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Sunscreen ingredient falls as ‘snow’ on hot exoplanet Kepler-13Ab

Kepler-13Ab / Kepler-13 stellar system (A, B, and C)

This illustration shows the seething hot planet Kepler-13Ab that circles very close to its host star, Kepler-13A. On the nighttime side, the planet’s immense gravity pulls down titanium oxide, which precipitates as snow. Seen in the background is the star’s binary companion, Kepler-13B, and the third member of the multiple-star system is the orange dwarf star, Kepler-13C. Image & Caption Credit: NASA / ESA / G. Bacon (STScI)

One of the hottest known exoplanets – Kepler-13Ab – experiences ‘snowfall’ composed of titanium dioxide, an active ingredient in sunscreen. However, the sunscreen “snow” only precipitates on the planet’s permanent dark side.

The “hot Jupiter” planet, about six times more massive than Jupiter itself, discovered by NASA’s Kepler exoplanet-hunting spacecraft, orbits so close to its parent star that it is tidally locked, with a dayside that always faces the star and a nightside that always faces away from it. The planet orbits the star Kepler-13A, one of a triple-star system located 1,730 light-years away, once every 1.8 Earth days.

A team of astronomers, led by Thomas Beatty of Pennsylvania State University, used the Hubble Space Telescope’s Wide Field Camera 3 (WFC3) to perform a spectroscopic study of the planet’s atmosphere in the near-infrared and conducted the observation as the planet passed behind its parent star. This type of transit, known as a secondary eclipse, can give scientists data on the temperature of the atmosphere and its components on the planet’s dayside.

Beatty’s team chose Kepler-13Ab because of its extreme heat, with dayside temperatures close to 5,000 degrees Fahrenheit (2,760 degrees Celsius).

While hot Jupiters typically have gaseous titanium dioxide in the atmospheres of their day sides, Kepler-13Ab instead has the substance fall as crystallized flakes or “snow” on its nightside.

Unlike most hot Jupiters, whose upper atmospheres contain titanium dioxide, which absorbs starlight and radiates it out as heat, Kepler-13Ab has an atmosphere that is cooler at its uppermost levels and warmer at its lower ones. This intrigued the research team, who attributed the cooler upper dayside atmosphere to the lack of titanium dioxide.

The team attributes the anomaly to high winds, which likely transport the titanium dioxide to the planet’s nightside, where it cools, condenses into clouds, and falls as snow into the lower atmosphere. Kepler-13Ab’s powerful gravity keeps the titanium dioxide snow trapped in the lower atmosphere in a process scientists refer to as a “cold trap”.

Kepler 13Ab / Solar System comparison

This is an artist’s impression of the gas giant planet Kepler-13Ab as compared in size to several of our solar system planets. The behemoth exoplanet is six times more massive than Jupiter. Kepler-13Ab is also one of the hottest known planets, with a dayside temperature of nearly 5,000 degrees Fahrenheit (2,760 °C). It orbits very close to the star Kepler-13A, which lies at a distance of 1,730 light-years from Earth. Image & Caption Credit: NASA / ESA / A. Feild (STScI)

Although this precipitation phenomenon has never been previously observed on an exoplanet, scientists several years ago theorized it could occur on very massive hot Jupiters.

“Presumably, this precipitation process is happening on most of the observed hot Jupiters, but those gas giants all have lower surface gravities than Kepler-13Ab,” Beatty said. “The titanium oxide snow doesn’t fall far enough in those atmospheres, and then it gets swept back to the hotter dayside, revaporizes, and returns to a gaseous state.”

On massive planets, gravity directly affects the atmospheric composition. “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,” Beatty explained. “When looking at these planets, you need to know not only how hot they are but [also] what their gravity is like.”

The techniques used to study the atmospheres of these giant planets will one day help scientists determine the habitability of Earth-sized exoplanets.

“Hot Jupiters provide us with the best views of what climates on other worlds are like. Understanding the atmospheres on these planets and how they work, which is not understood in detail, will help us when we study these smaller planets that are harder to see and have more complicated features in their atmospheres,” Beatty said.

Findings of the study have been published in The Astronomical Journal.



Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.

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