Researchers model nightside ice on near-synchronously rotating exoplanets

Sarah Silverman, listed as the submitter, posted a preprint to arXiv on Oct. 24, 2026 alongside coauthors Thomas Navarro, Martin Turbet, Nicolas B. Cowan, Natalya Gomez, Siddharth Bhatnagar and Emeline Bolmont, using a three-dimensional global climate model to test how water behaves on rocky planets in near-synchronous orbit around M-dwarf stars, choosing TRAPPIST-1e as the test case.

The researchers treat TRAPPIST-1e as a representative near-synchronous land planet because scientists commonly assume that planets around M-dwarf stars keep one face permanently to the star. In compact M-dwarf systems, gravitational interactions between planets can drive the substellar point, the place on the surface directly beneath the star, to librate or circulate over millennial timescales, and it can sporadically shift direction or speed. This planetary gravitational forcing is why the authors model near-synchronous rotation, where the substellar point moves, rather than a fixed substellar point as in perfect synchronous rotation. The team models a moving substellar point rather than a planet that permanently shows one face to its star.

The team’s three-dimensional global climate model measures how effectively near-synchronously rotating exoplanets trap water, the first such quantification for this class of planet. It sweeps across a range of synodic day lengths, the interval between successive noons at a given longitude, with TRAPPIST-1e as the test case.

Across the tested span of synodic day lengths, the model produces a permanent ice reservoir at the poles, and the result holds over the full range of day lengths the team examined. At lower latitudes, transient ice on the nightside builds up increasingly as the day lengthens while the polar ice reservoir persists.

The dayside remains free of a permanent ice sheet under the modeled conditions. That setup preserves an area where temperatures could allow liquid surface water even as ice accumulates on the nightside, so ice concentrates on the nightside and poles without covering the whole planet.

Feedbacks between ice height, atmospheric temperature structure, and bedrock adjustment lower the model’s predicted nightside ice buildup by a factor of ~3. The ice-elevation feedback couples ice height to atmospheric temperature and bedrock response.

The study revises nightside ice accumulation on a synchronously rotating exoplanet using a coupled atmosphere-plus-ice numerical model. The model includes atmospheric lapse rate, the way temperature changes with altitude, and isostatic adjustment, the bedrock’s response to the weight of ice. It treats ice elevation and bedrock deformation together with the atmosphere, going beyond simpler treatments of nightside ice trapping.

Taken together, the authors conclude that ice sheets trap less water than earlier work indicated for land planets around M-dwarf stars, so the habitability risk from nightside ice trapping is smaller than earlier work implied. They reach that judgment by combining the near-synchronous circulation results with the revised synchronous-rotation estimate.

The authors caution that this steady-state ice distribution may never occur. If an equilibrium ice pattern exists, the time ice needs to build up may lag behind changes in the planet’s spin regime. Shifts in spin could stop the planet from ever reaching that pattern.

The new study builds on earlier work by Thomas Navarro, Nicolas B. Cowan, Natalya Gomez and Timothy Merlis in the Bulletin of the AAS, dated Oct. 20, 2022. Titled Coupled atmospheric-ice sheet model for tidally locked exoplanets, that paper appeared in Vol. 54, Issue 8 of the Bulletin of the AAS. Navarro and Cowan and Gomez were at McGill University, Merlis was at Princeton University, and the paper carried a CC-BY 4.0 license.

The earlier paper coupled a GCM, a general circulation model that simulates the atmosphere in three dimensions, to an ice-sheet elevation and bedrock deformation model. It used that coupled system to study water ice trapped on the permanent nightside of tidally locked exoplanets. That 2022 work is the direct predecessor to the new near-synchronous analysis.

The earlier team explains that M-dwarf stars are the most common stars because of their cosmic abundance and long lifetimes. The team holds that planets in an M-dwarf habitable zone are most likely in synchronous rotation, with major consequences for climate and habitability. These worlds have no counterpart in the Solar System, and studying terrestrial exoplanets remains difficult, which keeps their climate and habitability uncertain. The abundance of M-dwarfs motivates studying planets in their habitable zones despite the lack of direct Solar System comparisons.

That earlier work identified a strong link between snowfall rates and ice elevation. It found that ice sheet elevation depends on temperatures through the whole troposphere, the lowest layer of the atmosphere. These results come from the same team’s 2022 coupled climate-ice study and relate to the ice-elevation feedback later examined in the new preprint.

The earlier work also estimated where nightside ice sheets hit a ceiling set by atmospheric water availability rather than melting at their base. That separates the constraint imposed by atmospheric water supply from the constraint imposed by basal melting. This distinction matches the 2022 work’s refinement of earlier water-trapping estimates.

The team concluded that judging whether synchronously rotating terrestrial planets are habitable calls for advanced geophysical modeling. The 2022 paper offers this conclusion as motivation for continued advanced modeling of synchronously rotating exoplanets, an effort the authors describe as still in its early days.

The arXiv record gives the citation as arXiv:2610.33884 [astro-ph.EP] and lists the paper under Earth and Planetary Astrophysics with the DOI https://doi.org/10.48550/arXiv.2610.33884. astro-ph.EP is the arXiv category for Earth and Planetary Astrophysics. The paper’s arXiv landing page lists the subject as Earth and Planetary Astrophysics and provides the DOI and citation string for the preprint.

Version v1 went up on Saturday, Oct. 24, 2026 at 19:51:12 UTC as a 5,542 KB file. The arXiv submission history names Sarah Silverman as the submitter and records these details for version 1 of the paper. The citation for this specific version is arXiv:2610.33884v1 [astro-ph.EP].

The ArXivSignals page reproduces the preprint’s abstract and assigns the paper a research-signal score on a 0 to 100 scale. The score is based on the paper’s own claims measured against a fixed rubric. The page says the score does not measure correctness or importance. ArXivSignals presents bibliometric-style scoring for the paper but explicitly disclaims that the score reflects correctness or importance, so it functions as a signal index rather than independent validation.

The findings bear on how scientists judge habitability for rocky planets around M-dwarf stars, which are the most common stars. In the authors’ framing, the study addresses how assumptions about tidal locking and climate stability shape habitability judgments for such worlds. The earlier team likewise described advanced geophysical modeling of synchronously rotating exoplanets as an effort still in its early days.