One star better than two?

The raw material for planets comes from disks of gas, dust, and ice particles that form around young stars. According to one leading theory, planets arise when diffuse, cold material within a disk collides and sticks together, gradually building boulder- and house-size objects into larger and larger bodies that slowly become planets.

In a competing theory, known as the gravitational-instability model, a sudden fragmentation of the disk can trigger the wholesale formation of giant planets, such as Jupiter, without any gradual buildup.

In both cases, theorists have assumed that planet formation is most likely to succeed in stars that don't have partners. The gravity of a nearby star might disrupt a disk, halting the planet-forming process. However, 30 of the 161 planets currently known beyond the solar system are found orbiting stars that have at least one partner.

Alan P. Boss of the Carnegie Institution of Washington (D.C.) now contends that when it comes to planet formation, two stars are at least as good as one and, in some cases, even better.

Boss' computer simulations of planet making indicate that if the gravity of a companion star only weakly disturbs the disk of its partner, planet formation should continue just as it does in a single star. In some cases, the disturbance could even trigger the fragmentation process required in the gravitational-instability model, hastening the formation of planets.

That's good news for planet hunters, he notes, because about two-thirds of the stars in our galaxy have companions. This result "increases the likelihood of the formation of planetary systems resembling our own, because binary stars are the rule in our galaxy, not the exception," says Boss.--R.C.