Shining solo.
A planet orbits a single red dwarf star in an artist's conception.

Stellar Stag Party

Of the stars apparent to the naked eye, more than half are actually two or more stars clumped in tight orbit around each other. Many astronomers have assumed that most stars in the galaxy have a stellar companion. Not so, according to a new census. The realization that single stars, such as our sun, are the rule and not the exception may mean that planets are more common than thought.

The stars most visible in the night sky are often bright, massive ones, of which 80% have companions. But recent studies of less massive stars, less than 1% as bright as the sun, have found they are not nearly as social as their bigger counterparts. In fact, three-quarters of red dwarfs--less than half the sun's mass--are alone.

Astronomer Charles Lada of Harvard Smithsonian Center for Astrophysics decided to combine star surveys to work out the overall fraction of stars with and without partners. In a paper submitted to Astrophysical Journal Letters, he reports that upwards of two-thirds of all stellar systems in the Milky Way are loners. The balance tips towards stellar solitude because red dwarfs are by far the most common type of star in our galaxy. Lada argues that this situation rules out theories that assume all stars form in multiple systems, from which single stars later escape. He thinks it more likely that big and small stars form in different kinds of environments: the former in massive clouds of turbulent gas and dust, which are prone to fragment into multiple stars, and the latter in smaller, more quiescent clouds that stay in one piece and form just one star.

Single stars are often touted as being more conducive for planets because there is no extra star to disrupt the planet-forming disk, Lada says. His new tally could mean that more of the galaxy is planet friendly.

Or maybe not: "I'm skeptical," says planet hunter Michel Mayor of the Observatory of Geneva in Switzerland. Mayor thinks that small stars--even if single--may have a hard time building up planets. Only four red dwarfs--out of 200 to 300 studied--have been found to harbor planets, he says. A fifth may have been detected last week (ScienceNOW, 25 January). Mayor plans to focus his team's research on red dwarfs, because more data on them are necessary to compare planet formation around big versus small stars.

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