Bizarro world: Huge exoplanet traps nearly all visible light

The Hubble Space Telescope has found a pitch black exoplanet that traps almost all visible starlight.
By David Sims | Sep 16, 2017
The Hubble Space Telescope has discovered a very large, very strange exoplanet that is completely black.

WASP-12b, known as a "hot Jupiter," is about twice the size of Jupiter and three times its radius. And it has what may be a unique ability to trap 94 percent of the visible starlight entering its atmosphere.

WASP-12b is located 1,400 light-years away in the constellation Auriga. The planet lies about 2 million miles from its star and completes an orbit once daily.

"We did not expect to find such a dark exoplanet," says lead researcher Taylor Bell of McGill University and the Institute for Research on Exoplanets, in a statement. "Most hot Jupiters reflect about 40 percent of starlight."

The study is published in the Astrophysical Journal Letters.

The researchers discovered the planet's light-absorbing ability using Hubble's Space Telescope Imaging Spectrograph to look for the small decrease in starlight as the planet passed behind its star. The lack of dimming revealed the absence of reflected light.

Because WASP-12b orbits so close to its star, it is tidally locked meaning its day and night sides are fixed. On the day side, the planet's atmospheric temperature is 4,600 degrees Fahrenheit, which makes it so hot that most molecules do not survive and clouds cannot form to reflect light back into space.

"This new Hubble research further demonstrates the vast diversity among the strange population of hot Jupiters," says Bell. "You can have planets like WASP-12b that are 4,600 degrees Fahrenheit and some that are 2,200 degrees Fahrenheit, and they're both called hot Jupiters. Past observations of hot Jupiters indicate that the temperature difference between the day and night sides of the planet increases with hotter day sides. This previous research suggests that more heat is being pumped into the day side of the planet, but the processes, such as winds, that carry the heat to the night side of the planet don't keep up the pace."

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