A team of astronomers may have discovered a "hot Jupiter" exoplanet with an atmosphere that snows titanium oxide – a kind of sunscreen – exclusively on the one side of the planet that is bathed in perpetual darkness. The study, which definitively proves that the universe has a great sense of humor, could one day be used to inform astronomers trying to judge the habitability of atmosphere-hosting Earth-sized planets.
Kepler-13Ab, the subject of the new study, was chosen to be observed by the Hubble Space Telescope because it represents one of the hottest exoplanets known to astronomers, with a scorching dayside temperature of almost 5,000 °F (2,760 °C).
The high temperatures experienced by Kepler-13Ab are a result of its tight orbit with its parent star, which has in turn led to the world becoming tidally locked with the stellar body. This means that one side of the exoplanet is constantly being scorched by its star, while another experiences an endless night.
In order to detect the characteristics of the atmosphere on the dayside of the planet, the astronomers directed Hubble to take spectroscopic, infrared data on Kepler-13Ab as it passed behind its parent star relative to Earth. This is known as a secondary eclipse.
Data collected during two of these secondary eclipses revealed that the upper atmosphere of Kepler-13Ab was far cooler than expected based on computer models of hot Jupiter exoplanets.
The scientists behind the research on Kepler-13Ab believe that the unexpected temperature results from a lack of the light-absorbing (and subsequently heat-emitting) gas, titanium oxide, which is often found in the upper atmosphere of hot Jupiter exoplanets.
It is possible that powerful winds are carrying the titanium oxide to the nightside of the planet, where it is able to condense into clouds, and then descend as snow. The crushing gravity of Kepler-13Ab, which is six times as powerful as that of Jupiter, pulls the sunscreen snow down, and traps it in the lower atmosphere.
This process is known as a "cold trap," and the recent observations of Kepler-13Ab represent the first time that the mechanism has been seen at work on an exoplanet.
"In many ways, the atmospheric studies we're doing on hot Jupiters now are testbeds for how we're going to do atmospheric studies on terrestrial, Earth-like planets," comments lead researcher Thomas Beatty of Pennsylvania State University. "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."
The team believes that the cold trap atmospheric process is likely occurring on most known hot Jupiter exoplanets. However, the distribution of material from the dayside to the nightside of a planet is only likely to occur on worlds with a high surface gravity. On less massive worlds, it is likely that the titanium oxide snow would be swept back up, and circulated back to the dayside of the exoplanet, where it would be vaporized and return to its original gaseous state.
So, in conclusion, if you're planning on visiting Kepler-13Ab, bring your own UV protection – the naturally-occurring sunscreen snow is useless because it falls on the wrong side of the planet. Also, just don't go there, because it's a gas giant, and also 1,730 light-years away, which means that even Elon Musk won't be presenting an ambitious plan to get you there any time soon. Probably.
A paper detailing the discovery has been published in The Astronomical Journal.