3D map of exoplanet atmosphere shows wacky climate

"This planet’s atmosphere behaves in ways that challenge our understanding of how weather works."

Astronomers have detected over 5,800 confirmed exoplanets. One extreme class is ultra-hot Jupiters, of particular interest because they can provide a unique window into planetary atmospheric dynamics. According to a new paper published in the journal Nature, astronomers have mapped the 3D structure of the layered atmosphere of one such ultra-hot Jupiter-size exoplanet, revealing powerful winds that create intricate weather patterns across that atmosphere. A companion paper published in the journal Astronomy and Astrophysics reported on the unexpected identification of titanium in the exoplanet's atmosphere as well.

As previously reported, thanks to the massive trove of exoplanets discovered by the Kepler mission, we now have a good idea of what kinds of planets are out there, where they orbit, and how common the different types are. What we lack is a good sense of what that implies in terms of the conditions on the planets themselves. Kepler can tell us how big a planet is, but it doesn't know what the planet is made of. And planets in the "habitable zone" around stars could be consistent with anything from a blazing hell to a frozen rock.

Like the Transiting Exoplanet Survey Satellite (TESS), Kepler identifies planets using the transit method. This works for systems in which the planets orbit in a plane that takes them between their host star and Earth. As this occurs, the planet blocks a small fraction of the starlight that we see from Earth (or nearby orbits). If these dips in light occur with regularity, they're diagnostic of something orbiting the star.

The frequency of the dips in the star's light tells us how long an orbit takes, which tells us how far the planet is from its host star. That, combined with the host star's brightness, tells us how much incoming light the planet receives, which will influence its temperature. (The range of distances at which temperatures are consistent with liquid water is called the habitable zone.) And we can use that, along with how much light is being blocked, to figure out how big the planet is.

Last year, astronomers discovered an unusual Earth-size exoplanet they believe has a hemisphere of molten lava, with its other hemisphere tidally locked in perpetual darkness. And at about the same time, a different group discovered a rare small, cold exoplanet with a massive outer companion 100 times the mass of Jupiter.

Meet Tylos

The different layers of the atmosphere on WASP-121b.

This latest research relied on observational data collected by the European South Observatory's (ESO) Very Large Telescope, specifically, a spectroscopic instrument called ESPRESSO that can process light collected from the four largest VLT telescope units into one signal. The target exoplanet, WASP-121b—aka Tylos—is located in the Puppis constellation about 900 light-years from Earth. One year on Tylos is equivalent to just 30 hours on Earth, thanks to the exoplanet's close proximity to its host star. Since one side is always facing the star, it is always scorching, while the exoplanet's other side is significantly colder.

Those extreme temperature contrasts make it challenging to figure out how energy is distributed in the atmospheric system, and mapping out the 3D structure can help, particularly with determining the vertical circulation patterns that are not easily replicated in our current crop of global circulation models, per the authors. For their analysis, they combined archival ESPRESSO data collected on November 30, 2018, with new data collected on September 23, 2023. They focused on three distinct chemical signatures to probe the deep atmosphere (iron), mid-atmosphere (sodium), and shallow atmosphere (hydrogen).

“What we found was surprising: A jet stream rotates material around the planet’s equator, while a separate flow at lower levels of the atmosphere moves gas from the hot side to the cooler side. This kind of climate has never been seen before on any planet,” said Julia Victoria Seidel of the European Southern Observatory (ESO) in Chile, as well as the Observatoire de la Côte d'Azur in France. "This planet’s atmosphere behaves in ways that challenge our understanding of how weather works—not just on Earth, but on all planets. It feels like something out of science fiction."

Nature, 2025. DOI: 10.1038/s41586-025-08664-1

Astronomy and Astrophysics, 2025. DOI: 10.1051/0004-6361/202452405  (About DOIs).

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