Hycean world Archives - Page 2 of 3 - Astrobiology

The comparison of reference sample with 𝐾𝑧𝑧 = 106 cm2 s−1 between GJ-176 and GJ-436. The difference between stars is not significant and only visible in the upper atmosphere, where UV flux has the biggest impact. It shows that for the purpose of our calculations, the choice of star is arbitrary, as long as it is the same stellar type. -- astro-ph.EP

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

Nitrogen Chemistry Of Hycean Worlds On The Example Of K2-18b

by Keith CowingSeptember 7, 2025September 7, 2025

A recent observation of the exoplanet K2-18b sparked interest among scientists – large amounts of carbon dioxide and methane were detected in an H2-rich background atmosphere.

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.

Modelled planetary bulk-density over time. The initial hydrogen inventory of each bulk-density evolution track is shown by the line colour. Blue/green shaded regions are reference densities for a planet of this mass (Zeng et al., 2019). Black dashed/dotted lines demarcate edges of the radius valley (at this planet mass) for low mass stars (Ho et al., 2024). The region ±1σ compatible with the estimated bulk-density of this planet is indicated by the blue errorbar (Rajpaul et al., 2024; Demangeon et al., 2021). Top panel: evolution of surface temperature (coloured) and stellar XUV energy flux (black). For visual clarity, this figure only shows cases with S/H=8. -- astro-ph.EP

Exoplanets, -moons, -comets, Status Report

Volatile-rich Evolution Of Molten Super-Earth L 98-59 d

by Keith CowingJuly 7, 2025September 18, 2025

Small low-density exoplanets are sculpted by strong stellar irradiation but their origin is unknown.

Envelope H2O mass fraction as a function of semi-major axis. The same dataset as in Figure 1 is shown. The left panel shows planets that predominantly formed inside the water ice line; the right panel shows those that formed outside. Classification is based on the accreted H2O mass fraction, with a threshold set at 5% of the total planetary mass. The colorbar indicates the molar bulk C/O ratio. Planets formed inside the ice line are systematically depleted in carbon due to the lack of volatile ice accretion and exhibit higher envelope H2O mass fractions. In contrast, planets formed beyond the ice line retain lower H2O content despite higher bulk volatile abundances. Each pie chart shows the mean mass fraction of hydrogen in H2 (gas), H (metal), H2 (silicate), H2O (gas), and H2O (silicate), normalized to the total mean hydrogen inventory for each population. Only components contributing more than 5% are labeled. Planets that formed beyond the ice line store most hydrogen as H2 (gas) and H (metal), while those that formed inside the ice line retain a larger share of hydrogen in H (metal) H2 (silicate) and H2O (gas + silicate). -- astro-ph.EP

Astrochemistry, Astrogeology, Atmospheres, Climate, Weather, Biochemistry & Organic Chemistry, Biogeochemical Cycles & Geobiology, Biosignatures & Paleobiology, Exoplanets, -moons, -comets, Habitable Zones, Ice Giant, Icy Worlds, Status Report, Water/Hycean Worlds & Oceanography

Sub-Neptunes Are Drier Than They Seem: Rethinking the Origins of Water-Rich Worlds

by Keith CowingJuly 3, 2025July 3, 2025

Recent claims of biosignature gases in sub-Neptune atmospheres have renewed interest in water-rich sub-Neptunes with surface oceans, often referred to as Hycean planets.

The HHZ (blue shaded regions) and dark HHZ (red shaded regions) around a 0.12M⊙ star for a 7M⊕, 1.7R⊕ hycean planet with tidal quality factor Q = 2. The low-opacity contours show the habitable zone locations without tidal heating (cf. N. Madhusudhan et al. 2021), and the high-opacity contours show the habitable zone location for our model where tidal heating is included. The dotted and dashed lines denote the optimistic and conservative HZ limits respectively for Earth-like planets according to the model of R. K. Kopparapu et al. (2013). The conservative HZ is bounded by their “Recent Venus” and “Early Mars” cases, while the optimistic HZ is bounded by their “Runaway Greenhouse” and “Maximum Greenhouse” cases. -- astro-ph.EP

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

Tides Tighten The Hycean Habitable Zone

by Keith CowingJune 17, 2025June 17, 2025

Hycean planets — exoplanets with substantial water ice layers, deep surface oceans, and hydrogen-rich atmospheres — are thought to be favorable environments for life.

Schematic diagram showing the three stages of cooling from Bondi radius delaminations. In (a), the planet forms a Bondi radius boundary layer of thickness ∆RB with a temperature contrast ∆TB across it. In (b) and (c), the movement of the planet through the protoplanetary disk causes the boundary layer to become unstable and delaminate. Diagram not to scale. -- astro-ph.EP

Astrogeology, Atmospheres, Climate, Weather, Lava / magma ocean / Volcanic Worlds, Status Report, Water/Hycean Worlds & Oceanography

On The Efficacy Of Ocean Formation With A Primordial Hydrogen Atmosphere

by Keith CowingJune 15, 2025August 18, 2025

It has been suggested that Earth’s present water budget formed from oxidation reactions between its initial hydrogen-rich primordial atmosphere and its magma ocean.

A top-down view of the TOI-2285 system architecture, where the scale of the figure is shown along each axis. The CHZ is shown in light green, the OHZ extensions to the HZ are shown in dark green, and the orbit of the known planet is shown as a solid line. -- astro-ph.EP

Atmospheres, Climate, Weather, Biogeochemical Cycles & Geobiology, Exoplanets, -moons, -comets, Habitable Zones, Imaging & Spectroscopy, Status Report, Venus, Water/Hycean Worlds & Oceanography

Interior And Climate Modeling of the Venus Zone Planet TOI-2285 b

by Keith CowingMay 21, 2025May 21, 2025

As the discovery of exoplanets progresses at a rapid pace, the large number of known planets provides a pathway to assess the stellar and planetary properties that govern the climate […]

Breakdown of our best-fit model for the transit spectrum of TOI-270 d. Absorption bands of CH4, CO2 are clearly visible and one isolated absorption band of CS2 contributes at 4.6 µm. The impact of water, CO and SO2 on the spectrum is negligible. The model has been smoothed for visual clarity. astro-ph.EP

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

Evidence For Sulfur Chemistry In The Atmosphere Of The Warm Sub-Neptune TOI-270 d

by Keith CowingApril 18, 2025April 18, 2025

Context: Recent JWST measurements allow access to the near-infrared spectrum of the sub-Neptune TOI-270 d, for which two different interpretations, a high-metallicity miscible envelope and a lower metallicity hycean world, […]

K2-18b - Cambridge University

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.