Not a planet?

Two years ago, European astronomers found what they hailed as the first image of an extrasolar planet. New observations of the object add to evidence that it’s not a planet after all.

The body, about eight times as heavy as Jupiter, resides next to a failed star, a brown dwarf dubbed 2M1207 (SN: 5/7/05, p. 291: Available to subscribers at Planetary Picture? Criteria for planethood cloud object’s identity). The brown dwarf is only about three times as heavy as its companion.

The European group suggested that the planetary-mass object coalesced from the outer reaches of a gas-and-dust disk surrounding the brown dwarf. But other astronomers argued that a massive planet probably couldn’t have formed in the sparse outer part of the disk.

At the June meeting of the American Astronomical Society in Calgary, Alberta, Ray Jayawardhana of the University of Toronto and his colleagues reported new observations of the companion to the brown dwarf. Their study indicates that the smaller object also has its own gas-and-dust disk.

This makes it likely that the body and the brown dwarf formed together, just as a pair of stars would, from the collapse of a single gas cloud, Jayawardhana says. If a planetary-mass object formed as a star does, rather than coalescing within a disk, then the object shouldn’t be called a planet, according to the International Astronomical Union.

“The discovery of a disk around the planetary-mass companion to 2M1207 should be a bit of a relief to planet-formation theorists” who doubted a planet could arise so far from a brown dwarf, says Alycia Weinberger of the Carnegie Institution of Washington (D.C.).

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