Life on WASP-33b would basically be hell—the titanic exoplanet’s atmosphere ranges in temperature from a searing 6,000 to a comparatively balmy 3,000 degrees Fahrenheit. But hey, at least you wouldn’t have to bring sunblock.

That’s because WASP-33b contains a familiar atmospheric layer called a stratosphere. And just as Earth’s stratosphere includes a thin sheet of ozone that shields life from damaging UV rays, recent observations using the Hubble Space Telescope show that WASP-33b’s stratosphere probably contains titanium dioxide, an even more powerful type of sunblock.

Earth’s atmosphere is much more than a bubble of air. It’s comprised of distinct layers that differ in some of their most basic physical properties. For instance, the troposphere that we live and breath in cools with increasing altitude. But high up in the overlying stratosphere, we see the opposite pattern. At 33 thousand feet, ozone absorbs UV light and re-radiates its energy into space, causing the temperature to rise with increasing altitude. This so-called ‘temperature inversion’ is also seen in the stratospheres of Jupiter and Saturn, which swap ozone for energy-absorbing hydrocarbons.


But on blazing hot exoplanets like WASP-33b, neither ozone nor hydrocarbons could survive, leading scientists to wonder whether stratospheres would exist at all. Now, we have our first observational evidence that they can.

In a new study, which appeared Friday in the Astrophysical Journal, astronomers took a fresh look at WASP-33b’s atmosphere using Hubble’s Wide Field 3 camera, which snaps a spectrum of near-infrared radiation. From a NIR spectra, astronomers can pick out the telltale signatures of different gases and determine their temperatures. In the case of WASP-33b’s furnace of an atmosphere, scientists used this data to identify a temperature inversion associated with water and titanium dioxide. Titanium dioxide, an active ingredient in sunblock, happens to be one of the few UV-shielding compounds that manages to hold itself together at thousands of degrees Fahrenheit.

“These two lines of evidence together make a very convincing case that we have detected a stratosphere on an exoplanet,” said lead study author Korey Haynes, in a statement.


The discovery is significant for two reasons. For starters, it’s one of the very first instances in which scientists have been able to decode the atmospheric chemistry of a distant world—something we hope to do much more of in the future, as bigger, more powerful telescopes come online. Secondly: even though we’d never want to live on WASP-33b, if and when we do settle on another planet, we’d very much like it to have a layer of sunblock, as well.

It’s a small sign, then, that our second home may be waiting out there.


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Top image: WASP-33b’s stratosphere was detected by measuring the drop in light as the planet passed behind its star (top). Temperatures in the low stratosphere rise because of molecules absorbing radiation from the star (right). Without a stratosphere, temperatures would cool down at higher altitudes (left). NASA / Goddard