September 10, 2004
A team of European and American astronomers has taken what may be the first image of a planet outside our solar system. In April, the team detected a faint energy source close to a brown dwarf designated 2MASSWJ1207334-393254, or 2M1207.
Brown dwarfs are more massive than planets, and astronomers usually set the line dividing the two classes at about 13 Jupiter masses. Unlike a star, a brown dwarf lacks the mass to produce energy by nuclear fusion; instead, it produces heat as a by-product of its gradual contraction.
What's causing all the excitement, however, is an object five magnitudes, or 100 times, fainter than the brown dwarf, imaged by a heat-sensitive detector. A spectrum of the object taken in June showed the presence of water molecules. A team led by Gael Chauvin of the European Southern Observatory made the discovery using the Very Large Telescope, which is located on Cerro Paranal in the Atacama Desert of northern Chile.
Both the brown dwarf and its possible planet lie in a group of stars known as the TW Hydrae association, about 230 light-years from Earth. The fainter object lies about 55 astronomical units — or 55 times the Earth-Sun distance — from its parent brown dwarf.
Although astronomers are not certain of the status of the object, no observation made so far eliminates the possibility that it's an exoplanet. In fact, the energy the object is radiating, the information from its spectrum, and current evolutionary models of planetary formation lead to the conclusion that a planet 5 times the mass of Jupiter orbits 2M1207.
Since 1998, the team has been studying the environments of young, nearby stellar associations — large conglomerates of mostly young stars and the dust and gas clouds from which they recently formed. The stars in these associations are ideal targets for the direct imaging of what astronomers call "sub-stellar companions" (planets and brown dwarfs). "Whatever their nature, sub-stellar objects are much hotter and brighter when young — tens of millions of years — and therefore can be more easily detected than older objects of similar mass," notes Chauvin.
Astronomy magazine contributing editor Ray Jayawardhana, associate professor of astronomy and astrophysics at the University of Toronto, Canada, is excited by this discovery. "The fact that the spectrum shows traces of water vapor rules out the possibility that this is a faint background star because water vapor indicates this is a cool object," he said. "No matter what definition of 'planet' you use, this is a planetary-mass object. The second part of the test will be to do follow-up imaging. It may take a year or so, but eventually we'll see if both objects are moving together," Jayawardhana explained. "That will prove the smaller body is a planet."
Both objects may have something to tell us about how brown dwarfs form. "It may be a process as simple as small cloud clumps making brown dwarfs and large clumps making stars, but then again it may be a lot more complex than that," he said.