Strange mini-solar systems revealed

Another strange system
At the other end of the life cycle for stars and planets, NASA's Spitzer Space Telescope has spotted a disk of potential planet-making material around a young brown dwarf.

Brown dwarfs are not massive enough to jump-start the thermonuclear fusion that fuels real stars. But they are warm enough to be detected by the infrared telescope.

This one, known as OTS 44, is 500 light-years away. It is among the smallest known brown dwarfs, just 15 times the mass of Jupiter. In terms of heft, it's at the border between stars and planets, insofar as astronomers have figured out where that border is.

The flat batch of orbiting gas and dust looks just like a protoplanetary disk. Apparently no planets have developed yet, but the disk has enough material to create one small gas-giant planet and a few Earth-sized, rocky worlds, astronomers said.

"We are seeing the ingredients for planets around a brown dwarf near the dividing line between planets and stars," said Giovanni Fazio, an astronomer at the Harvard Smithsonian Center for Astrophysics. "This raises the tantalizing possibility of planet formation around objects that themselves have planetary masses."

NASA / JPL / Caltech A swirling disk of planet-building gas and dust surrounds a brown dwarf in this artist's rendering.

No definition
In fact, astronomers have no formal definition for the term "planet" and in recent years have been arguing what exactly constitutes one at both ends of the spectrum.

Several objects known to be half the size of Pluto or bigger and circling our sun should be considered planets, some say. Or, others argue, Pluto should not be thought of as a planet. And on the upper end, theorists wonder whether an object must orbit a normal star to be considered a planet, and just how massive a planet can be before it's a dim star.

About the only thing astronomers can say for sure is that almost everything they find in space seems to have the potential for an orbiting companion of some sort. And it is becoming evident that planet formation as it is thought to have happened in our solar system may occur in a similar manner in significantly different environments.

"There may be a host of miniature solar systems out there, in which planets orbit brown dwarfs," said the Center for Astrophysics' Kevin Luhman.

That speculation is bolstered by the recent announcements of what are likely the first photographs of an extrasolar planet, an object that appears to orbit a brown dwarf. (All other discoveries of planets around other stars involve indirect detection methods.)

For Luhman, "this leads to all sorts of new questions, like, 'Could life exist on such planets?' or 'What do you call a planet circling a planet-sized body? A moon or a planet?'"

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