Blazing hot exoplanet has 'sunscreen' layer
NASA/Goddard

Blazing hot exoplanet has 'sunscreen' layer

Astronomers have found a stratosphere—a protective atmospheric layer—around a blisteringly hot exoplanet once thought to be too toasty to have one, they report in a paper to be published in The Astrophysical Journal. On Earth, ozone in the stratosphere provides the “sunscreen” that allows life to exist: It absorbs harmful ultraviolet light from the sun and converts it into heat. Planets like Jupiter and Saturn also have stratospheres that convert UV with hydrocarbons instead of ozone. But neither ozone nor hydrocarbons can survive around the hot, giant planets often found close to other stars. Now, a team of astronomers studying WASP-33b, an exoplanet 4.5 times the mass of Jupiter, found that it, too, has a key signal of a stratosphere: temperature inversion. Because the UV-converting molecules in the upper layers of the stratosphere bear the full brunt of the sun’s rays, the upper layers are warmer than the bottom layers. So temperature rises with higher altitude, the opposite of what happens in Earth’s troposphere (the layer directly under the stratosphere where weather happens). In the case of WASP-33b, researchers used images from the Hubble Space Telescope to detect light from different altitudes in the planet’s atmosphere as it passed behind its parent star. Low down in the atmosphere, the light showed a signal of water at 1600°C, whereas higher up it was aabout 3200°C. What was causing the temperature inversion? Titanium oxide detected in the atmosphere, the team guesses—one of only a few compounds that can both absorb UV and exist as a gas without breaking up at such a high temperature.