K2-18 b Archives

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 […]

Hycean world K2-18 b -- Grok via Astrobiology.com

Atmospheres, Climate, Weather, Biochemistry & Organic Chemistry, Biogeochemical Cycles & Geobiology, Biosignatures & Paleobiology, Exoplanets, -moons, -comets, Habitable Zones, Origin & Evolution of Life, Status Report, Water/Hycean Worlds & Oceanography

Prospects For Biological Evolution On Hycean Worlds

by Keith CowingFebruary 13, 2025February 13, 2025

Recent detections of carbon-bearing molecules in the atmosphere of a candidate Hycean world, K2-18 b, with JWST are opening the prospects for characterising potential biospheres on temperate exoplanets.

Maximal (red) and minimal (blue) atmospheric heating factor vs instellation, for three values of the heat redistribution parameter: f = 0.2 (solid), 0.5 (dashed), and 1 (dotted). The locations of the four planets in Table 1 considering their instellation are marked by vertical green stripes. Modified from Fig. 11 of Wandel (2018). The allowed habitable Hatm ranges of Earth, Mars and Venus for f = 1 are marked by vertical blue strips. The effective values of Hatm for these terrestrial planets are represented by blue dots. -- astro-ph.EP

Astrogeology, Astronomy & Telescopes, Atmospheres, Climate, Weather, Exoplanets, -moons, -comets, Habitable Zones, Icy Worlds, Status Report, Water/Hycean Worlds & Oceanography

Potential Interior Structures and Habitability of Super-Earth Exoplanets LHS 1140 b, K2-18 b, TOI-1452 b and TOI-1468 c

by Keith CowingDecember 12, 2024December 12, 2024

We analyze four super-Earth exoplanets, LHS 1140 b, K2-18 b, TOI-1452 b, and TOI-1468 c, which orbit M-dwarf stars in the habitable zone.

Hycean and mini-Neptune scenarios with two photochemical models. Top row: Original simulations from Wogan et al. (2024), showing the mixing ratio of molecules against atmospheric pressure in bar. Bottom row: VULCAN simulations of the same scenarios with equivalent assumptions (see main text). The scenarios are: an uninhabited Hycean (left column), an inhabited Hycean (middle column), and a mini-Neptune case (right column). The given molecular constraints are from the “One offset” case (Madhusudhan et al. 2023b) as this case gives the best fit to the observed data. 1-σ error bars are given for detected molecules (CH4 - orange; CO2 - green), and 2-σ upper limits are given for CO (purple), H2O (blue), and NH3 (grey), which were not detected by Madhusudhan et al. (2023b). The photosphere, the region in the atmosphere which impacts the transmission spectrum observed, is given in a light grey shade between 10−2 – 10−4 bar. A variety of simulations were run with both the W24 setup (see Table A1) and VULCAN (see Table A2). All of the simulations shown in this figure use the GJ 176 spectrum. Only the inhabited Hycean scenarios in both models are consistent with the retrieved abundances. -- astro-ph.EP

Atmospheres, Climate, Weather, Biogeochemical Cycles & Geobiology, Status Report, Water/Hycean Worlds & Oceanography

Considerations for Photochemical Modeling of Possible Hycean Worlds

by Keith CowingOctober 11, 2024September 18, 2025

JWST is revolutionising the study of temperate sub-Neptunes, starting with the first detection of carbon-bearing molecules in the habitable-zone sub-Neptune K2-18 b. The retrieved abundances of CH4 and CO2 and […]

Simulated VMR profiles for the main chemical species for 1 bar and 1000 bar LHS 1140 b with A = 0.3 using KINETICS and VULCAN. Note that the in KINETICS runs the VMRC3H4 is the sum of VMRCH3C2H and VMRCH2CCH2 -- astro-ph.EP

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

Probing Cold-to-Temperate Exoplanetary Atmospheres: The Role of Water Condensation on Surface Identification with JWST

by Keith CowingSeptember 18, 2024September 18, 2024

Understanding the surface temperature and interior structure of cold-to-temperate sub-Neptunes is critical for assessing their habitability, yet direct observations are challenging.

Possible conceptual structures for K2-18 b that are explored previously (Hu 2021; Hu et al. 2021; Tsai et al. 2021; Yu et al. 2021; Madhusudhan et al. 2023b; Shorttle et al. 2024; Wogan et al. 2024) and in this work. -- astro-ph.EP

Astrochemistry, Astrogeology, Biogeochemical Cycles & Geobiology, Biosignatures & Paleobiology, Exoplanets, -moons, -comets, Habitable Zones, Icy Worlds, Imaging & Spectroscopy, Neptune, Status Report, Water/Hycean Worlds & Oceanography

