Hellish Exoplanet Rains Hot Pebbles, Has Lava Oceans

The first rocky planet ever discovered outside our solar system has a hellish environment where hot pebbles rain down on oceans of lava, a new study suggests. Located about 500 light-years away in the constellation Monoceros, CoRoT-7b was first discovered in February by the French and European space agencies' CoRoT space telescope.

CoRoT-7b is about twice the size and five times the mass of Earth, and it's separated from its star by only 1.5 million miles (2.5 million kilometers)—that's about 23 times closer than Mercury is to our sun. On CoRoT-7b a year lasts only 20.4 hours.

"It's actually the closest orbit of any exoplanet that's been discovered," said study team member Laura Schaefer, an astronomer at Washington University in St. Louis, Missouri.

Rocky Rain

Because the newfound exoplanet orbits so close to its star, it's gravitationally locked to the smaller body. Half of the planet always faces its star, just as one side of the moon always faces Earth.

A new computer model of CoRoT-7b created by Schaefer and her team suggests the planet's surface on the star-facing side is a scorching 4,220 degrees Fahrenheit (2,327 degrees Celsius), and is covered largely by a lava ocean or scattered lava lakes.

(Related: "Most Earthlike Planet Yet Found May Have Liquid Oceans.")

And while Earth has a water cycle, CoRoT-7b has a rock cycle, the study says.

Vaporized rocks and minerals from the star-heated side rise to create a thin atmosphere. At higher altitudes, this material condenses into "rock clouds" that rain hot pebbles of different mineral compositions onto the planet's surface.

The pebbles are probably fairly small, Schaefer said.

"We're probably talking about dust size."

If the pebbles fall into the lava lakes or oceans, they get recycled back into the planet's rock cycle.

But if the rock clouds are pushed by stellar winds to the planet's frigid dark side, the pebbles remain solid once they hit the surface, Schaefer said.