explain. If the object, dubbed 51 Pegasi B, is a planet, researchers are hard-pressed to understand how it could have formed so close to its parent star. Doug N.C. Lin of the University of California, Santa Cruz proposes that 51 Pegasi B originated much farther from its parent. He suggests that it spiraled inward as it gradually gave up angular momentum to the disk of gas and dust from which it arose. In this scenario, other planets that haven't lost as much angular momentum should reside at greater distances from the star.

That prediction dovetails neatly with a second, far more tentative finding from the Swiss team. The researchers report evidence of a smaller wobble, which could signify a second, more distant planet with a lower mass and an orbital period greater than 18 months. Queloz cautions that he and Mayor haven't studied Pegasi 51 long enough to determine whether this long-term wobble repeats itself, as it must if a planet is the culprit. Last month, other astronomers announced that they had found a hint of a similar wobble, using a spectrograph at the Oak Ridge Observatory in Harvard, Mass.

The data "could mean that 51 Pegasi actually has a system of planets," notes Gordon A.H. Walker of the University of British Columbia in Vancouver. "Now that would be exciting indeed!"

Shrinivas Kulkarni of the California Institute of Technology in Pasadena had a similar planet system in mind when he examined images of GL229 and its surroundings taken on Nov. 17 by Hubble. Kulkarni and his colleagues had previously used a ground-based telescope to image in the near-infrared an object 20 to 50 times as massive as Jupiter. This body lies farther from its star than Pluto does from the sun.

Hubble didn't find any bodies closer to GL229, but the telescope did take several high-resolution images of the recently discovered object, including the first in visible light, he told Science News. Hubble observations planned for 6 months from now should reveal whether or not the faint body, believed to be a cool brown dwarf, truly orbits GL229, Kulkarni says.
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Title Annotation:Science News of the Week; new information about recently discovered planets circling 51 Pegasi and GL229
Author:Cowen, Ron
Publication:Science News
Article Type:Brief Article
Date:Nov 25, 1995
Words:630
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