The strongest known evidence has been gathered for a stratosphere on an exoplanet about 900 light years from the Earth, known as WASP-121b. The planet is categorised as a "Hot-Jupiter" exoplanet, and is about 1.2 times the mass of Jupiter.
Jupiter takes 12 years to orbit the Sun, but WASP-121b takes only 1.3 Earth days to revolve around its host star, and is really close to the star. If it were to get any closer, the gravity of the host star would start pulling the planet apart.
The upper atmosphere on the exoplanet is about 2,500 degrees Celsius, hot enough to boil some metals. While previous research indicated the possibility of Hot-Jupiter exoplanets having a stratosphere, the new finding is the best evidence yet as researchers observed a signature of water molecules for the first time. The signature indicated that the water molecules in the upper atmosphere were glowing,
The findings are in line with theoretical models that suggested a class of ultra-hot planets defined by their stratospheres. On the Earth, the Ozone layer traps some of the radiation from the sun, rising the temperatures in the stratosphere.
The stratosphere of Titan, a moon of Jupiter, is heated up by the methane in its atmosphere. On WASP-121b, the temperatures are increasing with the altitude, and follow up observations by Hubble will allow scientists to figure out on which latitudes the behavior is occurring.
WASP-121b is also a prime candidate for observations by the yet to be launched James Webb Space Telescope, the successor to the Hubble Space Telescope, which will be one of the most advanced astronomical instruments deployed.
Hannah Wakeford, co-author of the study says, "This super-hot exoplanet is going to be a benchmark for our atmospheric models, and it will be a great observational target moving into the Webb era." The finding has been reported in Nature.