K2-18 b Archives

Schematics of the MELTYQ coupled magma-atmosphere retrieval. The model is valid for sub-Neptune hosting a thick H2-rich atmosphere with enough surface temperature to melt rocks. The model splits the planet in three regions: a rocky core, a deep atmosphere, and an upper atmosphere. The chemistry is coupled allowing to retrieve the composition of the rocky magma via it’s influence on the observable atmosphere. -- astro-ph.EP

Astrogeology, Astronomy & Telescopes, Atmospheres, Climate, Weather, Biogeochemical Cycles & Geobiology, Exoplanets, -moons, -comets, Lava / magma ocean / Volcanic Worlds, Status Report

Coupling Magma-ocean And Atmospheres In Spectral Retrievals Of Sub-Neptunes

by Keith CowingMay 14, 2026May 14, 2026

Recent high-precision atmospheric observations with JWST is enabling detailed characterization of sub-Neptune atmospheres and motivating efforts to understand and constrain their interiors.

Transit spectrum of K2-18 b based on the detection with Kepler (Montet et al. 2015) and characterisation with HST/WFC (Benneke et al. 2017), Spitzer (Benneke et al. 2019), JWST/NIRISS &/NIRSpec (Madhusudhan et al. 2023; Schmidt et al. 2025) and JWST/MIRI (Madhusudhan et al. 2025; Liu et al. 2025). Different panels compare different pipelines for the NIRISS and NIRSpec analyses. -- astro-ph.EP

Astrochemistry, Astronomy & Telescopes, Atmospheres, Climate, Weather, Biosignatures & Paleobiology, Exoplanets, -moons, -comets, Icy Worlds, Imaging & Spectroscopy, Status Report

The Atmosphere Of K2-18 b: The Role Of Hazes, Clouds And Photoelectrons

by Keith CowingMarch 30, 2026March 30, 2026

The atmospheric characterisation of temperate exoplanets is becoming accessible with JWST, providing a critical connection between Solar System planets and the more commonly observed hot-Jupiters.

The volume mixing ratios of key gases in the Hycean and sub-Neptune atmospheres, averaged across 0.1 – 10−5 bar to represent the expected abundances of photochemical products probed by transmission spectroscopy. CH4 and CO2 are prescribed manually at the lower boundary in the Hycean model, and the DMS distribution is also imposed for our consistency tests, as described in Sections 3.2.1 and 3.2.2. -- astro-ph.EP

Astronomy & Telescopes, Exoplanets, -moons, -comets, Ice Giant, Imaging & Spectroscopy, Status Report

Three Outstanding Physical Questions For K2-18 b And Other Temperate sub-Neptunes

by Keith CowingMarch 24, 2026June 11, 2026

Recent transmission spectra of the temperate sub-Neptune K2-18 b obtained with JWST have attracted significant attention. Debates have quickly arisen over the interpretation of the spectral data, particularly the recent […]

The absorption cross sections of common absorbers in exoplanet atmospheres considered in this work, shown at a pressure and temperature of 0.1 mbar and 1000 K. The wavelength coverage of Pandora’s NIR detector (NIRDA), JWST’s NIRCam F322W2 and F444W filters, and HST’s WFC3 instrument (for the G141 grism) are shown. Pandora/NIRDA covers absorption bands of H2O, CH4, NH3 and the wing of the K doublet, making it most sensitive to these absorbers. -- NASA

Astronomy & Telescopes, Atmospheres, Climate, Weather, Biosignatures & Paleobiology, Cubesats and smallsats, Exoplanets, -moons, -comets, Imaging & Spectroscopy, Status Report

NASA’s Pandora SmallSat Mission: Simulated Modeling and Retrieval of Near-Infrared Exoplanet Transmission Spectra

by Keith CowingMarch 9, 2026March 9, 2026

Pandora is a SmallSat mission dedicated to understanding exoplanets and their host stars by disentangling the impact of stellar heterogeneity on exoplanet transmission spectra.

Mass-radius diagram of small exoplanets (with radii ranging from 1–4 R⊕) with precise densities from the PlanetS catalog.

Astronomy & Telescopes, Exoplanets, -moons, -comets, Imaging & Spectroscopy, Status Report

Densities Of Small Planets Around The M dwarfs TOI-4336 A And TOI-4342 With ESPRESSO: Three Sub-Neptunes, One Super-Earth, And A Neptune-mass Candidate

by Keith CowingFebruary 18, 2026February 18, 2026

We present the characterization of two planetary systems orbiting the M dwarfs TOI-4336 A (M3.5V) and TOI-4342 (M0V), each hosting two transiting planets previously validated with TESS and ground-based observations.

