New brown dwarfs show link to planets.

Planets and brown dwarfs wouldn't seem to have much in common. The former are thought to form from the flattened disks of gas and dust that surround newborn stars. The latter are thought to arise as stars do, by the agglomeration of gas in part of an interstellar cloud. Moreover, brown dwarfs, also called failed stars because they don't have enough mass to shine brightly, weigh much more than planets.

Since last fall, astronomers have reported indirect evidence of six planets orbiting nearby, sunlike stars (SN: 7/6/96, p. 11). The biggest of them has a mass about eight times that of Jupiter. The smallest brown dwarf, on the other hand, has about 80 times Jupiter's mass. Planet hunter Geoffrey W. Marcy of San Francisco State University has called the apparent gap in mass a desert that neither planets nor brown dwarfs populate.

In early July, however, Michel Mayor of Geneva Observatory and his colleagues reported at a bioastronomy meeting in Capri, Italy, that they had discovered or confirmed the existence of six brown dwarfs orbiting nearby stars and having masses between 16 and 40 times Jupiter's. The discovery of brown dwarfs with masses much closer to those of planets indicates that a desert does not exist, says Mayor. All six dwarfs have elliptical orbits, but five of six recently described planets have circular paths. He suggests that ellipticity may distinguish a brown dwarf from a planet.

Although they haven't seen the data, Marcy and his collaborator, R. Paul Butler of San Francisco State and the University of California, Berkeley, say they have no reason to doubt Mayor's findings. Mayor's team has detected 8 brown dwarfs among a sample of 570 stars and 1 planet among a sample of 142 stars. In contrast, Marcy and Butler have so far found 6 planets among a sample of 120 stars but not a single brown dwarf. Given that brown dwarfs are easier to detect because they exert a larger tug on their parent stars, Marcy and Butler say the statistics indicate that planets are more common than brown dwarfs around nearby stars.