It Snows Sunscreen On Planet Kepler-13Ab

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 falls 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. Credits: NASA, ESA, and G. Bacon (STScI)

NASA’s Hubble Space Telescope has found a blisteringly hot planet outside our solar system where it snows sunscreen. But only on the planet’s nightside. Because the dayside is spectacularly sunny and blisteringly hot, those visitors would best stop in the shade first, bottle up some of the stuff, then apply heavily before crossing over to the sunny side. Mineral-based sunscreens contain titanium oxide, the very material found in Kepler-13b’s atmosphere. It’s also the material that makes white paint white. The substance works by reflecting away and scattering ultraviolet light, preventing it from reaching your skin and causing sunburn. Titanium oxide is also found in the atmospheres of large, cool stars.

The planet, Kepler-13Ab, 1,730 light years from Earth, is one of the hottest exoplanets known with a daytime temperature of nearly 5,000°F (2,760°C). It’s more than 6 times more massive than Jupiter and orbits very close to its host sun, completing one orbit in just under 2 days. Kepler-13Ab, is a classic “hot Jupiter,”, a massive body sizzling close to its host star and the most commonly found type of exoplanet. Why? Because a massive, close planet causes the light of its host star to fade more dramatically than a tiny planet, and they tug harder on their stars. Both abilities make them much easier to detect.

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. Credit: NASA, ESA, and A. Feild (STScI)

Hubble astronomers suggest that powerful winds carry the titanium oxide gas around to the colder nighttime side, where it condenses into crystalline flakes, forms clouds, and falls as snow. Kepler-13Ab’s strong surface gravity — six times greater than Jupiter’s — pulls the titanium oxide snow out of the upper atmosphere and traps it in the lower atmosphere. Kepler-13Ab is so close to its parent star that it’s “tidally locked” — one side of the planet always faces the star; the other side is in permanent darkness.

Without the titanium oxide gas to absorb incoming starlight on the daytime side, the atmospheric temperature grows colder with increasing altitude. Normally, titanium oxide in the atmospheres of hot Jupiters absorbs light and radiates it away as heat, making the atmosphere grow warmer at higher altitudes. Instead, Kepler-13Ab’s atmosphere was colder than expected, leading astronomers to conclude that the gas had been blown over to the permanently shadowed side. Oh, I almost forgot to mention that the planet is so close to its parent star that it’s “tidally locked” with one side in permanent sunlight and the other in permanent darkness. The difference in temperature makes for blustery winds indeed!

The observations confirm a theory from several years ago that this kind of precipitation could occur on massive, hot planets with powerful gravity.

In science fiction, alien planets are often depicted as places future humans would mine for important metals or use their materials to make water for survival. It’s nice to know that far from Earth, space travelers need never worry about running out of sunscreen.