Investigating the Mystery of Migrating 'Hot Jupiters'

NASA | MARCH 30, 2016

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.

The turbulent atmosphere of a hot, gaseous planet known as HD 80606b is shown in this simulation based on data from NASA's Spitzer Space Telescope. Image credit: NASA/JPL-Caltech

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.

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.

The Spitzer results show that HD 80606b does not dissipate much heat when it is squeezed by gravity during its close encounters - and thus is not squishy, but rather stiffer as a whole. This suggests the planet is not circularizing its orbit as fast as expected, and may take another 10 billion years or more to complete.

The Spitzer study suggests that competing theories for hot Jupiter formation -- in which gas giants form "in situ," or close to their stars, or smoothly spiral inward with the help of planet-forming disks -- may be preferred.

The new study is also the first to measure the rotation rate of an exoplanet orbiting a sun-like star. Spitzer observed changes in the planet's brightness as the planet spun on its axis, finding a rotation period of 90 hours.

A rotation rate of 90 hours is much slower than what is predicted for HD 80606b, puzzling astronomers, and adding to the enduring mystique of hot Jupiters.

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