steam world Archives - Astrobiology

Mass–radius distribution of planets from Model I compared with the observed planets around G-type host stars from the TEPCat catalog Southworth (2011). Top: Weighted mass–radius distribution of simulated rocky planets and water worlds, shown with a color map of water mass fraction (WMF). Bottom: The same weighted distribution shown without the WMF color map, with weight contours highlighted to illustrate regions of higher model likelihood and observational completeness. The model broadly agrees with the observed distribution for planets smaller than ∼ 3 R⊕ in regions of high weights but fails to explain planets larger than ∼ 3 R⊕, suggesting the presence of a H/He-rich population, comprising ∼ 15% of the full TEPCat sample and ∼ 20% of the TEPCat sub-Neptunes. -- astro-ph.EP

Astrogeology, Atmospheres, Climate, Weather, Biosignatures & Paleobiology, Exoplanets, -moons, -comets, Ice Giant, Status Report, Water/Hycean Worlds & Oceanography

The Radius Cliff is a Waterfall: Explaining Sub-Neptune Exoplanets With Steam Worlds

by Keith CowingFebruary 17, 2026February 17, 2026

The demographics of Kepler planets provide a key testbed for models of planet formation and evolution, particularly for explaining the radius valley separating super-Earths and sub-Neptunes.

Sub-Neptune exoplanets tend to be much closer to their host star than Earth is to the Sun and are thought to have atmospheres made of steam over layers of an exotic phase of water that behaves like neither gas nor liquid. (Image by Artem Aguichine UC Santa Cruz)

Exoplanets, -moons, -comets, Ice Giant, Icy Worlds, Press Release

New Model Aims To Demystify Steam Worlds Beyond Our Solar System

by Keith CowingAugust 26, 2025

For astrobiologists, the search for life beyond our solar system could be likened to where one would look in a vast desert—essentially, where there’s water. And it turns out that […]

Visualisation Of A Steam World — NASA

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

Evolution Of Steam Worlds: Energetic Aspects

by Keith CowingDecember 27, 2024

Sub-Neptunes occupy an intriguing region of planetary mass-radius space, where theoretical models of interior structure predict that they could be water-rich, where water is in steam and supercritical state.

Top panels: Results from the white light curve fit of the supreme-SPOON reduction for Visits 1 (left) and 2 (right), and fitted systematics models. The order 1 systematics-corrected light-curves are shown (teal points), binned for visual purposes, along with the best-fit transit model (black) and the systematics model including the GP component (red, with the GP scaled for visualization purposes). The vertical blue shaded regions indicate the three candidate flares with the highest signal-to-noise spectra and following typical flare evolution profiles (see Section C). Bottom panels: Spectroscopic light-curve fits to the two NIRISS/SOSS transits of GJ 9827 d from the supreme-SPOON reduction. The left panel corresponds to the first visit and the right panel to the second visit. We show the detrended broadband light-curve for order 2, and seven spectroscopic light-curves from order 1, from top to bottom. The best-fit astrophysical model is shown for each light-curve (black) and light-curves are offset relative to each other for clarity. -- astro-ph.EP

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

JWST/NIRISS Reveals The Water-rich Steam World Atmosphere of GJ 9827 d

by Keith CowingOctober 7, 2024October 7, 2024

With sizable volatile envelopes but smaller radii than the solar system ice giants, sub-Neptunes have been revealed as one of the most common types of planet in the galaxy.

Artistic representation of an exoplanet whose water ice on the surface is increasingly vaporizing and forming an atmosphere during its approach to the central star of the planetary system. This process increases the measured planetary radius compared to the value the planet would have at its place of origin. CREDIT Thomas Müller (MPIA)

Astronomy & Telescopes, Exoplanets, -moons, -comets, Gas Giants, Press Release, Water/Hycean Worlds & Oceanography

Migration Solves An Exoplanet Puzzle

by Keith CowingFebruary 9, 2024February 9, 2024

Ordinarily, planets in evolved planetary systems, such as the Solar System, follow stable orbits around their central star. However, many indications suggest that some planets might depart from their birthplaces […]

Radius histograms of synthetic planets with orbital periods shorter than 100 days for different model assumptions without any bias applied. For each panel, the full distribution is shown in gray and colored histograms are showing different compositional subsets (pure rocky in green, water-rich without any H/He in blue, with some H/He in red). The left panel shows the nominal model, in the middle panel water was assumed to be condensed out in a solid ice layer resistant to evaporation below H/He, and the right panel uses the nominal treatment of water but excludes evaporation. The dotted histogram shows the full distribution from the nominal simulations. As in Fig. 1, bin counts are normalized by the total number of planets in the different samples. -- astro-ph.EP

Astrogeology, Atmospheres, Climate, Weather, Biogeochemical Cycles & Geobiology, Exoplanets, -moons, -comets, Status Report

A Radius Valley Between Migrated Steam Worlds And Evaporated Rocky Cores

by Keith CowingJanuary 11, 2024January 10, 2024

The radius valley (or gap) in the observed distribution of exoplanet radii, which separates smaller super-Earths from larger sub-Neptunes, is a key feature that theoretical models must explain. Conventionally, it […]