The planet WASP-121b is extreme. It’s a gas giant almost twice as big as Jupiter orbiting extremely close to its star–50 times closer than the Earth does around the Sun. WASP-121b is so close to its star that tidal forces have locked its rotation in a “resonance”: the planet always shows the same face to its star, like the Moon to the Earth. Therefore, one side of WASP-121b constantly bakes in light whereas the other is in perpetual night. This difference causes huge variations in temperature across the planet. It can be more than 3,000°C on one side and drop 1,500°C on the other.
This huge temperature contrast is the source of violent winds, blowing several kilometres per second, which try to redistribute the energy from day to night. Until now, we had to guess the strength and direction of the winds with indirect measurements, such as measurements of the planet’s temperature. In recent years, with the arrival of new instruments on giant telescopes, we’ve been able to directly measure the wind speed of certain exoplanets, including WASP-121b.
In our study published in the journal Nature that was conducted by my colleague, Julia Seidel, we not only looked at wind speed on an exoplanet, but also at how these winds vary with altitude. We were able to measure for the first time that winds in the deepest layers of the atmosphere are very different from those at higher altitudes. Put it this way: on Earth, winds blowing a few dozen kilometres per hour already make it hard to ride a bike; on WASP-121b, pedalling would be impossible, because the winds are a hundred times faster.
Our measurements reveal the behaviour of a pivotal zone of the atmosphere that forms the link between the deep atmosphere–usually surveyed by telescopes such as the James Webb Space Telescope–and the outer zones where the atmosphere escapes into space, blown by the wind coming from its star.
How did we measure the atmosphere of a planet millions of billions of kilometres away?
To make our measurements, we used one of the most precise spectrographs on Earth, mounted on the largest telescope available to us: ESPRESSO at the European Southern Observatory (ESO) Very Large Telescope (VLT), located in the Atacama desert in Chile. To collect as much light as possible, we combined the light from the VLT’s four 8-metre diameter telescopes. Thanks to this combination, which is still being tested, we collected as much light as would a 16-metre diameter telescope–which would be larger than any optical telescope on Earth.
The ultra-precise ESPRESSO spectrograph then enabled us to separate the light from the planet into 1.3 million wavelengths. This allows us to observe as many colours in the visible spectrum. This precision is necessary to detect different types of atoms in the planet’s atmosphere. This time, we studied how three different types of atoms–absorb light from the star: hydrogen, sodium and iron (all in a gaseous state, given the very high temperatures).
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.
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.
To do this, we need to understand the fundamental mechanisms that govern the atmosphere of these planets. In the solar system, winds can be measured directly by, for example, looking at how fast clouds move. On exoplanets, we cannot see any details directly.
In particular, “hot Jupiters” orbit so close to their stars that we cannot separate them spatially and take photos of the exoplanets. Instead, from among the thousands of known exoplanets, we select those that have the good taste to periodically pass between their star and us. During this “transit”, light from the star is filtered by the planet’s atmosphere, which allows us to measure the signs of absorption by different atoms or molecules. In general, the data we obtain are not good enough to separate the light that passes on one side of the planet from the other, and we end up with an average of what the atmosphere has absorbed. As conditions along the atmospheric limb (i.e., the slice of atmosphere surrounding a planet as observed from space) can vary drastically, interpreting the final average is often a headache.
This time, by using a telescope that, in effect, is larger than any other optical telescope on Earth, and combining it with an extremely precise spectrograph, we were able to separate the signal absorbed by the eastern side of the planet’s limb from the signal absorbed by the western side. This allowed us to measure the spatial variation of the winds in the planet.
The future of atmospheric study of exoplanets
Europe is currently building the next generation of telescopes, led by the ESO’s Extremely Large Telescope, which is scheduled for 2030. The ELT will have a mirror 30 metres in diameter, twice the size of the telescope we obtained by combining the light from the four 8-metre telescopes of the VLT.
This giant telescope will gather even more precise details about the atmospheres of exoplanets. In particular, it will measure the winds in exoplanets both smaller and colder than “hot Jupiters”.
But what we are all really waiting for is the ELT’s ability to measure the presence of molecules in the atmosphere of rocky planets orbiting in the habitable zone of their star, where water may be present in a liquid state.
The EXOWINDS project is supported by the French National Research Agency (ANR), which funds project-based research in France. Its mission is to support and promote the development of fundamental and applied research in all disciplines, and to strengthen the dialogue between science and society. For more information, visit the ANR website.
Vivien Parmentier received funding from the French National Research Agency (exowinds, ANR-23-CE31-0001-01).
Julia Victoria Seidel is an ESO (European Southern Observatory) Research Fellow.
This article was originally published on The Conversation. Read the original article.
