But in the case of WASP-121b, temperatures in the stratosphere increase by about 560 °C (1,000 °F). If the water vapor was at a cooler temperature than the air below it, the vapor would have blocked a certain set of wavelengths; but if the water vapor was hotter, it would glow at those wavelengths, instead.
By examining the wavelengths of light that emits into space from the gas on the planet, researchers can determine how much water vapor there is in the atmosphere and what temperature it likely is. But while Jupiter revolves around our sun once every 12 years, WASP-121b has an orbital period of just 1.3 days.
According to the Exeter team, the upper atmosphere reaches temperatures as high as 2,500° C (4,500° F), which is near the boiling point of iron. In fact, if this exoplanet were any closer to its star, it is estimated that WASP-121's gravity would begin to tear it apart.
Stratospheres are awesome. They're the only layers in low atmospheres that increase in temperature with altitude; even neater, Earth's stratosphere is responsible for the life-enabling ozone layer. As a result, WASP-121b temperatures soar to 4,500 degrees Fahrenheit. At higher temperatures, water molecules radiate or shimmer.
NASA says it's the strongest evidence yet that an exoplanet could have a stratosphere.
"Theoretical models have suggested stratospheres may define a distinct class of ultra-hot planets, with important implications for their atmospheric physics and chemistry", the study's lead author, Tom Evans, said in a press release.
The Hubble Space Telescope has detected a freakish exoplanet with a watery atmosphere that also emits an incandescent glow from its stratosphere.
"When it comes to distant exoplanets, which we can't see in the same detail as other planets here in our own Solar System, we have to rely on proxy techniques to reveal their structure", says astronomer Drake Deming from the University of Maryland.
This light held the key to identifying WASP-121b's stratosphere. 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. "We're excited to explore at what longitudes this behaviour persists with upcoming Hubble observations". Those molecules give off radiation as infrared light as they lose energy. STScI is operated for NASA by the Association of Universities for Research in Astronomy, Inc., in Washington. Jupiter, and Saturn's moon Titan, have stratospheres too, as do several other objects in the solar system.
"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", said Nikolay Nikolov, co-author and research fellow at the University of Exeter.
Artist's concept of "hot Jupiter" exoplanet, a gas giant that orbits very close to its star. Vanadium oxide and titanium oxide are candidates, as they are commonly seen in brown dwarfs.
"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", said Hannah Wakeford, study co-author who worked on this research while at NASA's Goddard Space Flight Center, Greenbelt, Maryland.
An exoplanet which is classified as hot Jupiter, WASP-121b was studied with the analysis of data from NASA's space telescope Hubble.