On This Planet, It Snows Sunscreen

Though Kepler-13Ab doesn't have an moon like the one in this image, it does have white snow — made from titanium dioxide — that falls from the sky. . (Photo: Mexrix/Shutterstock)

If you decided to explore the planet Kepler-13Ab — let's imagine you had already donned a special suit to protect you from its 5,000-degree Fahrenheit surface temps — the days would literally be endless. That's because the planet is located so close to its nearest star that it is tidally locked, which means that one side of the planet always faces its sun and the other is forever shrouded in darkness. So day and night only exist if you travel around the planet from the light to dark sides.

But the fact that this planet doesn't rotate is hardly the weirdest thing about it. If you were to walk to the always-dark side of Kepler-13Ab, you could do some snowshoeing through eternal night — except that the snow would be composed of titanium dioxide, not hydrogen and oxygen. Titanium dioxide is the same white stuff that blocks the sun's rays in certain kinds of sunscreen. But on Kepler-13Ab it falls as snow on the dark side of this planet — something that scientists have just discovered using the Hubble Space Telescope.

Sound bizarre? Well, once you know the science behind this alternative snow, it makes sense.

The observations were first made by a team at Pennsylvania State University, who are studying the weather on exoplanets to learn more about what makes a planet habitable to life (including human life). Their recently published study on the unique weather events on Kepler 13-Ab are test cases for looking at and understanding other planets.

"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," Thomas Beatty, an assistant professor of astronomy at Penn State told Space Daily.

Caused by a 'cold trap'

Scientists attribute the unique snow to a "cold trap." Kepler 13-Ab's atmosphere is cooler at higher altitudes, which is unusual. On other similar planets, gaseous titanium dioxide helps warm the upper atmosphere, but on this planet, it gets blown around by high winds and cannot affect the atmospheric temperature gradient.

When the titanium dioxide gets pushed higher into the atmosphere on the cold side of planet, it condenses into clouds of crystal-shaped flakes. The gravity of the planet (which is six times larger than Jupiter, so its gravity is also six times stronger) pulls the heavier molecules out of suspension and down to the ground in a snowstorm of sorts. There, it eventually gets blown up into the atmosphere on the planet's hot side, where it forms a gas and goes through the whole cycle again.

Scientists figured this all out using data obtained from doing spectrographic measurements with Hubble's Wide Field Camera 3. Measurements taken during a special transit allowed them to get information on temperature on the planet — even though it's 1,730 light years from Earth.