A puny way to make planets

Brown dwarfs are failures in the star-making business, but they appear to be successes in the planet-making arena.

Astronomers have had evidence for several years that brown dwarfs—objects too heavy to be planets but too small to shine as stars do—are sometimes surrounded by swirling disks of gas and dust (SN: 2/5/05, p. 83: Available to subscribers at Puny Parent? Planets may form around tiny orbs). Similar disks that swaddle young stars are known to spawn planets.

Whether the material surrounding brown dwarfs could coalesce into planets had remained unclear. “We did not know, for example, if these disks are dense enough to allow the coagulation of dust grains or to allow any other [planet-making] process taking place,” notes Dániel Apai of the University of Arizona in Tucson.

Now, infrared observations reveal that five of six brown-dwarf disks examined by the Spitzer Space Telescope are in the early stages of planet making. The telescope detected microscopic clumps of dust grains and tiny crystals orbiting the brown dwarfs. Spectra taken by Spitzer also indicate that the dust grains have settled toward the middle layer of each disk, another hallmark of planet-forming disks.

Apai and his colleagues describe their discoveries in the Nov. 4 Science.

The findings, Apai says, “emphasize the possibility to have planetary systems around brown dwarfs, which are among our closest neighbors.” These failed stars may also be more common than bona fide stars, upping the chances of finding many more planets beyond our solar system.