A striking discovery about the atmospheres of temperate sub-Neptune exoplanets is reshaping our understanding of planetary diversity beyond our solar system. Recent observations utilizing the powerful capabilities of the James Webb Space Telescope (JWST) have brought to light a dramatic contrast between planets that, on the surface, appear quite similar. While earlier studies had revealed strong molecular fingerprints in the atmospheres of some sub-Neptunes, suggesting relatively clear skies and rich chemical compositions, the new data from a third temperate sub-Neptune, LP 791-18 c, challenges these expectations and signals a more complex picture of atmospheric evolution.
Temperate sub-Neptunes—planets about two to four times the size of Earth with equilibrium temperatures typically between 250 and 400 Kelvin—are exciting frontiers for atmospheric research primarily because their temperatures straddle the threshold between Earth-like and gas giant conditions. Prior landmark observations of two such worlds, K2-18 b and TOI-270 d, had revealed significant molecular absorption lines, particularly from water vapor and carbon dioxide. These clear atmospheric signatures not only strengthened confidence in detecting key molecules on smaller, cooler planets but also suggested that their upper atmospheres are largely free of aerosols and clouds that might obscure underlying gases.
However, the new transmission spectrum of LP 791-18 c—another temperate sub-Neptune that orbits a cool M6-type star—paints a more enigmatic and diverse atmospheric portrait. Despite having a size, mass, and temperature in between those of K2-18 b and TOI-270 d, LP 791-18 c’s atmospheric signature diverges sharply. Observations carried out using JWST’s NIRSpec/PRISM instrument, which spans 0.7 to 5.4 micrometers, reveal the dominance of strong haze scattering in the planet’s atmosphere, making it markedly hazier than its counterparts.
This haziness, caused by microscopic particles suspended high in the atmosphere, washes out the typical absorption features that astronomers rely upon to identify molecules. While methane—a greenhouse gas commonly expected in sub-Neptune atmospheres—was detected on LP 791-18 c, the anticipated signature of carbon dioxide was conspicuously absent. This absence is particularly noteworthy given the high CO2 abundances found on K2-18 b and TOI-270 d, which implied those planets might have formed in water-rich environments beyond the ice line, where water ice is abundant.
The lack of CO2 and the prevalence of haze on LP 791-18 c imply a different planetary story. The analysis suggests a deep, metal-enriched atmosphere, with metallicity—an astronomer’s term for elements heavier than hydrogen and helium—between 246 to 415 times that of our Sun. Such an enriched composition indicates that LP 791-18 c likely accumulated fewer volatiles such as water during its formation, supporting the hypothesis that it formed inside the water-ice line. In other words, LP 791-18 c might be comparatively dry beneath its thick atmospheric envelope, distinguishing its chemical and formation history starkly from other similarly sized sub-Neptunes orbiting temperate stars.
These findings have profound implications for theories of planetary formation and atmospheric evolution. They illuminate the intrinsic diversity of sub-Neptunes, showing that even planets with near-identical bulk densities and equilibrium temperatures can exhibit radically different atmospheric chemistries and cloud or haze properties. This diversity challenges simplistic models that suggest atmospheric characteristics scale primarily with temperature alone and instead point towards more complex processes influenced by formation location, accretion history, and perhaps the host star’s properties.
The rich haze layers enveloping LP 791-18 c may also influence the planet’s habitability prospects and climate regimes. Aerosols and hazes can affect radiative transfer, impacting temperature distributions and atmospheric circulation patterns. Moreover, such particles complicate remote sensing attempts—the very tools used to decipher exoplanet atmospheres—by masking molecular absorption features critical to understanding atmospheric composition and potential biosignatures.
The contrast between LP 791-18 c and the previously studied temperate sub-Neptunes exemplifies the power of JWST’s new observational capability to probe the atmospheres of smaller, cooler planets. JWST’s NIRSpec/PRISM spectrum, with its broad wavelength coverage and heightened sensitivity, allows astronomers to detect subtle signatures and distinguish between atmospheric scenarios that were previously beyond reach. This breakthrough ushers in a new era in exoplanet characterization, revealing complexities that demand more nuanced interpretative frameworks and enhanced theoretical models.
Overall, the study emphasizes that temperate sub-Neptunes are not monolithic but reside within a spectrum of atmospheric states and compositions. The variations in haze abundance, metallicity, and molecular ratios underscore the need to observe a broader sample of such worlds to understand planetary atmospheres in the context of their formation and evolutionary pathways. Future observations are crucial to resolve whether LP 791-18 c represents an outlier or a common atmospheric archetype among temperate sub-Neptunes.
Intriguingly, the research underscores the necessity of multi-wavelength studies, combining data from JWST with complementary inputs from ground-based observatories and upcoming missions. Such integrated approaches will refine constraints on aerosol properties, trace gas abundances, and temperature-pressure profiles, enabling a fuller picture of these distant worlds. Each new atmosphere investigated adds a vital piece to the puzzle of planetary diversity and exoplanet climatology.
The discovery of LP 791-18 c’s hazy and metal-rich atmosphere opens new avenues for understanding planet formation beyond the water-ice line and raises questions about how atmospheric processing and photochemistry operate under varying stellar influences. It also invites speculation on the implications for small planets’ potential to harbor life since aerosol properties and atmospheric composition directly influence surface conditions and radiation environments.
As the field of exoplanet atmospheric science matures, these nuanced findings remind us that planetary atmospheres are dynamic tapestries woven by complex interplay among physical, chemical, and environmental factors. LP 791-18 c’s unique atmospheric fingerprint is a testament to the diversity that lies beyond the simple variables of size and temperature, urging scientists to embrace complexity and refine their models accordingly.
This investigation is a landmark in the quest to understand the nature of sub-Neptunes, a dominant class of exoplanets whose bulk characteristics often straddle the line between Earth-like and gas-dominated worlds. The revelations from LP 791-18 c enrich our knowledge of this population and underscore the vital role of next-generation telescopes in uncovering the universe’s planetary kaleidoscope.
In sum, the transmission spectrum of LP 791-18 c is a compelling case study in the chemical and morphological diversity of temperate sub-Neptune atmospheres. Characterized by elaborately hazy sky conditions rather than clear molecular windows, this planet challenges preconceived notions and ignites fresh scientific discussions about planet formation, migration, and atmospheric composition across cosmic neighborhoods.
The ongoing efforts to unravel the mysteries of LP 791-18 c and other similar planets herald a new chapter in the exploration of exoplanets, with each observation bringing us closer to answering fundamental questions about planetary origins, evolution, and the potential habitability of distant worlds in our galaxy.
Subject of Research: Atmospheric composition and haziness diversity in temperate sub-Neptune exoplanets, focusing on LP 791-18 c.
Article Title: Diversity in the haziness and chemistry of temperate sub-Neptunes.
Article References:
Roy, PA., Benneke, B., Fournier-Tondreau, M. et al. Diversity in the haziness and chemistry of temperate sub-Neptunes. Nat Astron (2025). https://doi.org/10.1038/s41550-025-02723-3
Image Credits: AI Generated
