Climate simulations classified 18 of 22 selected Earth-like exoplanets as habitable under one shared planetary setup, including nine Hot worlds and nine Eyeball worlds. Four planets fell into the frozen Snowball class.
That label came from the model's temperature rules. It called a planet Hot when modeled surface temperature exceeded 270 K, Eyeball when it fell between 180 K and 270 K, and Snowball when it was below 180 K. The classification was a way of comparing simulated climates under stated assumptions.
A common planetary setup
The planets were drawn from the conservative Habitable Worlds Catalog. After seven candidates were discarded because of tidal-locking, mass-constraint or PLASIM-analysis issues, the final sample contained 22 planets orbiting 16 M dwarfs.
The team used PLASIM-LSG, an intermediate-complexity climate model with a shallow slab ocean and parameterized horizontal ocean heat transport. Every run treated each world as an aquaplanet, an idealized ocean-covered setup, with a 50-m slab ocean, a fixed CO2 mixing ratio of 360 ppm, zero obliquity and eccentricity, and a 1:1 spin-orbit resonance.
Atmospheric pressure was varied across 0.5, 1 and 5 times Earth's surface pressure while the CO2 mixing ratio remained fixed at 360 ppm. Because the ratio was fixed, changing pressure also changed the total CO2 mass, a built-in caveat when reading the pressure results.
Starlight set the broad climate pattern
The broad split between the Eyeball and Hot regimes tracked the model's emission temperature, or Te, a measure of a planet's outgoing energy. The discussion placed the dividing line between 250 K and 260 K and described it as remarkably independent of pressure within this simplified framework.
That led the authors to propose a screening rule for future work: prioritize planets with Te below 255 K and instellation, the stellar energy received, below 1,460 watts per square metre for habitability and biosignature surveys. The paper presents those numbers as a model-based prioritization, not a validated observational threshold.
Rotation was less informative in this sample. The analysis found no statistically significant correlations between orbital or rotational periods and the habitability indices, and interpreted rotational dynamics as secondary to incident stellar flux. Most planets were slow rotators, however, which limited firm conclusions about other rotation regimes.
Pressure changed the hot worlds most
Pressure mattered most for the Hot class. Lower-pressure cases were generally colder and higher-pressure cases hotter. Habitability changed little for Eyeball planets, while Hot planets had a higher h50 score at 0.5 P⊕. At 5 P⊕, eight of the nine Hot planets had h50 = 0, indicating that the modeled surface fraction below the index's 50°C threshold had fallen to zero.
That pressure sensitivity could be large without changing the broad climate labels. In TOI-700 d, moving from 0.5 to 5 bar shifted the global mean temperature by up to 93 K. The average shifts were 9.9 K for Snowball planets, 24.3 K for Eyeball planets and 84.5 K for Hot planets, while the three class distinctions remained robust.
Water availability narrowed the picture
Temperature alone also overstated the habitable area in some cases. When precipitation and evaporation were added to the test, the estimated habitable area fell, especially for Hot planets. Rainfall generally exceeded the 250 millimetres a year threshold, so the limiting regions were usually those where evaporation was greater than precipitation.
A map for follow-up, not a final verdict
Those results come with substantial modeling limits. PLASIM-LSG used only three spectral bands, showed Gibbs phenomenon artifacts, and lacked the photochemistry needed to represent ozone formation and long-term hydrogen-escape water loss. The simulations also imposed the same aquaplanet, fixed-composition and synchronous-rotation framework on every planet, so the classifications remain conditional on those choices.
The study is therefore best read as a map for choosing which worlds deserve closer atmospheric scrutiny. It suggests that incident stellar flux is the strongest first guide in this selected sample, that pressure deserves particular attention on Hot planets, and that the Te and instellation cutoffs could help focus future habitability and biosignature surveys. Those priorities remain model-dependent because the comparison used fixed assumptions for every planet.
Paper data and sources
Original title: Assessing the Climate and Habitability of Tidally Locked Rocky Exoplanets
Authors: Erica Bisesi, Giuseppe Murante, Jost von Hardenberg et al.
Journal/Repository: Astrobiology. 2026;0(0)
Status: Peer-reviewed
First online: 2026-08-26
DOI: 10.1177/15311074261479167
Original paper · Full text