Exoplanets in Detail
In the hunt for exoplanets, Hubble isn’t the first space telescope that comes to mind. NASA’s Kepler spacecraft, launched in 2009, has found thousands of exoplanet candidates. “The strength of Kepler is in the statistics—you learn a lot when you can detect thousands of planets,” says Paul Kalas of the University of California at Berkeley. “The strength of Hubble is in being able to study a few systems in exquisite detail.”
Kalas and his team began using Hubble to look for planets around the star Fomalhaut in 2004, and found an enormous dust belt. The detailed images, however, showed that the belt was an unusual shape, likely disturbed by the gravity of at least one planet, so in 2006 they went back to look again. Comparing the images, they found “a tiny dot of light, 10 billion times fainter than the star.” Fomalhaut b is the first exoplanet to have its picture taken.
But even a picture isn’t definitive proof for everyone. Kalas extrapolated an orbit, but it didn’t explain the debris ring shape, as the planet’s gravity would push and pull on it. What’s more, Kalas says some scientists were concerned that the planet didn’t appear brighter in the infrared, as a planet orbiting a youthful 440-million-year-old star should.
Kalas went back to Hubble and found that Fomalhaut b’s orbit is extremely elliptical, similar to those of comets in our solar system, which follow paths that swing inside close to the sun and then far out beyond Pluto into the Kuiper Belt. At the farthest point from its star, Fomalhaut b is 350 times farther than Earth is from the sun. In about 20 years, the planet should travel far enough out to cross the debris ring. “Fomalhaut b would then become extremely interesting to monitor because there are no other examples that we know of where a planet would be penetrating its Kuiper Belt,” says Kalas, who plans to look for a more massive planet in the system that nudged Fomalhaut b into its eccentric orbit.