Discovered in 2016 by astronomers at the WASP-South survey, WASP-121b is 1.87 times bigger than Jupiter and 1.18 times more massive.
Its host star, WASP-121 (TYC 7630-352-1), is an active F6-type main-sequence star about 1.5 times the size of the Sun.
The WASP-121 system is located 881 light-years away in the constellation Puppis.
WASP-121b is a so-called ‘hot Jupiter’ and takes just 1.3 days to orbit WASP-121. It is so close to the parent star that if it got any closer, the star’s gravity would start ripping it apart.
Astronomers estimate the planet’s temperature to be about 4,600 degrees Fahrenheit (2,500 degrees Celsius), hot enough to boil some metals.
Previous research found possible signs of a stratosphere on WASP-33b as well as some other hot Jupiters.
The new study presents the best evidence yet because of the signature of hot water molecules that astronomers observed for the first time.
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.
Theoretical models have suggested stratospheres may define a distinct class of ultra-hot planets, with important implications for their atmospheric physics and chemistry.
These observations support this picture.
To study the stratosphere of WASP-121b, the team observed a secondary eclipse of the planet using the Wide Field Camera onboard the NASA/ESA Hubble Space Telescope and the Infrared Array Camera onboard NASA’s Spitzer space telescope.
The researchers analysed how different molecules in the stratosphere react to particular wavelengths of light.
Water vapour in the planet’s atmosphere, for example, behaves in predictable ways in response to certain wavelengths of light, depending on the temperature of the water.
Starlight is able to penetrate deep into a planet’s atmosphere, where it raises the temperature of the gas there.
This gas then radiates its heat into space as infrared light.
However, if there is cooler water vapour at the top of the atmosphere, the water molecules will prevent certain wavelengths of this light from escaping to space.
But if the water molecules at the top of the atmosphere have a higher temperature, they will glow at the same wavelengths.
The emission of light from water means the temperature is increasing with height.
In Earth’s stratosphere, ozone gas traps ultraviolet radiation from the Sun, which raises the temperature of this layer of atmosphere.
Other solar system bodies have stratospheres, too; methane is responsible for heating in the stratospheres of Jupiter and Saturn’s moon Titan, for example.
In solar system planets, the change in temperature within a stratosphere is typically around 100 degrees Fahrenheit (about 56 degrees Celsius).
On WASP-121b, the temperature in the stratosphere rises by 1,000 degrees Fahrenheit (560 degrees Celsius).
The astronomers do not yet know what chemicals are causing the temperature increase in WASP-121b’s atmosphere.
Vanadium oxide and titanium oxide are candidates, as they are commonly seen in brown dwarfs.
Such compounds are expected to be present only on the hottest of hot Jupiters, as high temperatures are needed to keep them in a gaseous state.
