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 8 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 December 10 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 (http://www.cfa.harvard.edu/press/pr0504.html).
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.
"Our goal is to determine the smallest `sun' with evidence for planet formation," said Luhman. "Here we have a sun that is so small it is the size of a planet. The question then becomes, what do we call any little bodies that might be born from this disk: planets or moons?"
Brown dwarfs are born like stars, condensing out of thick clouds of gas and dust. But unlike stars, brown dwarfs do not have enough mass to sustain nuclear fusion. They remain relatively cool objects visible in lower-energy wavelengths such as infrared.
With Spitzer, the science team spotted Cha 110913-773444 about 500 light-years away in the southern constellation Chamaeleon. This brown dwarf is young, only about 2 million years old. The team studied properties of the brown dwarf with infrared instruments at other observatories, but the cool, dim protoplanetary disk was only detectable with Spitzer's Infrared Array Camera, which was developed at CfA.
This release is being issued jointly with Pennsylvania State University.
NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer mission for NASA's Science Mission Directorate. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology in Pasadena.
Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.