NASA Spies Distant Hot Jupiter’s Eccentric Orbit To Answer Questions About Its Origin

Every 111 days, HD 80606b — an exoplanet belonging to a class of objects known as “hot Jupiters” — plunges toward its parent star in a solar system 190 light-years from Earth. As a result of its bizarre and highly eccentric orbit, the side of the planet facing the star quickly heats up to more than 2,000 degrees Fahrenheit at its closest approach.

“This planet is thought to be caught in the act of migrating inward,” Julien de Wit from the Massachusetts Institute of Technology and lead author of a new study on hot Jupiter formation, said in a statement released Monday. “By studying it, we are able to test theories of hot Jupiter formation.”

So far, nearly 2,000 exoplanets have been discovered. Of these, many are believed to be hot Jupiters — large planets locked in an extremely close orbit around their parent stars. Although scientists now believe that hot Jupiters are extremely common across alien solar systems, they have continued to puzzle scientists, who are still not entirely certain how these massive planets form or how they wind up so close to their stars.

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. These planets start out in eccentric orbits before they settle down into tight, circular orbits — over a period of hundreds of millions of years.

Scientists now hope that studying HD 80606b, which was detected by NASA's Spitzer Space Telescope in 2009, would provide insights into how hot Jupiters’ orbits shift from a nearly flattened ellipse to a circle.

The Spitzer results show that HD 80606b does not dissipate much heat when it is squeezed by gravity of its parent star during its close encounters, indicating that it is not “squishy.” If the planet is squishier, or more pliable, it can better dissipate gravitational energy as heat, which, in turn, speeds up the transition to a circular orbit — a process known as “circularization.”

Observations suggest that HD 80606b is not circularizing its orbit as fast as expected, and may take another 10 billion years or more 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,” co-author Greg Laughlin from the University of California said in the statement.