Look closely at the red speck of light in this false-color photo. It's the first image ever of a planet outside our solar system. The 8-million-year old world, about the size of Jupiter but five times as massive, has water vapor in its atmosphere and circles its mother brown dwarf star every 2,500 years or so at a distance of 8 billion kilometers. The whole system is 230 light-years away in the constellation Hydra. Astronomers are calling the planet 2M1207b, but that may change.
A European-American team of astronomers led by Gaël Chauvin of the European Southern Observatory (ESO) took the infrared photo in April 2004 using ESO's 8.2-meter Very Large Telescope (VLT) in Chile, outfitted with a revolutionary system to compensate for atmospheric turbulence (ScienceNOW: 13 September, 2004). Until now the team couldn't rule out the possibility that the red dot was a background object, unrelated to the brown dwarf. But new VLT measurements, reported in an upcoming issue of Astronomy & Astrophysics, confirm that the two objects are moving through space together, and independent Hubble Space Telescope data released on 2 May at an exoplanet workshop in Baltimore, Maryland, all but clinch the case. "At the 99.9% level, I agree this is probably the first image of an extrasolar planet," says Eric Becklin of the University of California at Los Angeles (UCLA), who was not involved in either study.
But is it really a planet and not, say, another brown dwarf star? According to theoretical models for inferring the mass of young low-mass objects from their infrared spectra, 2M1207b is only five times as massive as Jupiter. That's well below the 13.6-Jupiter-mass cutoff the International Astronomical Union uses to distinguish planets from brown dwarfs. "The possibility that this object is a brown dwarf is out of the box," says Glenn Schneider of the University of Arizona, who presented the Hubble results. If anything, "the models may well overestimate the mass," says Gibor Basri of the University of California, Berkeley, who helped develop a new way of determining masses of substellar objects by deducing their surface gravity from detailed spectroscopic measurements.With its claim to fame assured, co-discoverer Benjamin Zuckerman of UCLA says the team hopes to give the planet a name better suited to its historic status. "Anyone with a bright idea is welcome to suggest it," he says.