The planet in question is WASP-121b, a gas giant approximately 900 light-years from Earth. Planets such as WASP-121b are often called "hot Jupiters": large gas giants that are very close to their stars. WASP-121b is as close to its star as it could possibly be without being torn apart, completing a rotation every 1.3 days. Its atmosphere is a balmy 4,500+ degrees Fahrenheit.
WASP-121b was studied by an international team from the University of Exeter, who published their findings in the journal Nature. In order to study the stratosphere of the gas giant, the scientists trained their spectrometer on it to study changes in the planet's brightness.
As water molecules in 121b's atmosphere were found to have a curious glow to them, astronomers began to realize that this was no ordinary Hot Jupiter. "Theoretical models have suggested that stratospheres may define a special class of ultra-hot exoplanets, with important implications for the atmospheric physics and chemistry," said Dr. Tom Evans, lead author and research fellow at the University of Exeter. "When we pointed Hubble at WASP-121b, we saw glowing water molecules, implying that the planet has a strong stratosphere."
The stratosphere contains, among other things, Earth's ozone layer. The much-discussed atmospheric phenomenon absorbs ultraviolet radiation from the sun, which is one of the main reasons life is able to grow and flourish on our planet. Jupiter, and Saturn's moon Titan, have stratospheres too, as do several other objects in the solar system.
But 121b is much closer to its star than anything that orbits our sun. It also has another strange feature: stratospheres are usually hotter than whatever lies underneath them, to the tune of 200 degrees Fahrenheit or so. On WASP-121b, the stratospheric temperature is 1800+ degrees Fahrenheit higher than what lies beneath it.
"We've measured a strong rise in the temperature of WASP-121b's atmosphere at higher altitudes, but we don't yet know what's causing this dramatic heating," says Nikolay Nikolov, co-author and research fellow at the University of Exeter. "We hope to address this mystery with upcoming observations at other wavelengths."
Scientists have never before discovered an exoplanetary stratosphere, and the astronomical community is excited about the potential discovery. "This new research is the smoking gun evidence scientists have been searching for when studying hot exoplanets. We have discovered this hot Jupiter has a stratosphere, a common feature seen in most of our solar system planets." says professor David Sing, co-author and associate professor of Astrophysics at Exeter.
"It's a truly exciting find as we're seeing dramatic differences planet-to-planet which is giving valuable clues in figuring out how planets behave under different conditions, and we're only just scratching the surface of all the new Hubble data."
WASP-121b will also be one of the first subjects of study for NASA's James Webb Space Telescope. "This super-hot exoplanet is going to be a benchmark for our atmospheric models, and will be a great observational target moving into the Webb era," said Hannah Wakeford, an Exeter research fellow who also participated in the research.
