The international team identified glowing water molecules in the stratosphere of the exoplanet WASP-121b, which is 900 light years from Earth.
Water vapour in a planet's atmosphere behaves in predictable ways depending on its temperature.
While cool water vapour blocks specific wavelengths of light, vapour at higher temperatures causes them to "glow."
An artist's impression issued by the University of Exeter of exoplanet WASP-121b
The radiation was in the form of infrared light indivisible to the human eye which was picked up by the Hubble Space Telescope.
Lead researcher Dr Tom Evans, from the University of Exeter, said: "Theoretical models have suggested that stratospheres may define a special class of ultra-hot exoplanets, with important implications for the atmospheric physics and chemistry.
"When we pointed Hubble at WASP-121b, we saw glowing water molecules, implying that the planet has a strong stratosphere."
WASP-121b is a "hot Jupiter" gas giant that hugs its star so closely one of its years lasts 1.3 days on Earth.
The top of its atmosphere is heated to a blazing 2,500C (4,532F), the point at which iron exists as a gas rather than a solid.
The research is published in the latest issue of the journal Nature.