Spitzer analyses hot Jupiter's wild orbit

The study of exoplanets over the past decade has revealed much about the properties of other plantaery systems and how they form. A new study using the Spitzer Space Telescope is providing clues as to the formation of a type of scorching gas giant called 'hot Jupiters'.


Animation showing the temperature swings of the hot Jupiter exoplanet HD 80606b
Credit: NASA/JPL-Caltech/MIT/Principia College

The study of a massive, scorching exoplanet is enabling NASA astronomers to learn more about how ‘hot Jupiters’ form.

Hot Jupiters are a class of exoplanet so-called because they are gas giants, like the one found in our Solar System, but orbit much closer to their host star and, as a result, are immensely hot.

Nearly 2,000 exoplanets in total have been confirmed so far, and the results show that hot Jupiters, rather than being a cosmic anomaly, are actually rather common.

"We thought our solar system was normal, but that's not so much the case," says astronomer Greg Laughlin, co-author of a study from NASA's Spitzer Space Telescope investigating how hot Jupiters are formed.

The Spitzer Space Telescope has been used to observed HD 80606b, a hot Jupiter 190 lightyears from Earth. This exoplanet has an unusual orbit because it swings very close to its host star, then out to a great distance, following an oblong shape as it does so. It does this every 111 days.

As a result of its close passes with its star, the side of the planet facing the star is thought to heat up to over 1,100°C.

The current theory as to how hot Jupiters form suggests that gas giants in distant orbits – like our own Jupiter – are driven into closer orbits by gravitational influences from nearby stars or planets. This creates an eccentric orbit, like that of HD 80606b, which eventually settles down to become a close, circular orbit.

The astronomers behind this latest study are now asking how long it would take for this hot Jupiter to develop a more uniform, circular orbit. One method of determining this is to examine how ‘soft’ the planet is. As the hot Jupiter orbits close to its star, the gravity from the star squeezes it. The ‘softer’ the planet is, the better it can dissipate this energy as heat. The more heat is dissipated, the faster it will develop a circular orbit.

Results from the Spitzer study show that HD 80606b does not dissipate much heat during these close passes with its star, meaning it is relatively stiff. This also suggests that the hot Jupiter is not developing a circular orbit as quickly as anticipated, and could take another 10 billion years or so to complete.

"The long time scales we are observing here suggest that a leading migration mechanism may not be as efficient for hot Jupiter formation as once believed," says Laughlin.

If this is the case, a competing theory that says hot Jupiters form close to their stars, or else that they spiral smoothly inward, could become the preferred explanation.

The study also found that the exoplanet’s rotation period is 90 hours, which is slower than predicted, adding to the mystery of how these hot Jupiters form.

"Fifty years ago, we were measuring the rotation rates of planets in our own solar system for the first time,” says Laughlin. “Now we are doing the same thing for planets orbiting other stars. That's pretty amazing.”


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