Astronomers discover 'sunscreen' snow on distant exoplanet

More than 1,700 light-years from Earth lies a sizzling hot planet orbiting another star. While temperatures can soar to 2,700 C, the planet also produces some unique precipitation in the form of "sunscreen" snow.

Kepler-13Ab is what astronomers call a "hot Jupiter": a gas giant like our solar system's biggest planet, but hotter and with an orbit that takes it very close to its parent star.

Astronomers were able to study the planet's composition using the Hubble Space Telescope. They found the planet, discovered in 2011, snows titanium oxide, the active ingredient in some sunscreens.

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Kepler-13Ab is the only planet in this three-star system. It is so close to its parent star Kepler 13A, that it is tidally locked, with one side always facing the star, in the same way we only see one face of the moon.

The researchers discovered the titanium oxide when trying to determine why the planet's atmosphere is cooler at higher altitudes, something that is directly opposite to what occurs in the atmospheres of other hot Jupiters. On those planets, the titanium oxide absorbs light and reradiates it, which in turn makes the atmosphere warmer at higher altitudes.

In the case of Kepler-13Ab, the titanium oxide is removed from the day side which produces the opposite effect.

In the study, published in the Astronomical Journal, the researchers suggest that strong winds on the planet carry the gas around where it crystalizes, forming clouds. And because the surface gravity is incredibly strong — more than eight times that of Earth's — it pulls the titanium oxide snow from the upper atmosphere into the lower atmosphere on the night side of the planet.

The scientists believe that, while this process may be occurring on other hot Jupiters, the snow doesn't fall far enough due to their weaker gravity. Instead, it gets carried back to the hot day side where it vaporizes into a gas.

Better understanding how this kind of "cold trap" on hot Jupiters work could advance the  study of other exoplanets, the researchers say.

"In many ways, the atmospheric studies we're doing now on these gaseous 'hot Jupiter' kinds of planets are test beds for how we're going to do atmospheric studies of terrestrial, Earth-like planets," Thomas Beatty, assistant research professor of astronomy at Penn State and the lead author of the study said in a statement.