JWST Uncovers Rare Ultra-Hot Neptune LTT 9779 b's Exotic Atmosphere

by University of Oxford

A team of international researchers, including Dr. Jake Taylor from the Department of Physics at the University of Oxford, has utilized the James Webb Space Telescope (JWST) to investigate the exotic atmosphere of LTT 9779 b, categorized as a rare "ultra-hot Neptune." The groundbreaking results were published on February 25, 2025, in Nature Astronomy (DOI:10.1038/s41550-025-02488-9).

Introduction

The study presents novel insights into the extreme weather patterns and atmospheric characteristics of LTT 9779 b, which resides in the so-called hot Neptune desert—a category of planets in which very few are known to exist. While giant planets orbiting very close to their host stars, often referred to as 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 graduate student Louis-Philippe Coulombe from the Université de Montréal's Trottier Institute for Research on Exoplanets (IREx), who led the study. "It's a testament to the diversity of planetary systems and presents a window into how planets evolve under extreme conditions."

A Unique Laboratory for Alien Weather

Orbiting its host star in less than a day, LTT 9779 b is subjected to intense temperatures approaching 2,000°C on its dayside. The planet is tidally locked, similar to Earth's moon, meaning one side perpetually faces its star while the opposite remains in constant darkness.

Despite these extreme temperatures, the research team discovered that the planet's dayside is characterized by reflective clouds situated on its cooler western hemisphere. This phenomenon creates a striking contrast when compared to the hotter eastern side.

Dr. Taylor collaborated with Coulombe in analyzing the data. Together, they had conducted an initial atmospheric analysis of the planet's spectrum, the findings of which were previously published in The Astrophysical Journal Letters in 2024. "Our original study of the transmission spectrum hinted at the need for high-altitude clouds to explain observations; the latest research confirms the existence of these clouds," he explains.

Mapping the Atmosphere of an Ultra-Hot Neptune

The research team employed a detailed analysis by investigating both the heat emitted from the planet and the reflected light from its star. The team observed the planet at multiple positions within its orbit, scrutinizing its properties at each phase individually. Among their discoveries were clouds composed of silicate minerals, which formed on the slightly cooler western side of the planet's dayside.

By merging the reflected light with heat emissions, the team constructed a detailed model of the planet's atmosphere. Their findings highlighted a delicate equilibrium between the intense heat received from the star and the planet's capacity to redistribute energy. Furthermore, they detected water vapor in the atmosphere, providing insights regarding the planet's composition and the processes governing 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 Professor Björn Benneke, a co-author of the study and Coulombe's research advisor.

Implications for Exoplanet Science

This rare planetary system challenges scientists' existing paradigms about how planets are formed, migrate, and endure under relentless stellar forces. The planet's reflective clouds and high metallicity could illuminate how atmospheres evolve in extreme environments. Consequently, LTT 9779 b serves as a remarkable laboratory for exploring these questions, providing insights into the overarching processes impacting the architecture of planetary systems across the galaxy.

Findings Summary

Future Research Directions

Looking forward, researchers aim to utilize various observatory instruments to gather more comprehensive information about LTT 9779 b. "We haven't wrapped up our assessment of this planet yet," concludes Dr. Taylor. "We are using the Hubble Space Telescope and the Very Large Telescope to examine the dayside cloud structure in greater detail and to expand our understanding as much as we can."

For More Information

• White dwarf stars may host more habitable exoplanets than expected
• Geysers on Saturn's icy moon Enceladus may form from a 'mushy zone'
• Jupiter-like exoplanet orbiting nearby star detected
• Astronomers discover a 'hot Neptune' in a tight orbit
• Astronomers use Webb to probe a 'steam world' in the constellation Pisces