Observations of the gas-giant exoplanet WASP-121b reveal near-infrared emission lines of water, suggesting that the planet has a stratosphere—a layer in the upper atmosphere where temperature increases with altitude.
Infrared radiation emitted from a planet contains information about the chemical composition and vertical temperature profile of its atmosphere1,2,3. If upper layers are cooler than lower layers, molecular gases will produce absorption features in the planetary thermal spectrum4,5. Conversely, if there is a stratosphere—where temperature increases with altitude—these molecular features will be observed in emission6,7,8. It has been suggested that stratospheres could form in highly irradiated exoplanets9,10, but the extent to which this occurs is unresolved both theoretically11,12 and observationally3,13,14,15. A previous claim for the presence of a stratosphere14 remains open to question, owing to the challenges posed by the highly variable host star and the low spectral resolution of the measurements3. Here we report a near-infrared thermal spectrum for the ultrahot gas giant WASP-121b, which has an equilibrium temperature of approximately 2,500 kelvin. Water is resolved in emission, providing a detection of an exoplanet stratosphere at 5σ confidence. These observations imply that a substantial fraction of incident stellar radiation is retained at high altitudes in the atmosphere, possibly by absorbing chemical species such as gaseous vanadium oxide and titanium oxide.