Titanium dioxide, ingredient in sunscreen, falls as snow on hot exoplanet

Understanding atmospheres of large exoplanets will help scientists better understand those of Earth-like worlds.
By Laurel Kornfeld | Oct 31, 2017
A hot Jupiter exoplanet discovered by NASA's Kepler spacecraft has an unusually structured atmosphere from which titanium dioxide, one of the active ingredients in sunscreen, falls onto its surface as snow.

Located 1,730 light years away, Kepler-13Ab is in a close orbit around a star that is part of a triple-star system. It circles its star, Kepler-13A, once every 1.8 Earth days and is about six times more massive than Jupiter.

Because it orbits so close to its parent star, the planet is tidally locked, with a day side that always faces the star and a night side that always faces away from it.

Day side temperatures can reach nearly 5,000 degrees Fahrenheit (2,760 degrees Celsius), making Kepler-13Ab one of the hottest exoplanets ever discovered.

Hot Jupiters typically have gaseous titanium dioxide in the upper atmospheres of their day sides, which warms those day sides by absorbing starlight and subsequently radiating it out as heat.

In contrast, Kepler-13Ab has an unusual atmosphere in which the uppermost levels are cooler than the lower ones, the opposite of what occurs on similar worlds.

Scientists who studied the planet using NASA's Hubble Space Telescope think high winds on its day side transport titanium dioxide to its night side, where the substance cools, condenses into clouds, and then falls into the lower atmosphere as snow.

Because of the planet's extreme gravity, the "snow" stays in the lower atmosphere, further cooling it.

"These observations of Kepler-13Ab are telling us how condensates and clouds form in the atmospheres of very hot Jupiters, and how gravity will affect the composition of an atmosphere," study lead author Thomas Beatty of Pennsylvania State University said in a public statement.

"When looking at these planets, you need to know not only how hot they are, but also what their gravity is like."

This particular atmospheric process, known as a "cold trap," has not been previously seen on any exoplanet.

Scientists view detailed studies of these giant planets' atmospheres as tests for similar studies they hope to eventually conduct of the atmospheres of Earth-like exoplanets.

"Understanding more about the atmospheres of these planets and how they work will help us when we study smaller planets that are harder to see and have more-complicated features in their atmospheres," Beatty noted.

A paper detailing the study has been published in the October 2017 issue of The Astronomical Journal.

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