Puny Parent? Planets may form around tiny orbs

Barely more massive than a planet itself, a failed star 500 light-years from Earth is nevertheless cloaked in a disk of gas and dust from which planets could coalesce. The finding highlights the possibility that planet formation may be even more common than researchers had previously suggested.

PLANET POTENTIAL. Artist’s depiction of a newly discovered protoplanetary disk around a faint, low-mass brown dwarf. Evidence for the disk came from the Spitzer Space Telescope. Luhman, et al./Harvard-Smithsonian, JPL/NASA

An infrared camera on the orbiting Spitzer Space Telescope took just 20 seconds to find evidence of the disk around the failed star, OTS 44, also known as a brown dwarf. Too small to sustain the burning of nuclear fuel at their cores, as bona fide stars do, brown dwarfs range in mass from about 13 to 80 times the mass of Jupiter.

The Spitzer Space Telescope lacks the resolution to directly image a disk around a brown dwarf, but the pattern of excess infrared radiation detected around OTS 44 reveals the existence of a dusty disk, says study collaborator Lee Hartmann of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

Although astronomers had previously spied protoplanetary disks around brown dwarfs, the latest find is the smallest brown dwarf shown to have a disk. OTS 44 is just 15 times Jupiter’s mass. With a temperature of 2,300 kelvins, this brown dwarf is also the coolest failed star known to have a disk, says Hartmann.

He and his colleagues, who include Kevin Luhman of Harvard-Smithsonian, describe their findings in the Feb. 10 Astrophysical Journal Letters. They’re also scheduled to present their results Feb. 7 at a meeting in Aspen, Colo., on planet formation and detection.

Finding signs of a protoplanetary disk around such a low-mass brown dwarf “may open up a new horizon” in the search for planets beyond the solar system, says Hartmann.

Indeed, it may prove easier to image planets orbiting brown dwarfs than to find planets orbiting stars, comments theorist Adam Burrows of the University of Arizona in Tucson. At infrared wavelengths, the faint glow from a planet may be swamped by the brilliant light of a star but not by the fainter glare from a brown dwarf.

As they have found more disks around brown dwarfs and low-mass stars, astronomers have become eager to learn how the composition, duration, and mass of the disks vary with the mass of the parent object. Determining these relationships could, in turn, shed new light on how planets can form from any disk, notes Hartmann.

Scientists don’t know the mass of the disk surrounding OTS 44, and it will be difficult to measure. Hartmann says that there might not be enough material in the disk to form a planet the size of Jupiter but there could be enough to form a body the mass of Earth, which is just one three-hundredth as heavy as Jupiter.

Because of the brown dwarf’s low temperature, to stay as warm as Earth and have water remain liquid, any Earthsize world that did form would have to orbit much closer to the dwarf than our planet does to the sun, Hartmann and his colleagues note.

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