By measuring the position of these spectral lines very precisely, we were able to directly measure the speed of these atoms. The Doppler effect tells us that an atom coming toward us will absorb more blue light, while an atom moving away from us will absorb more red light. By measuring the absorption wavelength of each of these atoms, we have as many different measurements of the wind speed on this planet.
We found that the lines of the different atoms tell different stories. Iron moves at 5 kilometres per second from the substellar point (the region of the planet closest to its host star) to the anti-stellar point (the most distant) in a very symmetrical way. Sodium, on the other hand, splits in two: some of the atoms move like iron, while the others move at the equator directly from east to west four times faster, at the staggering speed of 20 kilometres per second. Finally, hydrogen seems to move with the east-west current of sodium but, also, vertically, no doubt allowing it to escape from the planet.
To reconcile all this, we calculated that these three different atoms are, in fact, in different parts of the atmosphere. While iron atoms lie at the deeper layers, where symmetrical circulation is expected, sodium and hydrogen let us probe much higher layers, where the planet’s atmosphere is blown by the wind coming from its host star. This stellar wind, combined with the rotation of the planet, probably carries the material asymmetrically, with a preferential direction given by the rotation of the planet.
WASP-121-b - atmosphere structure - eso2504c
There are violent winds in the atmosphere of WASP-121b. The three types of atoms travel at different speeds, helping to reconstruct the structure of the atmosphere, even though the planet is millions of billions of kilometres away from Earth. (ESO/M. Kornmesser)
Why study the atmospheres of exoplanets?
WASP-121b is one of those giant gaseous planets with temperatures of over 1,000°C that are known as “hot Jupiters”. The first observation of these planets by Michel Mayor and Didier Queloz (which later earned them a Nobel Prize in Physics) came as a surprise in 1995, particularly because planetary formation models predicted that these giant planets could not form so close to their star. Mayor and Queloz’s observation made us realise that planets do not necessarily form where they are currently located. Instead, they can migrate, i.e., move around in their youth.
How far from their star do “hot Jupiters” form? Over what distances do these objects migrate in their infancy? Why did the Jupiter in our solar system not migrate toward the Sun? (We’re lucky it didn’t, because it would have sent Earth into our star at the same time.)
Some answers to these questions may lie in the atmosphere of exoplanets, which exhibit traces of the conditions of their formation. However, variations in temperature or chemical composition within each atmosphere can radically skew the abundance measurements that we are trying to take with large telescopes such as the James Webb. In order to exploit our measurements, we first need to grasp how complex these atmospheres are.