by R. Cowen
Astronomers this week reported that one of the suns nearest neighborsa star just 15 light-years from Earthpossesses a planet at least 1.6 times as massive as Jupiter.
The unseen planet betrayed its presence through the characteristic wobble of its parent, a star called Gliese 876. Of all the stars thought to have planets, Gliese 876 is the closest to Earth. Weighing in at just one-third the mass of the sun, it is also the lightest of these stars.
In hunting planets, researchers had previously focused on stars similar in mass to the sun. Finding a planet around one of the first low-mass stars to be examined offers a hint that planetary systems "may be a common occurrence among stars that are quite different from the sun," says Geoffrey W. Marcy of San Francisco State University and the University of California, Berkeley. He presented the findings at a symposium of the International Astronomical Union in Victoria, British Columbia.
The newly detected planet takes 61 days to go around the star, and its average distance from its parent is one-fifth the separation between the sun and Earth. Thats closer to the star than Mercury, the innermost planet of the solar system, is to the sun. Marcy says that this discovery brings to 12 the number of planets revealed by the wobble in a stars motion (SN: 5/17/97, p. 305).
Theorist Didier Saumon of Vanderbilt University in Nashville, Tenn., told Science News that preliminary calculations indicate the surface of the planetpresumed to be gaseoushas a temperature around -75°C. Thats far below the freezing point of water. The calculations also suggest, however, that in warmer layers not far below the surface, water could exist as liquid droplets.
Although liquid water is thought to be a key ingredient for the development of life, "we shouldnt go into a feeding frenzy about this," cautions Marcy. He notes that within a gaseous planet, water cannot puddle and form an environment that can readily support organisms. Saumon adds, however, that if the planet has solid moons, they might lie within a temperate zone and could offer a foothold for living material. Jupiters moon Europa, for example, is suspected of harboring a vast ocean and is a likely place to look for life (SN: 11/1/97, p. 284).
Marcy and his colleagues, R. Paul Butler of the Anglo-Australian Observatory in Epping, Steven S. Vogt of the University of California, Santa Cruz, and Debra Fischer of San Francisco State, began studying Gliese 876 in 1994 at Lick Observatory on Mt. Hamilton in California. The faintness of this low-mass star, which has only about one-fortieth the intrinsic luminosity of the sun, made it difficult to analyze. Last year, the researchers began using one of the worlds largest visible-light detectors, the Keck I Telescope atop Hawaiis Mauna Kea, to study 400 nearby stars, including Gliese 876.
Observations with a high-resolution spectrometer attached to Keck I, including data recorded on June 18 and 19, revealed the presence of a planet. "It was the data from Keck that really nailed this planetary companion down," says Marcy.
Two hours after Marcy announced the results at the Victoria meeting on June 22, a colleague presented him with a startling e-mail. Another team, led by Xavier Delfosse of Geneva Observatory in Switzerland and Grenoble University in France, wrote that it had independently found evidence of the same planet. The researchers analyzed light from Gliese 876 using spectrometers at the Haute-Provence Observatory in France and the European Southern Observatory in La Serena, Chile.
"Its very convincing that they have confirmed [the finding]," says Marcy.
Marcy notes that although gravitational interactions between a star and a nearby planet tend to make orbits circular, the new planet follows a path more elongated than that of Pluto. He told Science News that another star under study by his team appears to have a planet in an equally close but even more elongated orbit. "All of the planets in our solar system have more nearly circular orbits than these two new planets. They raise questions about how common the architecture of our solar system is," he says.
One notion is that an elliptical orbit arises from the gravitational interaction of two giant planets that initially lie near each other but far from their parent star. After a close encounter between the planets, one ejects the other and then heads inward, in a smaller elliptical orbit (SN: 11/23/96, p. 328).
In 2 weeks, a team led by astronomers in Geneva plans to announce the discovery of two planets orbiting stars farther from Earth than Gliese 876.
From Science News, Vol. 153, No. 26, June 27, 1998, p. 405.
Copyright Ó 1998 by Science Service.
Marcy, G.W. 1998. Precise stellar radial velocities. Meeting of the International Astronomical Union. June. Victoria, British Columbia.
1996. One way into the hot seat. Science News 150(Nov. 23):328.
Cowen, R. 1997. Cest la vie. Science News 152(Nov. 1):284.
______. 1997. More planets. Science News 151(May 17):305.
Geoffrey W. Marcy
University of California, Berkeley
Department of Astronomy
Berkeley, CA 94720
Department of Physics and Astronomy
Nashville, TN 37235
copyright 1998 ScienceService