Webb reveals a hazy world that shows not all planet twins are identical

By UdeMnouvelles Dec 12th, 2025

In 5 seconds A Canadian-led team discovers that exoplanet LP 791-18 c, a temperate sub-Neptune, hides a hazy atmosphere unlike any of its planetary siblings.

An artistic representation of hazy exoplanet LP 791-18 c.
Photo : NASA Eyes on Exoplanets

Astronomers from the Trottier Institute for Research on Exoplanets (IREx) at the Université de Montréal have used the James Webb Space Telescope (JWST) to uncover the surprising nature of a world that looked, at first glance, like a near-twin of several well-studied exoplanets.

LP 791-18 c — a small sub-Neptune between the size of Earth and Neptune orbiting a cool red dwarf only 86 light-years away — was expected to resemble other temperate sub-Neptunes of similar size and temperature, such as K2-18 b or TOI-270 d. But instead of the cloud-free, methane- and carbon-dioxide-rich atmospheres of those worlds, Webb revealed something entirely different: a haze-dominated atmosphere rich in methane and with no signs of carbon dioxyde.

The discovery, led by a Canadian team, shows that planets that appear nearly identical at first glance can diverge dramatically in their cloudiness, chemistry, and history — and that only JWST’s extraordinary sensitivity can reveal that hidden diversity. The findings were published in Nature Astronomy.

A familiar planet with an unexpected chemistry

LP 791-18 c is about 2.5 times the radius of Earth and orbits within the “temperate” zone of its tiny host star — with equilibrium temperatures roughly between -20° and 140° Celsius and receiving roughly the same irradiation from its star as some of the best-studied sub-Neptunes. That similarity made it a prime target for comparison with K2-18 b and TOI-270 d, both of which show clear or lightly clouded atmospheres rich in methane, but also in carbon dioxide — a sign of water-rich interiors.

But when the Canadian-led team observed LP 791-18 c with JWST’s Near Infrared Spectrograph (NIRSpec), the spectrum told a different story.

“One might have expected a near-identical atmosphere to previously studied temperate sub-Neptunes: mostly methane, carbon dioxide, and no signs of clouds in the upper atmosphere. Instead, we found a planet with a completely different cloud regime, and even with a potentially different chemistry,” says Pierre-Alexis Roy, the study’s lead author, who completed this work as a Ph.D. student at IREx/UdeM and is now a postdoctoral researcher at UCLA. “LP 791-18 c is a hazy world. Its atmosphere is rich in methane and photochemical aerosols, with almost no signs of water or carbon dioxide — a striking contrast to its counterparts.”

The JWST observations revealed a strong methane absorption signature at high significance, while the signatures of carbon dioxide detected on the other two temperate sub-Neptunes, was surprisingly absent. Instead, the spectrum showed the broad, muted shape typical of atmospheres blanketed by complex hazes or clouds.

A world sculpted by photochemistry and starlight

The haze veiling LP 791-18 c is likely created through photochemistry, where ultraviolet radiation from the planet’s active red-dwarf star breaks apart methane molecules high in the planet’s atmosphere. The fragments recombine into heavier hydrocarbons. This same process gives Saturn’s moon Titan its orange smog.

This photochemical haze can block or mute the molecular “fingerprints” that astronomers rely on to decode a planet’s atmosphere. Yet even with this veil, JWST was able to identify methane clearly and place constraints on what is missing.

Another important result: the team demonstrated that the spectrum remains consistent regardless of whether stellar activity is included in the analysis or not. That means the unusual atmospheric chemistry is not an illusion caused by star spots or flares — a rampant issue in recent exoplanet atmosphere studies — but is intrinsic to the planet.

“Webb gives us a clean, direct look,” explains Prof. Björn Benneke (UCLA/UdeM), co-author and Pierre-Alexis Roy’s Ph.D. supervisor. “Even when we account for all the ways the star might influence the signal, the atmospheric picture stays the same. LP 791-18 c genuinely shows these abundant hazes and that strong methane absorption band.”

A clue to where the planet was born

If planets with the same size and temperature can have radically different atmospheres, the obvious question is why? The answer may lie billions of years in the past.

The chemical fingerprint of LP 791-18 c, rich in methane yet depleted in oxygen-bearing molecules, suggests it formed inside the protoplanetary disk’s water-ice line, the region close to the star where temperatures are too warm for icy grains to survive. Planets forming farther out tend to incorporate more water and volatile ices, leading to CO₂- and H₂O-rich atmospheres like those found on K2-18 b and TOI-270 d.

By contrast, LP 791-18 c appears to have been built from drier, carbon-rich material. Such a fundamental difference can leave a lasting imprint on the planet’s composition.

“This is the strongest evidence yet that formation history and evolution, not just temperature or size, shape the atmospheres of sub-Neptunes,” says Roy. “Two planets can look identical from afar, but their chemistry can reveal completely different origins.”

The study highlights how JWST is enabling “comparative planetology” at a level that was impossible before: identical-looking planets can now be distinguished by their deepest atmospheric traits.

“We know that the atmospheres of small terrestrial planets show an intrinsic diversity,” states Roy. “One simply has to consider Earth and Venus, which despite forming together in the habitable zone of the Sun, host drastically different atmospheres. But here, we are starting to observe such a diversity for gaseous sub-Neptunes as well.”

A miniature planetary system full of surprises

LP 791-18 c orbits within a compact, dynamic planetary system that has already captivated astronomers. In 2023, another UdeM-led team revealed that the system’s inner terrestrial planet LP 791-18 d likely experiences intense volcanic activity triggered by gravitational interactions, making it one of the best candidates for a volcano world outside the Solar System.

Now, with LP 791-18 c showing an entirely different form of atmospheric complexity, the system has become a showcase for the diversity of planets orbiting small stars.

“You’ve got a volcanic Earth-sized planet and a hazy methane-rich sub-Neptune in the same system,” notes Benneke. “It really underscores how wildly diverse planets can be, even when they form around the same star.”