In the late 1990s, astronomers noticed a distinct warp in the disk of dust and gas orbiting a young star some 60 light-years from Earth. Now, using new analytical tools, researchers have discovered a giant planet lurking within the dusty haze. About nine times as massive as Jupiter and composed mainly of gas, the planet is only a few million years old, proving that such enormous planetary bodies can form rapidly.
The star, β Pictoris, first drew attention 25 years ago when a sky survey by the NASA spacecraft Interface Region Imaging Spectrograph found that it was emitting unusually intense infrared radiation. That observation suggested that it was shrouded in hot dust. Further observations revealed a disk of dust and gas, a possible crucible for planet formation. When researchers discovered that some of the gas was falling into the star, it was a clue that there might be a massive object lodged within the disk whose gravitational pull was sucking gas from the outer edges of the disk toward the star. This suspicion grew stronger still in 1997 when pictures of the disk showed a warp, leading astronomers to predict from modeling that a giant planet or a low-mass companion star was hiding somewhere within the disk.
In 2003, astrophysicist Anne-Marie Lagrange of the Grenoble Observatory in France and her colleagues took pictures of the star using a newly installed adaptive optics system on the European Southern Observatory's Very Large Telescope, but they couldn't spot any objects. In 2008, however, Lagrange took a second look at the data using new computational tools, and this time her team detected a faint dot in the northeastern part of the dust disk. The team confirmed the presence of the object in late 2009 and found that it was circling the star at a distance of between 8 to 15 Astronomical Units. (An astronomical unit is the Earth's distance from the sun, about 150 million kilometers.) Matching this observation to the models they'd built to explain the warping of the disk, the researchers determined that the object could not be a low-mass star. They had found a planet, dubbed β Pictoris b, with an orbital period between 17 and 35 years.
β Pictoris is only 10 million to 12 million years old, and the dust and gas in its surrounding disk are fairly young as well. The gas in such disks tends to get blown away from the star relatively quickly, so researchers have wondered whether planets can coalesce in the short time the gas is available. "Now we have direct proof that giant planets can form rapidly—in just a few million years," says Lagrange, whose team reports its find online today in Science.
Lagrange says the finding is consistent with a planet formation model known as core accretion in which the planet starts out as a rocky core that gravitationally acquires more matter from the surrounding swarm of dust and gas. That's how the giant planets in our solar system are believed to have formed. Another implication of the find, Lagrange says, is that "stars surrounded by debris disks are really good places to look for planets."
"This is exciting, wonderful news," says James Graham, an astronomer at the University of California, Berkeley. But he says that detecting the planet is only the first step. Next, he says, astronomers should pin down the planet's size, shape, and the composition of its atmosphere. "Are there clouds like Jupiter? Is there weather?"