Free-floaters: Images of planets?

Recent reports have sparked a vigorous debate about just what constitutes a planet.

The controversy escalated last summer when astronomers began unveiling images of star-forming regions showing a multitude of low-mass objects. Based on estimates of their mass alone, the bodies would seem to qualify as planets (SN: 10/7/00, p. 228). Yet, they roam freely and don't orbit a parent star.

Last month, British astronomers led by Philip W. Lucas of the University of Hertfordshire in Hatfield and Patrick F. Roche of the University of Oxford announced that infrared spectra of the roughly 20 roaming bodies they found in the Orion nebula contain signs of water vapor. This indicated that these objects are extremely lightweight, a hallmark of planethood. The team reports its findings in an upcoming MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY.

So, are they planets?

"Of course not," says theorist Alan R Boss of the Carnegie Institution of Washington (D.C.). He asserts that the objects are just lightweight bodies that formed as stars do, from the collapse of a cloud of cold gas. In contrast, a bona fide planet would orbit a star and have formed from the disk of gas and dust that swaddled that star when it was young, he says.

In theory, the roaming bodies and more than 100 other recently discovered "free-floaters," as researchers are calling them, could have orbited stars until sibling planets pushed them out. Boss argues that such an explanation can't account for the multitude of these nomads.

A decade ago, he and other astronomers calculated that if an object did indeed form from the collapse of a cloud of cold gas, it must be at least as heavy as three Jupiters and probably quite a bit heavier. That would imply that the least massive of the free-floaters could not have formed as stars do.

Boss has since done new calculations that incorporate the role of magnetic fields in the gas clouds. As it turns out, the tension in the magnetic fields causes the clouds to fragment into smaller pieces than Boss had realized. He now says that gas clouds can condense into bodies as lightweight as Saturn, or one-third the mass of Jupiter. He describes his work in the April 20 ASTROPHYSICAL JOURNAL LETTERS.

In deference to the ambiguous origins of free-floaters, Lucas and Roche propose calling them "planetars." Boss says he prefers the term "sub-brown dwarfs" because they appear to be light weight versions of brown dwarfs, which are failed stars.

Although brown dwarfs form as stars do, they aren't as massive and can't sustain nuclear fusion at their cores. They have just enough heft--13 to 80 times Jupiter's mass--to briefly shine by burning deuterium.

Conventional wisdom has it that no object more massive than 13 Jupiters is a planet. But conventional wisdom could be wrong, notes Adam S. Burrows of the University of Arizona in Tucson. He says that material within the disk ringing a newborn star may gather into planets that weigh more than 13 Jupiters.

"Thirteen Jupiter masses is not necessarily the boundary between a planet and a brown dwarf," Burrows cautions. Ultimately, he says, astronomers may be able to use such properties as rotation, the abundance of heavy elements, and orbital motion to distinguish a planet from an object that formed in a starlike manner. For now, however, "we don't know in detail how stars [and planets] form," says Burrows.