Today's forecast Partially cloudy skies on an ultra-hot Neptune

Today's forecast Partially cloudy skies on an ultra-hot Neptune
by Clarence Oxford
Los Angeles CA (SPX) Feb 26, 2025

New insights into the extreme atmosphere of LTT 9779 b, a rare ultra-hot Neptune, have emerged from observations conducted using the James Webb Space Telescope (JWST). Led by Louis-Philippe Coulombe, a graduate researcher at Universite de Montreal's Trottier Institute for Research on Exoplanets (IREx), the study sheds light on the planet's unique weather patterns and atmospheric composition.

Published in Nature Astronomy, the research reveals that LTT 9779 b, which orbits its star in under 24 hours, experiences intense heating on its permanent dayside, where temperatures soar to nearly 2,000 C. Despite these extreme conditions, Coulombe and his team identified bright, reflective clouds concentrated in the planet's western hemisphere, contrasting with the hotter, cloud-free 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," said Coulombe.

A Weather System Marked by Asymmetry

Observations conducted with JWST revealed a distinct asymmetry in LTT 9779 b's dayside brightness. The research team attributes this imbalance to strong eastward winds that distribute heat unevenly across the planet's surface. By analyzing both the emitted thermal radiation and reflected light, they refined existing models of atmospheric heat circulation and cloud formation.

Through multi-phase analysis of the planet's orbit, the team confirmed the presence of silicate-based clouds on the western hemisphere. These reflective clouds are responsible for the planet's high brightness in visible wavelengths, as they effectively bounce back a significant portion of stellar radiation.

In addition to the cloud formations, the study detected water vapor in LTT 9779 b's atmosphere, offering further insights into its composition and climate dynamics.

"By modeling LTT 9779 b's atmosphere in detail, we're starting to unlock the processes driving its alien weather patterns," stated Coulombe's research advisor Bjorn Benneke, a Universite de Montreal astronomy professor and co-author of the study.

Unprecedented Observations with JWST

The JWST's unparalleled capabilities were instrumental in obtaining these findings. The observations, conducted using its Canadian-built Near Infrared Imager and Slitless Spectrograph (NIRISS), spanned 22 hours, covering an entire orbit of the planet. This included key events such as two secondary eclipses (when the planet moved behind its star) and a primary transit (when it passed in front of the star).

The brightness variations recorded throughout the orbit allowed researchers to construct a detailed map of the planet's temperature, composition, and cloud distribution. The dayside's intense illumination revealed how heat redistribution functions in such extreme conditions.

Michael Radica, a postdoctoral researcher at the University of Chicago and second author of the study, emphasized the significance of the findings. "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 under the NEAT (NIRISS Exploration of Atmospheric Diversity of Transiting Exoplanets) Guaranteed Time Observation program, overseen by IREx's David Lafreniere, an astrophysics professor at Universite de Montreal. He highlighted JWST's unique ability to observe exoplanets across a broad spectrum of wavelengths, separating thermal emission from reflected light. "This is exactly the kind of groundbreaking work JWST was designed to enable."

A Rare Find in the Hot Neptune Desert

LTT 9779 b resides in what astronomers call the "hot Neptune desert"-a region where planets of this type are scarcely found. Unlike hot Jupiters, which are frequently detected in close proximity to their stars, ultra-hot Neptunes remain rare, challenging existing theories of planetary formation and survival.

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

The findings contribute to a broader understanding of how planetary atmospheres respond to extreme stellar radiation. The high metallicity and reflective clouds observed on LTT 9779 b provide clues to the evolution of planetary atmospheres in harsh environments.

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

Research Report:
">Highly-reflective clouds on the western dayside of an exo-Neptune identified with phase-resolved reflected-light and thermal-emission spectroscopy