Astronomers discover sunscreen snow falling on hot exoplanet
These Hubble observations are the first detections of this "snow-out" process, called a "cold trap," on an exoplanet.
This discovery, and other observations made by the Penn State team, provide insight into the complexity of weather and atmospheric composition on exoplanets, and may someday be useful for gauging the habitability of Earth-size planets.
"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," said Thomas Beatty, assistant research professor of astronomy at Penn State and the lead author of the study.
"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."
The team's results are published in October 2017 issue of The Astronomical Journal.
Beatty's team targeted planet Kepler-13Ab because it is one of the hottest of the known exoplanets. Its dayside temperature is nearly 5,000 degrees Fahrenheit.
Kepler-13Ab is so close to its parent star that it is tidally locked, so one side always faces the star while the other side is in permanent darkness.
The team discovered that the sunscreen snowfall happens only on the planet's permanent nighttime side. Any visitors to this exoplanet would need to bottle up some of that sunscreen, because they won't find it on the sizzling-hot daytime side.
The astronomers didn't go looking for titanium oxide specifically. Instead, their studies revealed that this giant planet's atmosphere is cooler at higher altitudes - which was surprising because it is the opposite of what happens on other hot Jupiters.
Titanium oxide in the atmospheres of other hot Jupiters absorbs light and reradiates it as heat, making the atmosphere grow warmer at higher altitudes. Even at their much colder temperatures, most of our solar system's gas giants also have warmer temperatures at higher altitudes.
Intrigued by this surprising discovery, researchers concluded that the light-absorbing gaseous form of titanium oxide has been removed from the dayside of planet Kepler-13Ab's atmosphere.
Without the titanium oxide gas to absorb incoming starlight on the daytime side, the atmospheric temperature there grows colder with increasing altitude.
The astronomers suggest that powerful winds on Kepler-13Ab carry the titanium oxide gas around, condensing it into crystalline flakes that form clouds.
Kepler-13Ab's strong surface gravity - six times greater than Jupiter's - then pulls the titanium oxide snow out of the upper atmosphere and traps it in the lower atmosphere on the nighttime side of the planet. ■