Researchers from the University of Exeter made the new discovery by observing glowing water molecules in the atmosphere of the exoplanet WASP-121b with NASA’s Hubble Space Telescope.
In order to study the gas giant’s stratosphere, a layer of the atmosphere where temperature increases with higher altitudes scientists used spectroscopy to analyse how the planet’s brightness changed at different wavelengths of light.
Water vapour in the planet’s atmosphere, for example, behaves in predictable ways in response to different wavelengths of light, depending on the temperature of the water.
At cooler temperatures, water vapour in the planet’s upper atmosphere blocks light of specific wavelengths radiating from deeper layers towards space. But at higher temperatures, the water molecules in the upper atmosphere glow at these wavelengths instead.
The phenomenon is similar to what happens with fireworks, which get their colours from chemicals emitting light.
When metallic substances are heated and vaporized, their electrons move into higher energy states. Depending on the material, these electrons will emit light at specific wavelengths as they lose energy: sodium produces orange-yellow and strontium produces red in this process, for example.
The water molecules in the atmosphere of WASP-121b similarly give off radiation as they lose energy, but it is in the form of infrared light, which the human eye is unable to detect.
The research is published in leading scientific journal Nature.