This exoplanet's extreme climate challenges atmospheric models

This exoplanet's extreme climate challenges atmospheric models
by Robert Schreiber
Berlin, Germany (SPX) Feb 20, 2025

An international team of astronomers has conducted an unprecedentedly precise observation of the exoplanet WASP-121b by utilizing the combined power of the four telescopes in the Very Large Telescope (VLT) array. Their findings reveal one of the most extreme planetary climates ever recorded, challenging current atmospheric models.

WASP-121b, a well-studied ultra-hot Jupiter, is known for its extreme conditions, including clouds of metal vapor. Researchers from the University of Geneva's (UNIGE) Department of Astronomy and the PlanetS National Research Center, in collaboration with the European Southern Observatory (ESO), leveraged the VLT to analyze multiple atmospheric layers of this gas giant. Their observations, published in *Nature*, unveiled the presence of powerful and unexpected atmospheric winds, fundamentally altering theoretical understandings of planetary weather systems.

Ultra-hot Jupiters, similar in mass to Jupiter but orbiting very close to their host stars, receive thousands of times the solar radiation that Earth experiences. These extreme conditions create uniquely severe climates, making such exoplanets crucial subjects for testing atmospheric and climate models. WASP-121b, also called Tylos, is located about 900 light-years away in the constellation Puppis. The planet completes an orbit in just 30 hours, transiting its star nearly daily.

During these transits, astronomers directed ESO's VLT, situated on Mount Paranal in Chile's Atacama Desert, at the exoplanet. The starlight filtering through WASP-121b's atmosphere was collected by four eight-meter-diameter mirrors and transmitted via optical fibers to the ESPRESSO instrument. This spectrograph, developed under UNIGE's supervision and installed in 2018, decomposes starlight into its component colors, allowing for highly detailed atmospheric analysis.

"ESPRESSO is a spectrograph, essentially a 'rainbow machine' that breaks down light into distinct colors. The more light we gather, the more refined our observations become. By combining four large telescopes, ESPRESSO achieves an unprecedented level of precision," explained David Ehrenreich, associate professor in UNIGE's Department of Astronomy and co-author of the study. "This allows us to explore this exoplanet's atmosphere in greater detail than ever before."

Through their analysis, the researchers mapped WASP-121b's atmospheric circulation, tracking the movement of iron, sodium, and hydrogen vapors at different altitudes. This produced a three-dimensional climate model that contradicts previous theoretical predictions. "This planet's atmosphere defies expectations, presenting behavior that challenges our current understanding of planetary weather patterns," said Julia Victoria Seidel, lead researcher on the study, formerly a doctoral student at UNIGE and now a scientist at ESO and the Lagrange laboratory in France.

One of the study's most striking discoveries was a powerful updraft that lifts iron vapors from the intensely heated dayside and carries them to the cooler nightside. This phenomenon is overshadowed by a high-speed jet stream transporting air around the planet's equator at intermediate altitudes. "Our models had predicted these layers would be arranged in the opposite order," Seidel remarked.

At the highest atmospheric levels, gases are further stirred and accelerated by the jet stream, eventually allowing lighter elements such as hydrogen to escape the planet's gravitational pull. "Even Neptune's ferocious winds, which reach 2,000 km/h, pale in comparison to WASP-121b's extreme gales, which reach speeds of 70,000 km/h," Seidel added.

These findings highlight the limits of existing planetary circulation models. Ultra-hot Jupiters, with their extreme conditions, push these models beyond their intended applications, necessitating new approaches that account for additional physical processes.

Future studies of exoplanetary climates, particularly those of potentially habitable worlds, will require even more advanced observational tools. The upcoming Extremely Large Telescope (ELT), currently under construction on Mount Armazones in Chile, is expected to provide groundbreaking insights by 2030. "With its 39-meter mirror, the ELT will usher in a new era of planetary research," Ehrenreich said. UNIGE is a key partner in the development of ANDES, the high-resolution spectrograph for the ELT. "ANDES will enable us to investigate Earth-like climates, and we anticipate many more surprises ahead."

Research Report:Vertical structure of an exoplanet's atmospheric jet stream