An article published in the journal “Nature” describes the observation of the stratosphere of an exoplanet called WASP-121b, a hot Jupiter with an orbit very close to its star. A team of researchers led by Tom Evans of the University of Exeter used the Hubble Space Telescope to study it and find evidence that it has a stratosphere, a layer of its atmosphere where the temperature gets higher than that of its lower layers. It could be the first exoplanet that was proven to have a stratosphere.

The discovery of the planet WASP-121b was officially announced in March 2016. It’s about 900 light-years away from Earth, with a mass slightly higher than Jupiter’s but its size is almost twice as much. That’s because of the very high temperatures at which the gas that forms it gets heated and inflates increasing its volume.

The class of hot Jupiter planets owes its name to those high temperatures, caused by the proximity of the planet to its star. WASP-121b is so close to its star that its year only lasts 1.27 Earth’s days and the situation is made worse by the fact that the star is a bit more massive and hotter than the Sun.

However, WASP-121b doesn’t seem particularly remarkable for these characteristics because astronomers discovered many hot Jupiters, and at the beginning of June, an article described the discovery of KELT-9b, whose surface temperature is even higher than that of most stars. However, studying WASP-121b with the Hubble Space Telescope, the researchers led by Tom Evans found evidence that the temperature in that planet’s atmosphere at a certain altitude starts rising up to nearly 2,500° Celsius (4,600° Fahrenheit).

Gas giants are mainly composed of hydrogen but even in hot Jupiters several molecules form or come from outside and among them there’s water. On WASP-121b water is in its vapor state and it’s possible to predict what kind of emissions come from it depending on its temperature. Thanks to Hubble’s spectroscopic capabilities, it’s been possible to find evidence that the planet has a stratosphere where water vapor has a temperature higher than that in the atmosphere’s lower layers.

In the past, there were already observations pointing to possible signs of the existence of a stratosphere on other exoplanets such as WASP-33b but in the case of WASP-121b, the evidence is definitely the most convincing found so far. The temperature rise in the stratosphere of this planet is about 560° Celsius (1,000° Fahrenheit), really remarkable. It’s a mystery because it’s been hypothesized that the cause is the presence of vanadium oxide and titanium oxide but specific observations are needed to understand it.

For the time being, the discovery of WASP-121b’s stratosphere could lead to the creation of the ultra-hot planet class. So far there were only models about exoplanets’ stratosphere and a real case is crucial. This is another proof that we’re still at the beginning of the study of the exoplanets and we still have to understand how many kinds there are.