Observations of the gas-giant exoplanet WASP-121b, reported in Nature this week, suggest that the ultrahot planet has a stratosphere - a layer in the upper atmosphere in which temperature increases with altitude.

Earth’s atmosphere consists of layers that can be delineated by temperature. The stratosphere is warmer than the troposphere, which is just above the surface. Exoplanets could, in theory, have stratospheres, but the only way to determine this is via the analysis of their chemical properties using remote sensing. A previous claim has been made for an exoplanet having a stratosphere, but this remains open to question owing to various challenges, including the high variability of its host star.

Thomas Evans and colleagues report observations of WASP-121b - a gas giant located about 270 parsecs (880 light years) away - made in November 2016 and January 2017 using the Hubble Space Telescope and the Spitzer space telescope, respectively. They report features in the emission lines corresponding to water, which leads them to conclude that the planet has a stratosphere. The authors say the observed temperature inversion is probably caused by the presence of vanadium oxide, with weaker evidence for titanium oxide. The WASP-121b spectrum is the first exoplanet to show spectrally resolved features of a stratosphere in emission.