Interior structure schematic for the interior composition of sub-Neptune models presented in this work. Color is an approximate indicator of temperature (specific values depend on the model). The envelope (∼5% by mass) is a mixture of hydrogen, helium, and heavy elements (Z) represented by either water (Section 3.1) or silicates (Sections 3.2, 3.3). The mantle composition is MgSiO3, and the core composition is Fe16Si. We maintain a 2:1 mass ratio between the core and the mantle. For this work, the EMB represents a steep compositional gradient and thus controls mantle cooling by transferring heat to the envelope via conduction. Since conductive flux alone is insufficient to transport the entire interior heat to the envelope, a steep temperature gradient forms at the EMB, as seen in all models in this work. -- astro-ph.EP

Astrogeology, Atmospheres, Climate, Weather, Exoplanets, -moons, -comets, Status Report

Sub-Neptune Memories I: Implications of Inefficient Mantle Cooling and Silicate Rain

by Keith CowingJanuary 12, 2026January 12, 2026

We explore the evolution of sub-Neptune (radii between ∼1.5 and 4 R⊕) exoplanet interior structures using our upgraded planetary evolution code, APPLE, which self-consistently couples the thermal and compositional evolution […]

Wind fields in our imposed TOA 𝐴𝑏 = 0.6 case.

Atmospheres, Climate, Weather, Biogeochemical Cycles & Geobiology, Exoplanets, -moons, -comets, Press Release, Water/Hycean Worlds & Oceanography

General Circulation Models Of Hycean Worlds

by Keith CowingNovember 12, 2025November 12, 2025

Sub-Neptunes represent the current frontier of exoplanet atmospheric characterisation. A proposed subset, Hycean planets, would have liquid water oceans and be potentially habitable, but there are many unanswered questions about […]

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

Astronomy & Telescopes, Exoplanets, -moons, -comets, Imaging & Spectroscopy, Status Report

Exploring The Sub-Neptune Frontier With JWST

by Keith CowingSeptember 25, 2025September 25, 2025

Sub-Neptune planets, with sizes and masses between those of Earth and Neptune, dominate the exoplanet population.

Possible interior compositions of 55 Cnc e. Compositions consistent within the 1σ, 2σ, and 3σ M-R uncertainties are shown in orange, dark teal, and light teal, respectively. The red dashed and solid lines show xcarbon = 12 wt% and 24 wt%, respectively, the upper bounds of xcarbon assuming no volatile loss (E. A. Bergin et al. 2023). The blue curve shows interior solution from N. Madhusudhan et al. (2012), to the right of which are allowed values (1σ uncertainty). Ternary diagrams are shown for two radius assumptions: (a) visible radius, 2.04 ± 0.15 R⊕, and (b) gray radius, 2.20 ± 0.12 R⊕, following N. Madhusudhan et al. (2012). Our interior models with updated carbon and SiC EOSs are consistent with earlier studies. -- astro-ph.EP

Astrochemistry, Astrogeology, Biogeochemical Cycles & Geobiology, Exoplanets, -moons, -comets, Imaging & Spectroscopy, Press Release

Carbon-rich Sub-Neptune Interiors Are Compatible With JWST Observations

by Keith CowingAugust 22, 2025

Many possible interior compositions exist for sub-Neptunes: ice-poor, ice-rich, and water-dominated interiors can all match the measured masses and radii.

Transit spectra of the best fit non-equilibrium 1D model at resolution 200 (black solid line). This is the approximate best fit on the simulated grid of metallicity, C/O and Kzz. Values are respectively 280.7, 90.0 and 106 cm2 .s−1 . It is compare to K2-18 b observation (Madhusudhan et al. 2023) low resolution representation with an offset of -41ppm between NIRISS and NIRSpec data. The contributions of considered molecules are represented with shaded colors. -- astro-ph.EP

Astronomy & Telescopes, Atmospheres, Climate, Weather, Biogeochemical Cycles & Geobiology, Biosignatures & Paleobiology, Exoplanets, -moons, -comets, Gaia - Planetary Perspectives, Habitable Zones, Imaging & Spectroscopy, Press Release

Unraveling The Non-equilibrium Chemistry Of The Temperate Sub-Neptune K2-18 b

by Keith CowingJuly 22, 2025July 22, 2025

The search for habitable, Earth-like exoplanets faces major observational challenges due to their small size and faint signals.

K2-18 b

Coupling Magma-ocean And Atmospheres In Spectral Retrievals Of Sub-Neptunes

The Atmosphere Of K2-18 b: The Role Of Hazes, Clouds And Photoelectrons

Three Outstanding Physical Questions For K2-18 b And Other Temperate sub-Neptunes

NASA’s Pandora SmallSat Mission: Simulated Modeling and Retrieval of Near-Infrared Exoplanet Transmission Spectra

Densities Of Small Planets Around The M dwarfs TOI-4336 A And TOI-4342 With ESPRESSO: Three Sub-Neptunes, One Super-Earth, And A Neptune-mass Candidate

Sub-Neptune Memories I: Implications of Inefficient Mantle Cooling and Silicate Rain

General Circulation Models Of Hycean Worlds

Exploring The Sub-Neptune Frontier With JWST

Carbon-rich Sub-Neptune Interiors Are Compatible With JWST Observations

Unraveling The Non-equilibrium Chemistry Of The Temperate Sub-Neptune K2-18 b

Dinosaur-killing Asteroid Also Created A Dust Cloud That Insulated Earth, Charbroiling Cretaceous Creatures

Current Water Trapping Micro-habitats On The Surface Of Mars

Methanotrophy Under Extreme Alkalinity In A Serpentinizing System

ANDES, The High-resolution Spectrograph Oof The ELT: Simulated Performance Of The CORO Module And Overview Of The High-contrast Capabilities For Exoplanet Observations

When Polymers Meet Primitive Membranes: How Molecular Cooperation May Have Helped Life Begin