New research reveals the structure of violent winds 1,300 light years away

Planet WASP-121B is extreme. It is a gas giant nearly twice the size of Jupiter, orbiting around very close to the stars, nearly 50 times more than the Earth does around the Sun. The WASP-121B is so close to that star that it locked its rotation with a “resonance.” Planets always show the same face to stars, like the moon on Earth. Therefore, one side of the WASP-121B always burns light, while the other one is an eternal night. This difference causes large variation in the temperature of the entire planet. You can drop above 3,000°C on one side and 1,500°C on the other side.

This enormous temperature contrast is a source of violent winds, blowing away several kilometers per second, attempting to redistribute energy from day to night. Previously, it was necessary to infer wind strength and direction through indirect measurements, such as measuring planetary temperatures. In recent years, new equipment arrived at the giant telescope and it was able to directly measure wind speeds on certain deplanets, including the WASP-121B.

In our study published in Nature, conducted by my colleague Julia Seidel, we not only looked at wind speeds in the explanet, but also how these winds change with altitude. For the first time, we were able to measure that the wind in the deepest layers of the atmosphere is very different from those of the higher layers. In this way, on Earth, when winds blow tens of kilometers per hour, it’s already harder to ride a bike. The WASP-121B winds 100 times faster, so pedaling is not possible.

Our measurements reveal the behavior of pivotal zones of the atmosphere that form the link between deep atmospheres. This is an outer zone that is frequently investigated by telescopes such as the James Webspace telescope, where the atmosphere escapes into space and blows away by the wind coming from the stars.

How did you measure the atmosphere of a planet millions of kilometers away?

To make the measurements, we used one of the most accurate spectrographs on Earth attached to the largest telescope available to us: the espresso of a very large telescope (VLT) in the Atacama Desert, Chile. To collect as much light as possible, we combined four 8-meter diameter telescopic light from the VLT. Thanks to this combination, which is still being tested, we collected as much light as a 16-meter-diameter telescope. This is bigger than any optical telescope on Earth.

Later, a super-advanced espresso spectrograph allowed light to be separated from the planet to 1.3 million wavelengths. This allows you to observe many colors within the visible spectrum. This accuracy is required to detect different types of atoms in the planet’s atmosphere. This time, how did you study three different types of atoms from the star, absorbed light from the star, hydrogen, sodium and iron (all in gaseous state given the very high temperatures)?

By measuring the position of these spectral lines very accurately, we were able to directly measure the velocity of these atoms. The Doppler effect shows that atoms coming towards us absorb more blue light, and atoms leaving us absorb more red light. By measuring the absorption wavelength of each of these atoms, there are various measurements of the wind speed of this planet.

I learned that different lines of atoms tell different stories. Iron travels from the sub-therapoint (the region of the planet closest to the host star) to the anti-therapoint (the farthest) in a very symmetrical way, at 5 km. Sodium, on the other hand, is divided into two parts. Some atoms move like iron, while others move four times faster along the equator directly from east to west, moving at an incredible speed of 20 kilometers per second. Finally, hydrogen appears to be moving along with the east-west current of sodium, but it also undoubtedly escapes the planet vertically.

To adjust all of this, we calculated that these three different atoms are actually different parts of the atmosphere. While iron atoms lie deeper layers than are expected for symmetric circulation, sodium and hydrogen allow them to probe much higher layers where the planet’s atmosphere is blown away by the wind coming from the host star. This star wind, combined with the rotation of the planet, carries the material asymmetrically, perhaps giving it a preferential orientation through the rotation of the planet.

Why study the atmosphere of exoplanets?

WASP-121B is one of the giant gaseous planets with temperatures above 1,000°C, known as “Hot Jupiter.” The first observations of these planets by Mayor Michel and Didier Quarz (who later won the Nobel Prize in Physics) came as a surprise, especially as planetary formation models predicted that these giant planets could not form very close to their stars. The mayor and Queloz’s observations made us realize that the planets do not necessarily form in their current location. Instead, they can move around when they were younger, meaning they can move around.

How far is “Hot Jupiter” from their stars? What distance do these objects travel in the early stages? Why did Jupiter in the solar system not move towards the Sun? (We weren’t lucky because we were sending Earth to our planet at the same time.)

Some answers to these questions may lie within the exoplanet atmosphere, which shows traces of the conditions of their formation. However, variations in temperature or chemical composition in each atmosphere can fundamentally distort the abundance of measurements that are being ingested by a larger telescope, such as James Webb. To utilize measurements, you first need to understand how complex these atmospheres are.

To do this, you need to understand the basic mechanisms that govern the atmosphere of these planets. In the solar system, wind can be measured directly by, for example, by looking at how fast the clouds move. Details cannot be viewed directly on explanets.

In particular, “Hot Jupiter” is closer to a star, and the closer it is to a star, it cannot be spatially separated and photographed by exoplanets. Instead, out of thousands of known exoplanets, we choose one that has a good taste that regularly passes between their stars and us. During this “transit”, light from the star is filtered by the planet’s atmosphere, allowing you to measure the signs of absorption by various atoms or molecules. In general, the data we obtain is not sufficient to separate the light across one side of the planet from the other, and is the average of what air absorbs. Interpreting the final average is often a headache, as conditions along the limbs of the atmosphere (i.e., slices of the atmosphere surrounding the planet observed from space) can change dramatically.

This time, by using a telescope that is practically larger than other optical telescopes on Earth, it was possible to combine it with a very accurate spectrometer to separate the signal absorbed to the east of the Earth’s limb from the signal absorbed to the west. This allowed us to measure the spatial variation of planetary winds.

The Future of Atmospheric Research at Xplanet

Europe is currently building the next generation of telescopes led by ESO’s extremely large telescopes scheduled for 2030. The ELT has a mirror twice as large as the telescope, 30 meters in diameter, obtained by combining light from the four 8 meters telescopes of VLT.

This giant telescope collects more accurate details about the exoplanet’s atmosphere. In particular, it measures the winds of exoplanets, smaller and colder than “Hot Jupiter.”

But what we all really awaits is the ability of ELT to measure the presence of molecules in the atmosphere of rocky planets orbiting in habitable zones of stars where water is present in liquid state.

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