The JWST transmission spectrum of TOI-1231 b shows strong CH4 detection and moderate CO2 evidence, consistent with a deep H2-rich atmosphere and no distinct surface.
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JWST Reveals CH$_4$, CO$_2$, and H$_2$O in a Metal-rich Miscible Atmosphere on a Two-Earth-Radius Exoplanet
32 Pith papers cite this work, alongside 25 external citations. Polarity classification is still indexing.
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external citations · Pith
abstract
Even though sub-Neptunes likely represent the most common outcome of planet formation, their natures remain poorly understood. In particular, planets near 1.5-2.5$\,R_\oplus$ often have bulk densities that can be explained equally well with widely different compositions and interior structures, resulting in grossly divergent implications for their formation. Here, we present the full 0.6-5.2$\,\mu \mathrm{m}$ JWST NIRISS/SOSS+NIRSpec/G395H transmission spectrum of the 2.2$\,R_\oplus$ TOI-270d ($4.78\,M_\oplus$, $T_\mathrm{eq}$=350-380 K), delivering unprecedented sensitivity for atmospheric characterization in the sub-Neptune regime. We detect five vibrational bands of CH$_4$ at 1.15, 1.4, 1.7, 2.3, and 3.3$\,\mu$m (9.4$\sigma$), the signature of CO$_2$ at 4.3$\,\mu$m (4.8$\sigma$), water vapor (2.5$\sigma$), and potential signatures of SO$_2$ at 4.0$\,\mu \mathrm{m}$ and CS$_2$ at 4.6$\,\mu\mathrm{m}$. Intriguingly, we find an overall highly metal-rich atmosphere, with a mean molecular weight of $5.47_{-1.14}^{+1.25}$. We infer an atmospheric metal mass fraction of $58_{-12}^{+8}\%$ and a C/O of $0.47_{-0.19}^{+0.16}$, indicating that approximately half the mass of the outer envelope is in high-molecular-weight volatiles (H$_2$O, CH$_4$, CO, CO$_2$) rather than H$_2$/He. We introduce a sub-Neptune classification scheme and identify TOI-270d as a "miscible-envelope sub-Neptune" in which H$_2$/He is well-mixed with the high-molecular-weight volatiles in a miscible supercritical metal-rich envelope. For a fully miscible envelope, we conclude that TOI-270d's interior is $90_{-4}^{+3}\,$wt$\,\%$ rock/iron, indicating that it formed as a rocky planet that accreted a few wt % of H$_2$/He, with the overall envelope metal content explained by magma-ocean/envelope reactions without the need for significant ice accretion. TOI-270d may well be an archetype of the overall population of sub-Neptunes.
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First statistically significant detection of dayside silicate clouds on a Neptunian-mass exoplanet, with CO and CO2 detections and C/O ratio of 0.984 from JWST spectra.
UV irradiation makes water-dominated exoplanet haze analogs more absorbing from 0.5-8 μm, producing a detectable difference in the 2.6 μm N-H feature in modeled transmission spectra for GJ 1214b.
High-resolution M-band spectroscopy detects super-stellar SiO in TWA 5 B, implying no significant magnesium-silicate clouds and formation consistent with core accretion beyond the CO snowline or gravitational instability with solid enrichment.
JWST NIRISS/SOSS data yield a transmission spectrum of the 23 Myr V1298 Tau c showing H2O absorption and an atmospheric metallicity of 14.8^{+56}_{-12} times solar, lower than mature planets of similar mass.
Transmission spectroscopy detects CS2 at >8σ in the H/He atmosphere of young exoplanet V1298 Tau e, contrasting with SO2 on sibling planet b.
MELTYQ couples magma-atmosphere equilibrium models with spectral retrievals to constrain sub-Neptune magma oxidation states and volatile inventories from transmission spectra.
The atmosphere of TOI-1130b shows high metallicity, low C/O, and elevated mean molecular weight consistent with ex-situ formation beyond the water ice line.
Hydrolyzed haze analogs from water-rich exoplanet conditions show higher absorptivity and a high imaginary refractive index that flattens spectral features in atmospheric models.
Coupled thermal-chemical models indicate that sub-Neptunes formed outside the water-ice line exhibit high atmospheric CH4, H2O, and C/O ratios while those formed inside show suppressed CH4 and low C/O.
Horizontal quenching at wind speeds ≳500 m/s, plus carbon-sulfur chemistry, explains the MIRI non-detection of night-side methane on WASP-43 b without requiring high metallicity.
A diffusion-limited microbial cell model predicts that smaller, longer-lived methanogens produce stronger methane biosignatures, and competition should drive alien life toward these traits.
Photochemical kinetic-transport models indicate CS2 abundance in gas giant exoplanets peaks at Teq 500-700 K through CH4-H2S chemistry with S2 photolysis as the driver.
Uranus and Neptune are reinterpreted as magma-ocean giants with hydrogen-rich interiors rather than traditional ice giants, consistent with observations via three fit parameters each.
GJ 3929 b's full four-eclipse JWST/MIRI data set yields an eclipse depth of 118±22 ppm, consistent with a bare rocky surface and ruling out thick CO2 atmospheres at >3σ.
Water- and metal-rich atmospheres on compact hot mini-Neptunes lose mass more slowly than H/He cases at high enrichment levels due to enhanced cooling and higher mean molecular weight.
Direct pixel-level extraction of NIRISS spectra for WASP-18b followed by cross-correlation yields 4.4σ CO, 3.4σ H2O, and 7.8σ OH detections, with improved abundance constraints from subsequent retrievals.
An open-source GCE code with a 100x faster solver demonstrates that refractory ratios Mg/Si and Fe/Si control carbon partitioning and atmospheric properties in water-accreting sub-Neptunes.
Water-hydrogen demixing occurs on warm sub-Neptunes with envelope metallicities of 150-700 times solar, including TOI-270 d, implying layered interiors and underestimated bulk metallicities when using fully-miscible models.
A quench-pressure parameterisation with two free parameters recovers unbiased C/O and [M/H] from synthetic JWST spectra with vertical mixing, and tentatively detects quenching and photochemical H2S depletion in HD 189733b NIRCam data.
New data break the eccentricity multimodality of TOI-2134 c to e=0.31±0.01, refine both planets' masses and radii, and yield a 4.7σ RM obliquity of 59±31° for planet c.
A simulated 2.5-year multi-band photometric survey is projected to detect ~100 young transiting planets, sufficient to measure their occurrence rate to 5% precision and differentiate gas-dwarf versus water-world formation scenarios.
Clouds drive over 1000 K heating at depth in sub-Neptune atmospheres, producing molten mantle interfaces for most planets in the sample and increasing abundances of O2, SiH4, and SiO by at least 36 percent.
Coupling Bern formation models with extended chemical equilibrium including S and N shows equilibration depletes atmospheric nitrogen, shifts C/O higher outside the ice line, generates Si species, and leaves sulfur abundances weakly dependent on formation location.
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The Influences of Hydrogen-Silicate-Iron Miscibility on the Demographics of Sub-Neptunes and Super-Earths
Models coupling hydrogen-silicate-iron miscibility with atmospheric escape reproduce the observed mass-radius occurrence density of sub-Neptunes and super-Earths.