A star that looks like a planet

Astronomers using NASA’s Spitzer Space Telescope have discovered a remarkably small brown dwarf surrounded by a dusty disk. The brown dwarf contains only about eight times the mass of Jupiter, making it one of the smallest known brown dwarfs. It is even smaller than several planets around other stars, leading to the question of whether any objects that form from the disk around it should be considered planets or moons.

“There are two camps when it comes to defining planets versus brown dwarfs,” said Giovanni Fazio of the Harvard-Smithsonian Center for Astrophysics (CfA), a member of the discovery team. “Some go by size, and others go by how the object formed. For instance, this new object would be called a planet based on its size, but a brown dwarf based on how it formed.”

The tiny brown dwarf, called Cha 110913-773444, is the smallest known brown dwarf to harbor what appears to be a planet-forming disk of rocky and gaseous debris. A team led by Kevin Luhman of Penn State University discusses this finding in the Dec. 10, 2005 issue of The Astrophysical Journal Letters. Luhman led a similar observation in 2004 that uncovered a 15- Jupiter-mass brown dwarf with a protoplanetary disk.

If the protoplanetary disk surrounding Cha 110913-773444 does form into planets, the whole system would be a miniaturized version of our solar system – with the central “sun,” the planets and their orbits all roughly 100 times smaller.