Planet-forming process playing out in miniature?

Astronomers have found what they say may be a miniature solar system in the making.

In the latest example of the apparent fecundity of nature, a tiny starlike object too small to be a star seems to be surrounded by a tiny disk of dust that could someday form planets, which could perhaps even be briefly capable of supporting life, according to observations.

The discovery raises the possibility that astronomers will soon discover actual planets and habitable abodes around objects that are barely larger than planets themselves. The starlike body, only about 15 times as massive as the planet Jupiter, is the smallest object yet to be found with such a disk. With a temperature of 3,600 degrees Fahrenheit, it is also the coolest. As such, the object is not really a star at all, but a failed star known as a brown dwarf that is only barely bigger than the largest giant planets.

"It's just incredible," said Kevin Luhman of the Harvard-Smithsonian Center for Astrophysics. "A little disk that could form planets around an object that is small enough itself to be a planet." Luhman led an international team of astronomers that observed the brown dwarf, known as OTS 44, using the Spitzer Space Telescope. He will be discussing his results this week at a meeting on extraterrestrial planets at the Aspen Center for Physics in Colorado. A paper describing the results will be published in The Astrophysical Journal Letters.

Geoffrey W. Marcy of the University of California, Berkeley, said the report opened a new possible abode for life, on Earth-like planets orbiting brown dwarfs. "While these earths would be warmed up for only a few million years," he said in an e-mail message, "it is interesting to imagine what biochemistry might flicker valiantly during that brief period when the brown dwarf is luminous enough to warm up the earth, only to freeze over before Darwinian evolution can kick in."

The work continues a recent trend in which astronomers using bigger and bigger telescopes have shifted their attention somewhat, from the biggest and brightest objects in the cosmos--supernova explosions, quasars and giant galaxies--to the smallest and dimmest. Little dim things far outnumber their more striking and violent cousins, and it is among these humbler denizens of the cosmos that astronomers must look for life and life-friendly conditions.

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The Spitzer Space Telescope, launched into orbit around the Sun in 2003, is the fourth and last of NASA's so-called Great Observatories. It is designed to see the infrared, or heat radiation emitted by celestial objects.

Cool dim objects like small stars, brown dwarfs and planets emit electromagnetic radiation most strongly in infrared wavelengths, which are longer than those of visible light, and which can pass through interstellar dust clouds that often shroud new stars, but Earth's atmosphere blocks infrared light. And so prospective infrared astronomers have had to go to space, starting with the Infrared Astronomical Satellite, IRAS, in 1983, which discovered disks of dust around several stars, including Vega.

Since then, disks have been detected or have been inferred to exist around thousands of stars, including a few brown dwarfs. In December, astronomers using Spitzer announced that they had detected the

signatures of dusty disks around half a dozen stars already known to harbor giant planets, thereby closing the loop between disks and planets.

By searching out the smallest dwarfs that have such disks, Luhman and his team hoped to learn more about how brown dwarfs formed and determine whether planets could exist around such objects.

The brown dwarf, OTS 44, is part of a cluster of very young stars about 500 light-years away in the constellation Chamaeleon. When the astronomers looked at it with the Spitzer, they found that the brown dwarf was putting out excess heat at certain wavelengths, a signature of cooler material surrounding the brown dwarf.

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The extra heat, they calculated, was coming from a disk of dust and perhaps larger materials, extending 300,000 to 900 million miles from the dwarf star, about a tenth the size of the disk that formed our own solar system. They estimated that there was enough material in the little disk to make a planet like Jupiter or several smaller Earth-like planets.

The disk around the brown dwarf is unlikely to have any planets, Dr. Luhman said; based on the ages of the stars in the cluster, the brown dwarf and its disk are only about a million years old. It could take 10 million years for the small bits and chunks of rock and dust to coagulate into planets.

But he added, "There could be little embryos we can't see."

A planet that orbited close enough to the brown dwarf could even be briefly habitable, meaning that liquid water could exist on it, Luhman said. This "habitable zone," as astronomers call it, would extend from 1 million to 4 million miles from the brown dwarf, well within the confines of the disk, according to theory.

But as the brown dwarf cools, Luhman explained, this zone will sweep in, quickly at first and then ever more slowly. That means that the fate of a planet depends on where it is. A planet in the center of the zone now could look forward to a few tens of millions of years of habitability, he said.

But he added that theorists had no idea how and if such planets would form to begin with. Nor do they know whether life could arise in the brief time allotted.

Other astronomers said it was a little too early to start speculating about life or prebiotic chemistry around a brown dwarf.

Alan Boss, a planetary theorist at the Carnegie Institution of Washington, said, "The biggest impact of this discovery may be to force theorists to consider the possibility of planet formation around objects which are so low in mass that they are on the verge of being called planets themselves."

It is not too soon, he said, to start worrying about what to call some of these things.

Four years ago, astronomers discovered a dozen or so objects with masses estimated at 5 to 15 Jupiters amid new stars and gas clouds in the Orion nebula. They named them free-floating "planets," to the dismay of most astronomers, who object that planets are things that revolve around stars. A committee of the International Astronomical Union came up instead with the name "sub-brown dwarfs."

The confusion has been compounded lately by the recent "weighing" of a putative brown dwarf in the constellation Doradus, which suggest that some of these cosmic small fry, like brown dwarfs, might be more massive than they look. If this is true, the free-floating planets may actually be brown dwarfs, and some brown dwarfs may actually be small stars.

Laird Close of the University of Arizona, who made those measurements, said that even given the uncertainty in the brown dwarf's mass, OTS 44 was the smallest object yet found with an infrared excess--and thus probably a circumstellar disk. But in an e-mail message, he cautioned that this did not prove that 15-Jupiter-mass objects could form their own solar systems.

Boss, chairman of the astronomical union's committee, said, "More classification headaches are likely to ensue as astronomers probe deeper and deeper into the backyards of the Sun's neighborhood."

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