Scientists have discovered the strongest evidence to date for a stratosphere, a mid-level layer of atmosphere in which temperature increases with altitude, on a planet outside our Solar System, or exoplanet.
The stratosphere was found around a hot Jupiter exoplanet, WASP-121b, using data taken by the Hubble Space Telescope.
With a mass 1.2 times that of Jupiter and a radius about 1.9 times Jupiter’s, Hubble discovered that WASP-121b had high temperatures in its stratosphere relative to its lower atmosphere.
“This result is exciting because it shows that a common trait of most of the atmospheres in our solar system – a warm stratosphere – also can be found in exoplanet atmospheres,” said Mark Marley, study co-author based at NASA’s Ames Research Center.
Reporting in the journal Nature, scientists were also able to determine that the exoplanet has an orbital period of a mere 1.3 days.
This exoplanet is so close to its star that its outer atmosphere is heated to a blazing 2,500 °C, hot enough to boil lead.
“This super-hot exoplanet is going to be a benchmark for our atmospheric models,” said Hannah Wakeford, study co-author who worked on this research while at NASA’s Goddard Space Flight Center.
Although previous research found possible signs of stratospheres on other hot Jupiters including WASP-33b, the new study presents the best evidence yet, as researchers found the signature of hot water molecules for the first time.
To study the stratosphere of WASP-121b, scientists analysed the wavelengths of light emitted by the water vapour in the exoplanet’s atmosphere relative to the ambient atmospheric temperature.
Able to determine the exoplanets atmospheric temperature from its proximity to its star, researchers noted that the infrared radiation reaching Hubble glowed at the same wavelength as expected from water vapour in the inner atmosphere.
Such signatures are only possible if the water molecules at the top of the atmosphere have a higher temperature than the molecules in the lower atmosphere.
If not, then the outgoing radiation will be reflected, preventing it from escaping out into space and being observed.
“The emission of light from water means the temperature is increasing with height,” said Tiffany Kataria, study co-author based at NASA’s Jet Propulsion Laboratory.