K2-18 b Archives

Logarithm of exomoon lifetime (log10 (𝑡max ); color coded) from stability simulations that vary a host planet’s mass, tidal constant time-lag, 𝜏p, and initial eccentricity while using Earth-like parameters for the 𝑘2 Love number and moment of inertia constant, 𝐶. -- astro-ph.EP

Astronomy & Telescopes, Exoplanets, -moons, -comets, Habitable Zones, Imaging & Spectroscopy, Status Report, Water/Hycean Worlds & Oceanography

Can Moons Exist Around The Habitable-zone Planet K2-18 b?

by Keith CowingJuly 18, 2025July 18, 2025

K2-18 b closely orbits a nearby M3 dwarf within its habitable zone, where this planet could be either a super-Earth or a mini-Neptune.

Illustration of the range of possible internal compositions for temperate sub-Neptunes like K2-18 b, and a roadmap to characterize them through atmospheric observations. A liquid-water ocean separating the atmosphere from the interior necessarily implies a small atmosphere, which may be composed of either H2 or heavier molecules. In contrast, a massive atmosphere is expected to be well-mixed with the volatile envelope underneath. We propose characterizing such massive envelopes along two principal axes: the H2-to-H2O ratio and the metallicities defined by carbon, nitrogen, and sulfur. In a standard H2-dominated envelope, the oxygen abundance tracks that of C, N, and S with the solar abundance ratio. In a mixed or steam-rich envelope, oxygen (and thus H2O) is more abundant; in a soot-rich envelope, it is less so. Nitrogen and sulfur may be depleted relative to carbon due to preferential partitioning into the mantle or core. The existence of a water-dominated layer between the envelope and the rocky interior remains uncertain, but is retained here as a possibility. Regardless of envelope composition, water may condense in the atmospheres of K2-18 b and cooler sub-Neptunes, rendering direct H2O measurements from transmission spectra unreliable as diagnostics of the bulk composition. Instead, detecting and quantifying a suite of gases, such as CH4, CO2, CO, NH3, and SO2, and their relative abundances offers a more robust pathway to distinguish among envelope compositions, physical states, and the potential presence of a liquid-water ocean. -- astro-ph.EP

Astrogeology, Atmospheres, Climate, Weather, Biogeochemical Cycles & Geobiology, Biosignatures & Paleobiology, Exoplanets, -moons, -comets, Icy Worlds, Imaging & Spectroscopy, Status Report, Water/Hycean Worlds & Oceanography

A Water-rich Interior In The Temperate Sub-Neptune K2-18 b Revealed By JWST

by Keith CowingJuly 18, 2025July 18, 2025

Temperate sub-Neptunes Are Compelling Targets For Detecting Liquid-water Oceans Beyond The Solar System. If water-rich and lacking massive hydrogen-helium envelopes, these planets could sustain liquid layers beneath their atmospheres despite […]

Potential interior structures of Uranus, Neptune, K2-18 b and TOI-270 d if they have undergone hydrogen-water demixing. We note that alternatives models are also possible (see Secs. 3 and 4). For Uranus and Neptune, the sketches correspond to the cases with Toffset = 1100 K, in which both planets are fully demixed. However, we can’t exclude scenarios in which demixing is still ongoing or does not occur at all. The sketch for K2-18 b shows a mini-Neptune scenario with ongoing demixing, while the one for TOI-270 d corresponds to a case of full demixing. -- astro-ph.EP

Ariel Mission, Astrochemistry, Astrogeology, Atmospheres, Climate, Weather, Exoplanets, -moons, -comets, Ice Giant, Icy Worlds, Neptune, Status Report, Uranus

The Possibility of Hydrogen-Water Demixing in Uranus, Neptune, K2-18b and TOI-270d

by Keith CowingJuly 13, 2025July 13, 2025

The internal structures of Uranus and Neptune remain unknown. In addition, sub-Neptunes are now thought to be the most common type of exoplanets.

Distribution of 339 planets in the stellar effective flux (𝑆eff) versus stellar temperature (𝑇eff) space. Red solid circles indicate the 17 planets orbiting within the EHZ, while gray points depict the remaining planets. Solar system planets are marked with purple circles. The green solid lines correspond to the inner and outer edges of the classical 1D habitable zone, as defined by Kopparapu et al. (2013). The blue dashed line represents the inner edge of the Extended Habitable Zone (EHZ) around M dwarf stars, assuming 30% heat transport efficiency (Wandel 2023a). The brown curve marks the outer edge of the EHZ, determined by the melting of ice sheets driven by geothermal heat (Wandel 2023b). The gray dashed line indicates the tidally locked radius around M dwarf stars. The green shaded region defines the classical HZ and the blue shaded region highlights the extended HZ for tidally locked exoplanets. -- astro-ph.EP

Atmospheres, Climate, Weather, Biogeochemical Cycles & Geobiology, Biosignatures & Paleobiology, Exoplanets, -moons, -comets, Gaia - Planetary Perspectives, Habitable Zones, Imaging & Spectroscopy, Status Report, Water/Hycean Worlds & Oceanography

Exploring The Habitability And Interior Composition Of Exoplanets Lying Within The Extended Habitable Zone

by Keith CowingJuly 8, 2025July 8, 2025

Studying the habitability, internal structure and composition of exoplanets is crucial for understanding their potential to sustain life beyond our solar system.

