Two stars, two planetary embryos.

In the search for planets outside the solar system, astronomers have focused on stars that go it alone in the cosmos. Theorists have reasoned that stars that occur in pairs are less likely to be surrounded by disks of gas and dust, the embryonic material from which planets arise. The gravitational tug-of-war between the two stars could prevent disks from forming or from lasting long enough to make planets.

New observations reveal that, at least in one case, a close stellar partnership hasn't prevented the formation of disks. Indeed, the orbiting duo may form planets faster than single stars do.

Studies with the Very Large Array radio telescope near Socorro, N.M., confirm that what had seemed a single star in the L1551 molecular cloud--a star-birth region 450 light-years from Earth--is in fact two stars. The stars are just slightly farther apart than the distance from the sun to Pluto.

Despite their closeness, each star has its own disk of gas and dust, report Luis E Rodriguez of the National Autonomous University in Mexico City and his colleagues in the Sept. 24 Nature.

The proximity of the two stars does seem to have affected the size of their dust disks. Each has a radius similar to the distance between the sun and Saturn--about one-tenth the typical size of disks encircling single stars. "Large outer planets like Neptune and Pluto will not form in these disks because the disks are simply not big enough," says Rodriguez.

Because paired stars are believed to be far more common than single stars, "the finding that the stars in this run-of-the-mill binary [have the potential] to form Earthlike planets suggests that planet formation is a very robust process that may be going on around most stars in the universe," Rodriguez notes.

The interaction of the two stars may downsize each other's disks, but it also seems to have made the disks more massive, notes theorist Alan P. Boss of the Carnegie Institution of Washington (D.C.). The observations suggest that each disk contains about 5 percent as much mass as the sun, more than double the amount in disks surrounding young, single stars.

According to Boss, when a disk is massive enough and cold enough, it becomes unstable and breaks up into large clumps. Such clumps may represent the rapid, wholesale formation of giant planets (SN: 8/8/98, p. 88). That's in contrast to the standard picture, in which planets build up little by little as material in the disk slowly packs together.

Boss adds that if a giant planet forms near the edge of one of the disks, it might be pulled out of orbit by the gravity of the other star. Such a scenario could bolster support for the controversial claim that the puzzling object TMR-1C is a wayward planet ejected by a pair of stars that lie nearby (SN: 6/6/98, p. 357).