The presence of a stratosphere can provide clues about the composition of a planet and how it formed. This atmospheric layer includes molecules that absorb ultraviolet and visible light, acting as a kind of ‘sunscreen’ for the planet it surrounds. Until now, scientists were uncertain whether these molecules would be found in the atmospheres of large, extremely hot planets in other star systems.

The results are published today (12 June) in The Astrophysical Journal.

“Detecting the presence of a stratosphere in an exoplanet and the chemical compound causing it is a major advancement in our ability to study exoplanetary atmospheres,” said co-author Dr Nikku Madhusudhan of the Institute of Astronomy at Cambridge.

In Earth’s atmosphere, the stratosphere sits above the troposphere – the turbulent, active-weather region that reaches from the ground to the altitude where nearly all clouds top out. In the troposphere, the temperature is warmer at the bottom – ground level – and cools down at higher altitudes.

The stratosphere is just the opposite. In this layer, the temperature increases with altitude, a phenomenon called temperature inversion. On Earth, temperature inversion occurs because ozone in the stratosphere absorbs much of the sun’s ultraviolet radiation, preventing it from reaching the surface, protecting the biosphere, and therefore warming the stratosphere instead.

Similar temperature inversions occur in the stratospheres of other planets in our solar system, such as Jupiter and Saturn. In these cases, the culprit is a different group of molecules called hydrocarbons. Neither ozone nor hydrocarbons, however, could survive at the high temperatures of most known exoplanets, which are planets outside our solar system. This leads to a debate as to whether stratospheres would exist on them at all.

“Some of these planets are so hot in their upper atmospheres, they’re essentially boiling off into space,” said 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 multi-layered structures.”

Using NASA’s Hubble Space Telescope, the researchers have settled this debate by identifying a temperature inversion in the atmosphere of WASP-33b, which has about four-and-a-half times the mass of Jupiter. Team members also think they know which molecule in WASP-33b’s atmosphere caused the inversion – titanium oxide.

“These two lines of evidence together make a very convincing case that we have detected a stratosphere on an exoplanet,” said Korey Haynes, lead author of the study. Haynes was a graduate student at George Mason University in Fairfax, Virginia, and was working at Goddard with Mandell when the research was conducted.

The researchers analysed observations made with Hubble’s Wide Field Camera 3 by co-author Drake Deming at the University of Maryland. Wide Field Camera 3 can capture a spectrum of the near-infrared region where the signature for water appears. Scientists can use the spectrum to identify water and other gases in a distant planet’s atmosphere and determine its temperature.

Haynes and her colleagues used the Hubble observations, and data from previous studies, to measure emission from water and compare it to emission from gas deeper in the atmosphere. The team determined that emission from water was produced in the stratosphere at about 3300 degrees Celsius. The rest of the emission came from gas lower in the atmosphere that was at a temperature about 1650 degrees Celsius.

The team also presented the first observational evidence that WASP-33b’s atmosphere contains titanium oxide, one of only a few compounds that is a strong absorber of visible and ultraviolet radiation and capable of remaining in gaseous form in an atmosphere as hot as this one.

“Understanding the links between stratospheres and chemical compositions is critical to studying atmospheric processes in exoplanets,” said Madhusudhan. “Our finding marks a key breakthrough in this direction.”

Inset image: NASA scientists detected a stratosphere and chemical compounds on WASP-33b by measuring light emitted from the dayside atmosphere of the planet observed as it passed behind its star (top). Temperatures in the stratosphere increase with height (right) because of molecules absorbing radiation from the star entering from the top and reemitting it locally; otherwise, temperatures would cool down at higher altitudes (left). Credit: NASA/GSFC