A family portrait of hydrocarbons in Titan’s transmission spectrum obtained with Cassini/VIMS. Best fit model (thick blue line) to Titan’s spectrum obtained during visit T10 (see Table 1) from Robinson et al. (2014), together with the individual contribution of different absorption molecules (see legend) included in the present retrieval (here, case 25, see Table 3). The best-fit residuals are reported as a function of scaled error together with their distribution in the bottom panels. -- astro-ph.EP

Astrochemistry, Astrogeology, Atmospheres, Climate, Weather, Biochemistry & Organic Chemistry, Biogeochemical Cycles & Geobiology, Biosignatures & Paleobiology, Exoplanets, -moons, -comets, Imaging & Spectroscopy, Missions & Hardware, Status Report, Titan

The Detection-vs-Retrieval Challenge: Titan As An Exoplanet

by Keith CowingJune 19, 2025June 19, 2025

Cassini’s observations of Titan’s atmosphere are exemplary benchmarks for exoplanet atmospheric studies owing to (1) their precision and (2) our independent knowledge of Titan.

Transmission spectra of K2-18 b from exoTEDRF (blue), JExoRES (black), SPARTA (orange) and Eureka! (green). For NIRISS and NIRSpec, faded points show higher resolution (R=100 and 200, respectively) spectra, and solid colors are binned for clarity. Smaller panels show error-normalized differences relative to exoTEDRF. The JExoRES spectra used different binning and are interpolated in the difference panels. -- astro-ph.EP

Astrochemistry, Astrogeology, Atmospheres, Climate, Weather, Exoplanets, -moons, -comets, Imaging & Spectroscopy, Status Report, Venus

Insufficient Evidence For DMS And DMDS In The Atmosphere Of K2-18 b

by Keith CowingMay 20, 2025September 18, 2025

Recent JWST observations of the temperate sub-Neptune K2-18 b have been interpreted as suggestive of a liquid water ocean with possible biological activity. Signatures of DMS and DMDS have been […]

Contributions of molecules in the JWST MIRI range for the transmission spectrum of K2-18 b, assuming 1% CO2 and CH4, and 10−5 by volume for each of DMS, diethyl sulfide, methacrylonitrile, trans-2-pentene, 2-butene and propyne. -- astro-ph.EP

Astrochemistry, Astronomy & Telescopes, Atmospheres, Climate, Weather, Biogeochemical Cycles & Geobiology, Biosignatures & Paleobiology, Exoplanets, -moons, -comets, Habitable Zones, Imaging & Spectroscopy, Status Report

A Systematic Search for Trace Molecules in Exoplanet K2-18 b

by Keith CowingMay 16, 2025May 16, 2025

The first transmission spectrum of the habitable-zone sub-Neptune K2-18 b with JWST has opened a new avenue for atmospheric characterisation of temperate low-mass exoplanets.

Astrochemistry, Astrogeology, Astronomy & Telescopes, Atmospheres, Climate, Weather, Biochemistry & Organic Chemistry, Biosignatures & Paleobiology, Exoplanets, -moons, -comets, Habitable Zones, Imaging & Spectroscopy, Press Release

Strongest Hints Yet Of Biological Activity Outside Our Solar System

by Keith CowingApril 17, 2025April 17, 2025

Astronomers have detected the most promising signs yet of a possible biosignature outside the solar system, although they remain cautious. Using data from the James Webb Space Telescope (JWST), the […]

The mid-infrared transmission spectrum of K2-18 b obtained with the JWST MIRI LRS instrument. The data points with error bars (in brown) show the observed spectrum as described in section 2.1. The horizontal errorbars correspond to the spectral bin width. The dark blue curve denotes the median retrieved spectral fit, while the two lighter shaded regions denote the 1- and 2-σ intervals. The prominent features of DMDS and DMS are identified. Both molecules have overlapping spectral features between 6.8-8 µm, with broader features between ∼9-10 µm for DMS and ∼10-11 µm for DMDS. The individual spectral contributions of these molecules are shown in Figure 3. -- astro-ph.EP

Astrochemistry, Astronomy & Telescopes, Atmospheres, Climate, Weather, Biochemistry & Organic Chemistry, Biosignatures & Paleobiology, Exoplanets, -moons, -comets, Imaging & Spectroscopy, Status Report, Water/Hycean Worlds & Oceanography

New Constraints On DMS and DMDS In The Atmosphere Of K2-18 b From JWST MIRI

by Keith CowingApril 17, 2025April 17, 2025

The sub-Neptune frontier has opened a new window into the rich diversity of planetary environments beyond the solar system.

Artist’s impression shows the planet K2-18b, its host star and an accompanying planet in this system. ESA/Hubble, M. Kornmesser

Astrogeology, Atmospheres, Climate, Weather, Biogeochemical Cycles & Geobiology, Biosignatures & Paleobiology, Exoplanets, -moons, -comets, Habitable Zones, Icy Worlds, Origin & Evolution of Life, Press Release, Water/Hycean Worlds & Oceanography

Atmospheres Of New Planets Might Have Unexpected Mixtures Of Hydrogen And Water

by Keith CowingMarch 28, 2025March 28, 2025

Recent studies show that the most common type of planets in our galaxy, those between the sizes of Earth and Neptune, typically form with a hydrogen atmosphere, resulting in conditions […]