Astronomers may have their long-coveted first image of an extrasolar planet, thanks to follow-up observations made by the Hubble Space Telescope. But it all depends on the definition of a "planet." The object in question does not orbit a normal star. Rather, it orbits a brown dwarf — an object containing insufficient mass to sustain the nuclear fusion reactions that power stars. The brown dwarf is only five times heavier than its companion.
A French team led by Gael Chauvin (European Southern Observatory) found the planet candidate in April 2004. They used the Very Large Telescope (VLT) in Chile to image the brown dwarf 2M 1207, located about 225 light-years away in Hydra. The team spotted a very faint companion glowing in the VLT infrared image, which was taken with adaptive optics to counter atmospheric turbulence. Given the companion's estimated age of 8 million years, low luminosity (1/100th that of the brown dwarf), and temperature (1,000 degrees C, cooler than a light-bulb filament), Chauvin's group calculated that 2M 1207b has 5 times the mass of Jupiter. The primary weighs in at about 25 Jupiters, so the pair seems more like an extremely low-mass binary star that what most astronomers would consider a planetary system.
But is this faint smudge gravitationally bound to the brown dwarf, or is it a background star that just happens to lie along the same line of sight? To find out, Glenn H. Schneider (University of Arizona) and his team used Hubble's near-infrared camera, NICMOS, to obtain an image of 2M 1207 and the fainter object nearby. Comparing the August 28, 2004 NICMOS image with the April 2004 discovery image shows that the two objects appear to be moving in the same direction and at the same speed across the sky.
There are still some small uncertainties in the motions, but Schneider's team says that there is only a 0.9 percent chance that 2M 1207b is a background interloper rather than a companion to the brown dwarf. "Stay tuned for the final confirmation, but it's looking pretty good for 2M 1207b," says Schneider.
"That image is by far the best candidate to be an actual image of an exoplanet," adds team member Eric E. Becklin (UCLA).
The next Hubble observations are planned for April 2005. If these observations confirm that the two objects are bound together, it will mean that they orbit each other every 2,500 years at a distance of at least 54 astronomical units — more than 38 percent farther than Pluto's average distance from the Sun.
Schneider and his colleagues presented their observations at the 2005 American Astronomical Society meeting in San Diego.