Volatile-rich Sub-Neptunes as Hydrothermal Worlds: The Case of K2-18 b

by Keith CowingSeptember 11, 2024September 11, 2024

Temperate exoplanets between the sizes of Earth and Neptune, known as “sub-Neptunes”, have emerged as intriguing targets for astrobiology. It is unknown whether these planets resemble Earth-like terrestrial worlds with […]

The transmission spectrum of the sub-Neptune K2-18 b, a candidate Hycean world (from Madhusudhan et al. 2023). The observed JWST spectrum is shown in the orange and red data obtained with the NIRISS and NIRSpec instruments on JWST as noted in the legend. The dark blue line denotes the median-fit model spectrum while medium and lighter blue regions denote the 1σ and 2σ contours, respectively. CH4 and CO2 are detected robustly at 5σ and 3σ confidence, respectively, as evident from their strong spectral features labelled in the figure. Only marginal evidence for DMS is found in regions of the spectrum overlapping with the strong CH4 and CO2 features as shown. The yellow circles show the median-fit model binned to the same resolution as the data. -- astro-ph.EP

Atmospheres, Climate, Weather, Exoplanets, -moons, -comets, Gaia - Planetary Perspectives, Habitable Zones, Origin & Evolution of Life, Status Report, Water/Hycean Worlds & Oceanography

The Hycean Paradigm in the Search for Life Elsewhere

by Keith CowingJune 19, 2024June 19, 2024

The search for habitable conditions and signs of life on exoplanets is a major frontier in modern astronomy. Detecting atmospheric signatures of Earth-like exoplanets is challenging due to their small […]

Comparisons between the theoretical transmission spectra generated by EPACRIS (solid lines) and the JWST observations for (Top) H2:H2O=50:50 and [C+N+S]=100×solar metallicity scenario of K2-18 b assuming Ab = 0.3 corresponding to the left panel of Figure 4, and (Bottom) H2:H2O=75:25 and [C+N+S]=100×solar metallicity scenario of TOI-270 d assuming Ab = 0 corresponding to the right panel of Figure 4. The grey symbols with error bars indicate JWST observations of corresponding exoplanets taken from Madhusudhan et al. (2023) for K2-18 b (Top), and Benneke et al. (2024) for TOI-270 d (Bottom). Each color represents a spectrum generated by excluding specific species: green for no CH4, red for no CO2, light blue for no OCS, blue for no H2O, teal for no CO, light brown for no NH3, and black for all species included. -- astro-ph.EP

Astronomy & Telescopes, Atmospheres, Climate, Weather, Biochemistry & Organic Chemistry, Biogeochemical Cycles & Geobiology, Biosignatures & Paleobiology, Exoplanets, -moons, -comets, Imaging & Spectroscopy, Status Report

Chemical Mapping of Temperate Sub-Neptune Atmospheres: Constraining The Deep-interior H2O/H2 Using The Atmospheric CO2/CH4

by Keith CowingJune 7, 2024June 7, 2024

Understanding the envelope composition of sub-Neptune-type exoplanets is challenging due to the inherent degeneracy in their interior composition scenarios. Particularly, the H2O/H2 ratio, or can be expressed as the O/H […]

This infographic compares the characteristics of three classes of stars in our galaxy: Sunlike stars are classified as G stars; stars less massive and cooler than our Sun are K dwarfs; and even fainter and cooler stars are the reddish M dwarfs. The size of the habitable zone is different for each class of star. In our solar system, the habitable zone begins just beyond the orbit of Venus and almost encompasses Mars. Credits: NASA, ESA and Z. Levy (STScI)

Astronomy & Telescopes, Atmospheres, Climate, Weather, Biosignatures & Paleobiology, Exoplanets, -moons, -comets, Habitable Zones, Imaging & Spectroscopy, Press Release

Reconnaissance Of Potentially Habitable Worlds With Webb Space Telescope

by Keith CowingJune 5, 2024June 5, 2024

Exoplanets are common in our galaxy, and some even orbit in the so-called habitable zone of their star. NASA’s James Webb Space Telescope has been busy observing a few of […]

Model transmission spectra used for cross-correlation. Top: Transmission spectra with contributions from H2O (blue), CH4 (brown) and NH3 (green), with the spectral baseline set by H2-H2 and H2-He collision-induced absorption. Bottom: Cloud-free and cloudy transmission spectra for the CH4 model with a grey-opacity cloud deck placed at various cloud top pressures. The cloud-free spectrum shown in brown is the same in both figures. -- astro-ph.EP

Astronomy & Telescopes, Atmospheres, Climate, Weather, Biogeochemical Cycles & Geobiology, Exoplanets, -moons, -comets, Status Report, Water/Hycean Worlds & Oceanography

High-resolution Spectroscopic Reconnaissance Of A Temperate Sub-Neptune

by Keith CowingMarch 29, 2024

The study of temperate sub-Neptunes is the new frontier in exoplanetary science. A major development in the past year has been the first detection of carbon-bearing molecules in the atmosphere […]