The last decade has seen a bonanza of exoplanet discoveries. Nearly 2,000 exoplanets — planets outside our solar system — have been confirmed so far, and more than 5,000 candidate exoplanets have been identified. Many of these exotic worlds belong to a class known as “hot Jupiters.” These are gas giants like Jupiter but much hotter, with orbits that take them feverishly close to their stars.
At first, hot Jupiters were considered oddballs, since we don’t have anything like them in our own solar system. But as more were found, in addition to many other smaller planets that orbit very closely to their stars, our solar system started to seem like the real misfit.
“We thought our solar system was normal, but that’s not so much the case,” said astronomer Greg Laughlin of the University of California, Santa Cruz, co-author of a new study from NASA’s Spitzer Space Telescope that investigates hot Jupiter formation.
As common as hot Jupiters are now known to be, they are still shrouded in mystery. How did these massive orbs form, and how did they wind up so shockingly close to their stars?
The Spitzer telescope found new clues by observing a hot Jupiter known as HD 80606b, situated 190 light-years from Earth. This planet is unusual in that it has a wildly eccentric orbit almost like that of a comet, swinging very close to its star and then back out to much greater distances over and over again every 111 days.
One side of the planet is thought to become dramatically hotter than the other during its harrowing close approaches. In fact, when the planet is closest to its host star, the side facing the star quickly heats up to more than 2,000 degrees Fahrenheit (1,100 degrees Celsius).
“As the planet gets closer to the star, it feels a burst of starlight, or radiation. The atmosphere becomes a cauldron of chemical reactions, and the winds ramp up far beyond hurricane force,” said Laughlin, a co-author on the Spitzer study, which is accepted for publication in The Astrophysical Journal Letters.
HD 80606b is thought to be in the process of migrating from a more distant orbit to a much tighter one typical of hot Jupiters. One of the leading theories of hot-Jupiter formation holds that gas giants in distant orbits become hot Jupiters when the gravitational influences from nearby stars or planets drive them into closer orbits. The planets start out in eccentric orbits, then, over a period of hundreds of millions of years, are thought to gradually settle down into tight, circular orbits.
“This planet is thought to be caught in the act of migrating inward,” said Julien de Wit of the Massachusetts Institute of Technology, Cambridge, lead author of the new study. “By studying it, we are able to test theories of hot Jupiter formation.”