What’s Hiding in the Sky of This Extreme Exoplanet?
Winds Faster Than a Bullet? The Extreme Storms of WASP-121b
WASP-121b is no ordinary planet. This massive gas giant, nearly twice the size of Jupiter, orbits its host star at a distance 50 times closer than Earth does around the Sun. This extreme proximity creates a planetary environment unlike any other—one side is in constant daylight, while the other remains trapped in perpetual darkness. The result? A staggering temperature contrast that fuels powerful atmospheric winds, reaching speeds of several kilometers per second.
Locked in Perpetual Heat and Darkness
Due to the planet’s tight orbit, tidal forces have locked its rotation into resonance, meaning one hemisphere always faces the star while the other remains in darkness. The temperature on the star-facing side can soar above 3,000°C, while the night side plunges by over 1,500°C. This imbalance sets the stage for extreme weather patterns, as high-speed winds work relentlessly to redistribute heat across the planet’s atmosphere.
Measuring the Unimaginable: Tracking Winds from Afar
Until recently, wind speeds on exoplanets could only be estimated through indirect temperature measurements. However, with advancements in astronomical instruments, direct measurements of WASP-121b’s atmospheric winds have now become possible. Using one of the world’s most advanced spectrographs—ESPRESSO, mounted on the Very Large Telescope (VLT) in Chile—researchers have managed to study how wind speeds vary with altitude, uncovering a dynamic and complex system of atmospheric layers.
A Layered Atmosphere: Different Winds at Different Heights
By analyzing the spectral signatures of various atoms in the atmosphere, scientists discovered a striking pattern. Each type of atom moves at different speeds and in distinct directions, depending on its altitude within the planet’s atmosphere:
Iron (Fe): Found in the deeper layers, iron moves symmetrically from the intensely heated substellar point to the cooler anti-stellar side at a speed of 5 kilometers per second.
Sodium (Na): This element behaves in two separate ways. Some sodium atoms move similarly to iron, while others are carried along the equator at an astonishing 20 kilometers per second, moving from east to west.
Hydrogen (H): Unlike the others, hydrogen atoms follow the east-west currents of sodium but also exhibit vertical motion, likely enabling them to escape into space.
The Role of Stellar Winds and Planetary Rotation
These variations suggest that WASP-121b’s atmosphere is influenced not only by the planet’s rotation but also by powerful stellar winds from its host star. The upper atmosphere is being stretched and shaped by these external forces, creating asymmetrical flows that disrupt the expected atmospheric circulation. The planet’s rapid rotation further adds complexity, shaping a highly dynamic and ever-changing weather system.
Unraveling the Secrets of an Alien Climate
Studying WASP-121b’s atmosphere is more than just an exercise in curiosity; it provides crucial insights into the behavior of extreme planetary environments. By probing different layers of its atmosphere, scientists are beginning to understand how gas giants respond to intense stellar radiation, extreme temperature gradients, and powerful winds. As telescope technology advances, even more precise measurements will shed light on how atmospheres behave on other distant worlds, bringing us closer to understanding the diversity of planetary climates beyond our solar system.
A Window into the Future of Exoplanet Exploration
The ability to measure and analyze winds on WASP-121b marks a major milestone in exoplanetary research. By combining cutting-edge spectrography with the power of the VLT, astronomers are uncovering the hidden dynamics of alien atmospheres. As observational techniques improve, the mysteries of distant worlds will continue to unfold, revealing more about the complex and extreme nature of planets beyond our own solar system.
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 realize 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 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 of 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 meters in diameter, twice the size of the telescope we obtained by combining the light from the four 8-meter 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.
Source: What’s Hiding in the Sky of This Extreme Exoplanet?
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What’s Hiding in the Sky of This Extreme Exoplanet?