Astronomers have used the Hubble Space Telescope to obtain detailed observations of the exoplanet Kepler-13Ab and have discovered that titanium oxide (the main ingredient in some sunscreens) “snows” through the planet’s hot atmosphere.
This is the first detection of such a process, technically called a cold trap, on an exoplanet. The team, from The Pennsylvania State University, have published the results in the Astronomical Journal.
Kepler-13Ab is a hellish world with a temperature of over 2,750°C (5,000°F). It’s tidally locked to its star with one side in constant sunlight, and it's over nine times heavier than Jupiter. And while it’s not a hospitable place, observing and understanding its atmosphere will help astronomers in the future.
"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," lead author Professor Thomas Beatty said in a statement.
"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 researchers didn’t set out to look for this particular effect and were puzzled by the observations. Hot Jupiters tend to have warmer upper atmospheres but this wasn’t the case for Kepler-13Ab’s day side. Titanium oxide is usually responsible for this increased temperature as it absorbs the stellar radiation, so the team suggested that powerful winds are taking the titanium oxide to the night side. Once it’s there it condenses and snows down under the exoplanet's strong gravitational pull. The observations suggest that this is how titanium oxide gets cold trapped.
"Seeing this cold-trap process in action provides us with a long sought and important piece of that puzzle," co-author Professor Jason Wright said.
"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 added. "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."
This study suggests that gravity plays a role in the atmospheric cycles on exoplanets, and it has to be taken into account when models are prepared. The complexity of these distant worlds continues to be absolutely fascinating.