This artist's concept shows hot Jupiter WASP-121b, which presents the best evidence yet of a stratosphere on an exoplanet. Credits: Engine House VFX, At-Bristol Science Centre, University of ExeterA gas giant exoplanet that orbits close to its parent star has been found to have a stratosphere–an upper atmosphere in which temperatures increase at higher altitudes–by a team of scientists who observed it using the Hubble Space Telescope.
Approximately 880 light years from Earth, planet WASP-121b, nearly twice the size of Jupiter and 1.2 times as massive, orbits its star once every 1.3 days and has surface temperatures close to 4,500 degrees Fahrenheit (2,500 degrees Celsius).
Led by Thomas Evans of the University of Exeter in England, the researchers, who observed the planet with Hubble, detected spectral signals of water molecules in its stratosphere.
Earlier studies have detected evidence of stratospheres on other exoplanets, but this is the first time anyone has found an exoplanet stratosphere with hot water molecules.
“Theoretical models have suggested stratospheres may define a distinct class of ultra-hot planets, with important implications for their atmospheric physics and chemistry,” Evans stated. “Our observations support this picture.”
Using spectroscopy, the researchers studied the reactions of different molecules in the planet’s stratosphere when exposed to various wavelengths of light.
As light from the parent star enters the planet’s atmosphere, it heats up the gases there, which then radiate heat into space in the form of infrared light.
If the water vapor in the stratosphere is cooler, it can keep specific wavelengths of light from radiating into space. In contrast, warm temperatures at the top of the stratosphere cause water molecules to glow in the same wavelengths to which they are exposed.
“The emission of light from water means the temperature is increasing with height,” explained Tiffany Kataria of NASA’s Jet Propulsion Laboratory in Pasadena, California, who took part in the study.
Warm stratospheres have already been found on planets in our own solar system. This discovery will enable scientists to compare the processes that occur in exoplanets’ stratospheres with those that occur on solar system planets, noted study participant Mark Marley of NASA’s Ames Research Center in California.
A key difference between stratospheres in our solar system and that of WASP-121b is the degree of temperature changes. For most planets in our solar system, stratosphere temperature rises by at most 100 degrees Fahrenheit (56 degrees Celsius).
On WASP-121-b, stratosphere temperature rises by as much as 1,000 degrees Fahrenheit (560 degrees Celsius). While scientists are still uncertain as to which chemicals cause such a large increase, they suspect vanadium oxide and titanium oxide, both of which are found in brown dwarfs.
Findings of the study, for which Evans is lead author and Kataria, Marley, and other scientists are co-authors, have been published in the journal Nature.
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