The haze veiling LP 791-18 c is likely created through photochemistry, where ultraviolet radiation from the planet\u2019s active red-dwarf star breaks apart methane molecules high in the planet\u2019s atmosphere. The fragments recombine into heavier hydrocarbons. This same process gives Saturn\u2019s moon Titan its orange smog.<\/p>\n
This photochemical haze can block or mute the molecular \u201cfingerprints\u201d that astronomers rely on to decode a planet\u2019s atmosphere. Yet even with this veil, JWST was able to identify methane clearly and place constraints on what is missing.<\/p>\n
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 \u2014 a rampant issue in recent exoplanet atmosphere studies \u2014 but is intrinsic to the planet.<\/p>\n
\u201cWebb gives us a clean, direct look,\u201d explains Prof. Bj\u00f6rn Benneke (UCLA\/UdeM), co-author and Pierre-Alexis Roy\u2019s Ph.D. supervisor. \u201cEven 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.\u201d<\/p>\n
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A clue to where the planet was born<\/p>\n
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.<\/p>\n
The chemical fingerprint of LP 791-18 c, rich in methane yet depleted in oxygen-bearing molecules, suggests it formed inside the protoplanetary disk\u2019s 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 CO\u2082- and H\u2082O-rich atmospheres like those found on K2-18 b and TOI-270 d.<\/p>\n
By contrast, LP 791-18 c appears to have been built from drier,\u00a0carbon-rich material. Such a fundamental difference can leave a lasting imprint on the planet\u2019s composition.<\/p>\n
\u201cThis is the strongest evidence yet that formation history and evolution, not just temperature or size, shape the atmospheres of sub-Neptunes,\u201d says Roy. \u201cTwo planets can look identical from afar, but their chemistry can reveal completely different origins.\u201d<\/p>\n
The study highlights how JWST is enabling \u201ccomparative planetology\u201d at a level that was impossible before: identical-looking planets can now be distinguished by their deepest atmospheric traits.<\/p>\n
\u201cWe know that the atmospheres of small terrestrial planets show an intrinsic diversity,\u201d states Roy. \u201cOne 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.\u201d<\/p>\n
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A miniature planetary system full of surprises<\/p>\n