WASP-33b's stratosphere was detected through a drop in light as it passed by its own star. Credit: NASA/Goddard
The Hubble Space Telescope has detected a stratosphere, just like Earth’s, on a huge and immensely hot exoplanet called WASP-33b.
The presence of the stratosphere could offer new clues about the formation and composition of the planet.
Molecules in the atmospheric layer absorb ultraviolet and visible light, acting as a ‘sunscreen’ for planets.
Earth has a stratosphere, but scientists had been uncertain whether these molecules could be found in the atmospheres of large, hot planets in other star systems.
In Earth’s atmosphere, the stratosphere is above the troposphere; an active-weather region stretching from the ground to the point where most clouds top out.
In this layer, the temperature is warmer at the bottom, i.e. ground level, and cools down at higher altitudes.
In the stratosphere, temperature increases with altitude. This process, known as temperature inversion, happens due to ozone in the stratosphere absorbing radiation from the Sun and preventing it from reaching the surface.
It is already known that similar temperature inversions occur in the stratospheres of Jupiter and Saturn. In these cases, it they are caused by molecules called hydrocarbons.
However, ozone and hydrocarbons would be unable to survive at the high temperatures of most known exoplanets, meaning it had previously been unclear whether stratospheres could exist on them.
“Some of these planets are so hot in their upper atmospheres, they’re essentially boiling off into space,” says Avi Mandell, a planetary scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and a co-author of the study.
“At these temperatures, we don’t necessarily expect to find an atmosphere that has molecules that can lead to these multilayered structures.”
But a temperature inversion has now been observed in the atmosphere of WASP-33b, about four-and-a-half times the mass of Jupiter. It is also thought that the inversion was caused by titanium oxide in the exoplanet’s atmosphere.
“These two lines of evidence together make a very convincing case that we have detected a stratosphere on an exoplanet,” says Korey Haynes, lead author of the study.
The team looked at observations made with Hubble’s Wide Field Camera 3 by co-author Drake Deming at the University of Maryland in College Park.
This camera can capture a spectrum of the near-infrared region where the signature for water appears.
Using this spectrum to identify water and other gases in a distant planet’s atmosphere enables scientists to determine its temperature.
“Understanding the links between stratospheres and chemical compositions is critical to studying atmospheric processes in exoplanets,” says co-author Nikku Madhusudhan of the University of Cambridge, United Kingdom.
“Our finding marks a key breakthrough in this direction.”