Now Reading: Exploring The Sub-Neptune Frontier With JWST
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Exploring The Sub-Neptune Frontier With JWST
A classification scheme for volatile-rich sub-Neptunes. The schematic compares different types of sub-Neptunes with H2 and H2O dominated atmospheres as a function of the equilibrium temperature and H2 content. Ice worlds: Cold subNeptunes where any water is frozen in a layer of ice atop high-pressure ice, with a thin H2-dominated atmosphere (65). Hycean worlds: Warmer than ice worlds, hycean worlds have a thin H2-rich atmosphere overlying an ocean of liquid water, which itself is above a layer of high-pressure ices. These planets have the potential to support habitable conditions (9). Mini-Neptunes: Sub-Neptunes with a deep H2-rich atmosphere followed by a mixed H2O-H2 supercritical layer below before reaching a layer of high-pressure ice and then a core (17, 65). Steam worlds: Water worlds with H2O dominated atmospheres. At equilibrium temperatures above Tcrit, a high proportion of H2O exists in the atmosphere, with a supercritical mixed-envelope at higher pressures. Supercritical mini-Neptunes: These planets are hotter than Tcrit, and contain interiors where H2 gas is highly soluble in supercritical H2O (46, 65). This mixed-envelope would lie over a layer of high-pressure ice above the core. Legend: The legend at the top shows the different compositions corresponding to the colors in the schematic, including the core, different phases of H2O and envelope composition; high-P ice refers to high-pressure ice. Note that the core has further distinct layers but we do not label them here. Sharp phase transitions are shown for ice-atmosphere and ocean-atmosphere transitions. Other phase transitions are marked by a gradient, as they may not be sharp phase transitions (83). — astro-ph.EP
Sub-Neptune planets, with sizes and masses between those of Earth and Neptune, dominate the exoplanet population.
Sub-Neptunes are expected to be the most diverse family of the exoplanet population, potentially including rocky gas dwarfs, water worlds, and mini-Neptunes, with a wide range of atmospheric, surface and interior conditions. With no analogue in the solar system, these planets open fundamental questions in planetary processes, origins, and habitability, and present new avenues in the search for life elsewhere.
Atmospheric observations with the James Webb Space Telescope (JWST) are enabling unprecedented characterization of sub-Neptunes, starting with the first detections of carbon-bearing molecules in the habitable zone sub-Neptune K2-18 b.
We survey the present landscape of JWST observations and atmospheric inferences of sub-Neptunes, which in turn provide key insights into their atmospheric processes, internal structures, surface conditions, formation pathways and potential habitability. The atmospheric abundance constraints reveal evidence of chemical disequilibria, and insights into the planetary mass-metallicity relation in the sub-Neptune regime.
Similarly, for sub-Neptunes with H2O-rich interiors, increasing atmospheric H2O abundances with the equilibrium temperature may indicate the existence of a critical temperature for transition from H2 dominated atmospheres with tropospheric cold traps to those with steamy atmospheres. The chemical abundances also provide initial evidence for diverse planet types, from potentially habitable hycean worlds to steam worlds with super critical water layers.
These planet types serve as benchmarks for an emerging taxonomy of volatile-rich sub-Neptunes as a function of their equilibrium temperature and atmospheric extent, heralding a new era of chemical classification of low-mass exoplanets with JWST.
Nikku Madhusudhan, Måns Holmberg, Savvas Constantinou, Gregory J. Cooke
Comments: Accepted for publication in PNAS special feature on Characterization of Exoplanets in the JWST Era
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM)
Cite as: arXiv:2509.19247 [astro-ph.EP] (or arXiv:2509.19247v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2509.19247
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Submission history
From: Madhusudhan Nikku
[v1] Tue, 23 Sep 2025 17:08:30 UTC (3,017 KB)
https://arxiv.org/abs/2509.19247
Astrobiology,
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