March 23, 2006—Astronomers have discovered a "cool" brown dwarf circling a star relatively close to Earth.
Brown dwarfs are neither stars nor planets. They are typically dozens of times more massive than Jupiter, the most massive planet in our solar system, but too small to shine like a star.
The newly discovered brown dwarf orbits a small star located about 12.7 light years from the sun, making it the third closest brown dwarf to Earth yet found.
"This is really very much in our neighborhood of the galaxy," said Laird Close, an astronomer at the University of Arizona in Tucson, whose team made the find.
Close added that this is a rare "cool" brown dwarf. It has a temperature of about 1,382ºF (750ºC). Most brown dwarfs are much hotter, he said.
Two other brown dwarfs have been discovered orbiting a star about 11.8 light years away. Of the seven brown dwarfs discovered within 20 light years of the sun, five have companion stars, the researchers say.
These findings hint that, at least close to the sun, brown dwarfs tend to pair up with stars rather than float in space alone.
"The interesting thing is the models for the most part suggest brown dwarfs should be free-floating in space, so it's a bit of a puzzle," Close said.
"The fact that we see more brown dwarfs as companions very near the sun suggests that our understanding of how brown dwarfs formed needs further theoretical work," he added.
The Arizona team discovered the brown dwarf with the European Southern Observatory's Very Large Telescope in Chile.
The discovery will be detailed in an upcoming issue of the Astrophysical Journal.
Illustration courtesy ESO