• 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.

Source:TH