In the previous year, astronomers made an astonishing discovery of an Earth-sized exoplanet characterized by one hemisphere entirely composed of molten lava, while the other remains in constant darkness due to tidal locking. Around the same period, another research team identified a rare, cold exoplanet that possesses a massive companion, weighing over 100 times that of Jupiter.

Introducing Tylos

This groundbreaking research utilized observational data gathered by the European Southern Observatory’s (ESO) Very Large Telescope (VLT), employing a specialized spectroscopic instrument known as ESPRESSO that synthesizes light from the four principal VLT units into a single signal. The exoplanet in question is WASP-121b, or Tylos, situated approximately 900 light-years away in the Puppis constellation. Interestingly, a single year on Tylos equates to merely 30 hours on Earth due to the exoplanet’s close orbit around its host star. As a result, one side remains continually exposed to the star’s heat, while the opposite side experiences significantly cooler temperatures.

These dramatic temperature differences make it difficult to comprehend how energy circulates within the atmospheric system. Mapping the three-dimensional atmospheric structure is crucial for understanding the vertical circulation patterns that current global circulation models cannot replicate. The research team combined archival ESPRESSO data from November 30, 2018, with fresh observations obtained on September 23, 2023, focusing on three distinct chemical signatures: iron (deep atmosphere), sodium (mid-atmosphere), and hydrogen (shallow atmosphere) .

“Our findings were unexpected: a jet stream circulates material around the planet’s equator, while a separate flow within the lower atmosphere transports gases from the hot side to the cooler side. This type of climatic behavior has never been documented on any planet before,” commented Julia Victoria Seidel from the European Southern Observatory in Chile and the Observatoire de la Côte d’Azur in France. “The atmospheric dynamics of this planet defy our understanding of weather systems—not only on Earth but across all celestial bodies. It seems almost like a narrative from science fiction.”

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

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