JWST Forecasts Partially Cloudy Skies on Ultra-Hot Neptune LTT 9779 b

Montréal, Canada—A team of researchers led by Louis-Philippe Coulombe, a graduate student at the Université de Montréal’s Trottier Institute for Research on Exoplanets (IREx), has used the James Webb Space Telescope (JWST) to explore the exotic atmosphere of LTT 9779 b, a rare “ultra-hot Neptune”. The results, to be published on February 25th in a compelling new study in Nature Astronomy, offer new insights into the extreme weather patterns and atmospheric properties of this fascinating exoplanet.

A Unique Laboratory for Alien Weather

Louis-Philippe Coulombe, lead author of this study. This photo was taken at taken at Tasman lake in New Zealand. In his spare time, Louis-Philippe likes to hike and do landscape photography. Photo credit: Courtesy.

Orbiting its host star in less than a day, LTT 9779 b is subjected to searing temperatures reaching almost 2,000°C on its dayside. The planet is tidally locked (similar to Earth’s Moon), meaning one side constantly faces its star while the other remains in perpetual darkness. Despite such extremes, Coulombe’s team discovered that the planet’s dayside hosts reflective clouds on its cooler western hemisphere, creating a striking contrast to the hotter eastern side. “This planet provides a unique laboratory to understand how clouds and the transport of heat interact in the atmospheres of highly irradiated worlds,” says Coulombe.

The team’s analysis, conducted using JWST, uncovered an asymmetry in the planet’s dayside reflectivity. The team proposed that the uneven distribution of heat and clouds is driven by powerful winds that transport heat around the planet. These findings help refine models describing how heat is transported across a planet and cloud formation in exoplanet atmospheres, helping bridge the gap between theory and observation.

Mapping the Atmosphere of an Ultra-Hot Neptune

The research team studied the atmosphere in detail by analyzing both the heat emitted by the planet and the light it reflects from its star. To create a clearer picture, they observed the planet at multiple positions in its orbit and analysed its properties at each phase individually. They discovered clouds made of materials like silicate minerals, which form on the slightly cooler western side of the planet’s dayside. These reflective clouds help explain why this planet is so bright at visible wavelengths, bouncing back much of the star’s light.

By combining this reflected light with heat emissions, the team was able to create a detailed model of the planet’s atmosphere. Their findings reveal a delicate balance between intense heat from the star and the planet’s ability to redistribute energy. The study also detected water vapor in the atmosphere, providing important clues about the planet’s composition and the processes that govern its extreme environment.

“By modeling LTT 9779 b’s atmosphere in detail, we’re starting to unlock the processes driving its alien weather patterns,” explains Prof. Björn Benneke, a co-author of the study and Coulombe’s research advisor.

A Trailblazing Study with JWST

Illustration of LTT 9779 b, the only known ultra-hot Neptune. This planet orbits so close to its star that its atmosphere is scorching hot, glowing from its own heat while also reflecting starlight. Because it is tidally locked – always showing the same side to its star – one half is permanently in daylight while the other remains in darkness. New JWST observations with NIRISS reveal a dynamic atmosphere: powerful winds sweep around the planet, shaping mineral clouds as they condense into a bright, white arc on the slightly cooler western side of the dayside. As these clouds move eastward, they evaporate under the intense heat, leaving the eastern dayside with clear skies. Image credit: Benoit Gougeon; Université de Montréal

The JWST has once again demonstrated its incredible power, allowing scientists to study the atmosphere of LTT 9779 b in unprecedented detail. This study used JWST’s Canadian instrument, Near Infrared Imager and Slitless Spectrograph (NIRISS), observing the planet for nearly 22 hours. The data captured the planet’s full orbit around its star, including two secondary eclipses (when the planet passes behind its star) and a primary transit (when the planet passes in front of its star).

For an exoplanet like LTT 9779 b, which is tidally locked to its star, the amount and type of light we observe changes as the planet rotates, showing us different parts of its surface. The dayside reflects and emits more light due to intense heating, while the cooler nightside emits less light. By capturing spectra at various phases, researchers can map out variations in temperature, composition, and even cloud coverage across the planet’s surface.

Dr. Michael Radica, a former PhD student at UdeM and now a postdoctoral researcher at the University of Chicago, was the second author of this study. Earlier this year, Dr. Radica published a detailed analysis of the planet’s light spectrum during transit. “It’s remarkable that both types of analyses paint such a clear and consistent picture of the planet’s atmosphere,” he noted.

The research was conducted as part of the NEAT (NIRISS Exploration of Atmospheric Diversity of Transiting Exoplanets) Guaranteed Time Observation program, led by IREx’s Prof. David Lafrenière.

The study highlights the importance of JWST’s ability to observe exoplanets across a wide wavelength range, allowing scientists to disentangle the contributions of reflected light and thermal emission. “This is exactly the kind of groundbreaking work JWST was designed to enable,” says Lafrenière.

Implications for Exoplanet Science

LTT 9779 b resides in the “hot Neptune desert,” a category of planets where exceptionally few are known to exist. While giant planets orbiting very close to their host stars – often called “hot Jupiters” – are commonly detected using current exoplanet-finding methods, ultra-hot Neptunes like LTT 9779 b remain remarkably rare.

“Finding a planet of this size so close to its host star is like finding a snowball that hasn’t melted in a fire,” says Coulombe. “It’s a testament to the diversity of planetary systems and offers a window into how planets evolve under extreme conditions.”

This rare planetary system continues to challenge scientists’ understanding of how planets form, migrate, and endure in the face of unrelenting stellar forces. The planet’s reflective clouds and high metallicity may shed light on how atmospheres evolve in extreme environments. LTT 9779 b is a remarkable laboratory for exploring these questions, offering insights into the broader processes that shape the architecture of planetary systems across the galaxy.

“These findings give us a new lens for understanding atmospheric dynamics on smaller gas giants,” says Coulombe. “This is just the beginning of what JWST will reveal about these fascinating worlds.”

About the Article

This research is part of the NEAT Guaranteed Time Observations program, led by Prof. David Lafrenière. The full article, titled Highly-reflective clouds on the western dayside of an exo-Neptune identified with phase-resolved reflected-light and thermal-emission spectroscopy, published on February 25, 2025 in Nature Astronomy, is available at https://www.nature.com/articles/s41550-025-02488-9.

The authors acknowledge financial support from the Canadian Space Agency for this study.

Science Contacts:

Louis-Philippe Coulombe

Trottier Institute for Research on Exoplanets

Université de Montréal

Ph.D. Candidate

louis-philippe.coulombe@umontreal.ca

Prof. Björn Benneke

Trottier Institute for Research on Exoplanets

Université de Montréal

bjorn.benneke@umontreal.ca

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