Scientists using the James Webb Space Telescope (JWST) have delved into the atmosphere of the scorching hot exoplanet LTT 9779 b.
Officially named Cuancoá, this Neptune-sized exoplanet was discovered in 2020 and orbits its sun-like star every 19 hours. With a mass about 29 times that of Earth, LTT 9779 b lies within the “hot Neptune desert” — a category of planets for which exceptionally few are known to exist, making it a rare and intriguing discovery.
“Finding a planet of this size so close to its host star is like finding a snowball that hasn’t melted in a fire,” said graduate student Louis-Philippe Coulombe from the Université de Montréal in a press release. “It’s a testament to the diversity of planetary systems and offers a window into how planets evolve under extreme conditions.”
The team used the Single Object Slitless Spectroscopy (SOSS) mode of JWST’s Near Infrared Imager and Slitless Spectrograph (NIRISS) to study LTT 9779 b. This instrument detects light in the near-infrared range — wavelengths just beyond visible light — which makes it particularly effective for analyzing exoplanet atmospheres, distant galaxies and faint celestial objects. These capabilities far surpass those of previous telescopes, allowing scientists to uncover details that were once out of reach.
For LTT 9779 b, the team used SOSS mode to detect water vapor and study light reflected from its clouds, which form on the exoplanet’s day side. Like Earth’s moon, LTT 9779 b is tidally locked, meaning one of its faces will always face its star — experiencing temperatures reaching almost 3,600 degrees Fahrenheit (2,000 Celsius) as a result of its close orbit — while its night side will remain in permanent darkness.
“This planet provides a unique laboratory to understand how clouds and the transport of heat interact in the atmospheres of highly irradiated worlds,” stated Coulombe.
On tidally locked gas giants, atmospheric circulation is driven by stark temperature differences between the permanent day and night sides. Hot air rises on the scorching day side, while cooler, denser air sinks on the night side, creating a convection-driven current. Due to the Coriolis effect caused by the planet’s rotation, this circulation generates a powerful eastward-flowing jet stream.
On LTT 9779 b, this heat transport results in a cooler western dayside, where temperatures drop low enough for clouds to form. The presence of these clouds is also thought to be linked to the planet’s higher atmospheric metallicity, which promotes the formation of reflective particles like aerosols. This hypothesis is supported by LTT 9779 b’s unusually high albedo, meaning it reflects more sunlight than other planets of similar temperature.
“This partial coverage of clouds over its dayside, which reflects a certain fraction of the stellar flux, probably affects the energy budget of the planet,” the research team wrote in their paper.
They also found signs of water vapor on the dayside of LTT 9779 b, confirming that scientists can study the atmospheres of cloudy exoplanets by analyzing the heat they give off.
These findings suggest that the planet’s thick clouds and high reflectivity may be linked to its atmospheric composition and circulation patterns.
“By modeling LTT 9779 b’s atmosphere in detail, we’re starting to unlock the processes driving its alien weather patterns,” said Björn Benneke, a co-author of the study.
The team is now working to refine their models using additional observations, aiming to better understand how clouds form and persist in such extreme environments.
“We haven’t finished piecing together the information about this planet yet,” concluded Jake Taylor from the Department of Physics at the University of Oxford. “We are currently using observations from the Hubble Space Telescope and the Very Large Telescope to study the dayside cloud structure in more detail to learn as much as possible.”
A study of LTT 9779 b’s atmosphere was published in the journal Nature Astronomy.