An international team of astronomers utilizing the NASA/ESA/CSA James Webb Space Telescope (JWST) has directly measured a striking atmospheric imbalance between the morning and evening hemispheres of WASP-107b, a low-density exoplanet located approximately 200 light-years from Earth in the constellation Virgo. Published in the journal Nature Astronomy, the study provides the most detailed space-based characterization of east-west limb asymmetry ever achieved for a planet beyond our solar system.
WASP-107b is classified by planetary scientists as a 'warm super-Neptune' or 'cotton-candy planet' because it is nearly as large in diameter as Jupiter—roughly 94% of Jupiter's radius—yet contains only about 10% of Jupiter's mass. Because the planet orbits extremely close to its orange dwarf host star, completing a full revolution every 5.7 days, it is tidally locked, with one hemisphere baked in perpetual daylight and the opposite side plunged into permanent night.
Separating Morning and Evening Starlight Across a Puffy Exoplanet
Led by researcher Matthew Murphy at the University of Arizona's Steward Observatory alongside European and Canadian collaborators, the team used JWST's Near-Infrared Camera (NIRCam) to observe WASP-107b as it transited across the face of its star. By isolating the tiny fraction of starlight filtering separately through the planet's leading eastern edge (the evening terminator) and trailing western edge (the morning terminator), scientists reconstructed separate atmospheric spectra for dawn and dusk.
The NIRCam spectra revealed that the planet's evening limb is roughly 180 kelvins (324 degrees Fahrenheit) hotter than its morning limb—reaching around 870 kelvins at dusk compared with 690 kelvins at dawn. This thermal contrast alters cloud formation and atmospheric expansion: the cooler morning side hosts thicker, higher-altitude silicate sand clouds and photochemical hazes that block certain infrared wavelengths, while the warmer evening side exhibits clearer skies and distinct water vapor, carbon dioxide, and sulfur dioxide signatures.
“For the first time from space, we can separate the morning and evening sides of an exoplanet's atmosphere and watch how winds, clouds, and sunlight interact across an alien day-night boundary.”
How Tidal Locking and Equatorial Winds Shape Alien Weather
Computer climate models had previously predicted that ultra-hot gas giants above 1,500 kelvins should exhibit the strongest morning-to-evening asymmetries, whereas cooler worlds like WASP-107b were expected to look relatively uniform. Finding such a pronounced disparity at moderate temperatures suggests that super-rotating equatorial jet streams—winds blowing faster than the planet's rotation—transport heat and aerosols around puffy, low-gravity atmospheres far more dynamically than standard 1D models assumed.
The research team plans follow-up observations with Webb's Mid-Infrared Instrument (MIRI) and NIRSpec to map WASP-107b's full three-dimensional circulation. Astronomers noted that accounting for east-west terminator differences will be essential for accurately measuring chemical abundances on dozens of smaller rocky and sub-Neptune exoplanets targeted by JWST and the European Space Agency's upcoming Ariel mission.
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