Cool brown dwarf may give birth to planets


'Mini-solar systems are an exciting possibility'
By William J. Cromie
Harvard News Office

Solar systems like our own may be forming around dim stars scattered all over the Milky Way. It's possible that some of these systems could harbor planets with water and even life.

The likelihood of such a possibility has increased with the discovery of a disk of gas and dust orbiting a star only about 15 times more massive than Earth's planetary neighbor, Jupiter. Such stars carry the undignified name "brown dwarfs," because they never grow big enough to shine like a sun. But they could warm planets that form out of the encircling disk of stardust the way Earth and its neighbor planets formed around the sun.

"Such mini-solar systems are an exciting possibility, one that hasn't been explored extensively because this is the first evidence for the building blocks of planets around such a small object," says astronomer Kevin Luhman. He led a group of colleagues at the Harvard-Smithsonian Center for Astrophysics, who found OTS 44, a brown dwarf about 3,000 trillion miles away in the southern constellation Chamaeleon.

Brown dwarfs are objects 15-70 times more massive than Jupiter, which is 318 times as massive as Earth. OTS 44 is the smallest, coolest one ever detected with a preplanetary disk. It's not all that much bigger than a planet itself. Brown dwarfs don't shine like stars because they are not hot enough. But, at 3,600 degrees, this dwarf is large enough to comfortably warm planets that condense out of its disk.

Due to this brown dwarf's small size and relatively cool temperature, any new, habitable planet would need to be much closer than the distance between Earth and its white hot sun, 93 million miles. Luhman estimates that liquid water and life could exist on a world 1 million to 4 million miles from OTS 44. The disk is wide enough to accommodate a planet at that distance.

Without starlike nuclear reactions to keep it going, the dwarf will gradually become cooler and dimmer. Any new planets would, at first, be too hot to sustain life, as we know it. But as time passed, these worlds would grow cooler and more hospitable. Given enough time in this zone, life might arise.

"It's an intriguing possibility, but purely speculative," Luhman admits. "However, finding a circumstellar disk around such a small brown dwarf certainly widens the possibilities for planet formation."

Searching for dwarfs

Luhman and his group were searching for brown dwarfs with the Spitzer Space Telescope, launched in August 2003, when they found OTS 44. Instruments aboard this satellite detect heavenly bodies by the heat they emit, enabling astronomers to find new stars and brown dwarfs hidden from view behind clouds of dark gas and dusky debris. The specific camera used to take the first images of OTS 44 was built by Giovanni Fazio, a senior scientist at the Smithsonian Astrophysical Observatory.

"Without the Spitzer's infrared (heat) camera, it would have been close to impossible to find such a small brown dwarf with an orbiting disk," Luhman says. "We quickly detected OTS 44 from its halo of heat, a sure sign of an accompanying disk of gas and dust." The dusty disk absorbs heat from the almost-star, and this heat is reradiated in wavelengths that create an unmistakable fingerprint.

Luhman and his colleagues believe that the brown dwarf is about 2 million years old, relatively young compared with the sun's age of about 5 billion years. It is difficult to determine the disk's size, but they speculate that it extends beyond 1 billion miles, or roughly 10 times as far as the Earth lies from the sun.

Because it's so young and warm, OTS 44 is reddish - redder than the sun. As it ages, the dwarf will become darker, eventually looking more like Jupiter than the sun.

Brown dwarfs, also known as "failed stars," are not unusual in the Milky Way and, presumably, in other galaxies. Disks encircle some of them. One large dwarf boasts a planetlike companion about five times as massive as Jupiter. However, Luhman doesn't think this super planet came from a disk, because it's too far away from the dwarf and too large.

Luhman's group is now searching for smaller brown dwarfs adorned with rings. They might exist in sizes of only 10, even five, Jupiter masses. Planets developing around these dwarf dwarfs would be difficult to detect with Spitzer's heat cameras, but more sophisticated instruments are planned for launch in the next decade aboard a replacement for the aging Hubble Space Telescope.

"I think it's likely that we'll find planets, and even miniature solar systems, around brown dwarfs elsewhere the Milky Way," Luhman says enthusiastically. What a sight that would be - miniature solar systems popping up all over the universe, vastly increasing the possibility of many more Earth-like planets.

"It is," says Luhman, "definitely an exciting possibility."