The result challenges a widely used idea known as the “cosmic shoreline,” a framework developed to describe the boundary between rocky planets that can retain atmospheres and those that are stripped bare by stellar radiation. In the simplest version of the concept, a planet’s distance from its star, combined with the star’s energy output and the planet’s gravity, determines whether its atmosphere survives. Worlds too close to their stars should lose their gases, while planets farther away have a better chance of keeping them. Yet observations have revealed several lava-covered planets located well inside the predicted atmospheric danger zone. These planets appear to possess substantial gaseous envelopes, forcing scientists to reconsider whether the cosmic shoreline is a fixed boundary or part of a more complex, evolving landscape.

The most prominent example is 55 Cancri e, a super-Earth nearly eight times as massive as our planet. It races around its host star at a distance roughly 20 times closer than Mercury’s orbit around the Sun, completing an orbit in less than a day. Its dayside is believed to be hot enough to maintain a global or nearly global ocean of molten rock. Under conventional expectations, radiation and charged particles from the star should drive atmospheric particles into space at extraordinary rates. However, observations made with the James Webb Space Telescope in 2024 indicated that 55 Cancri e has a remarkably thick atmosphere. The planet is not alone: recent observations have begun to reveal a growing population of similarly hot rocky worlds whose atmospheres seem to defy the conditions that should destroy them.

“A major question has been why these planets can still have atmospheres when they are so close to their stars,” said Barron Nguyen, a graduate student in the laboratory of Laura Schaefer at Stanford’s Doerr School of Sustainability and lead author of the study. “These lava worlds have pointed to something being wrong with the cosmic shoreline boundary, but we’ve found a way for them to preserve their atmospheres by proposing a new regime beyond it.” The key, according to the study, is that the atmosphere of a lava world is not simply a passive layer being eroded from above. It is part of a constantly changing system in which the molten surface, the planet’s interior, and the atmosphere exchange gases while the star attempts to remove them.

Nguyen and his colleagues constructed a model that follows this exchange over planetary timescales. The simulation accounts for atmospheric escape, the release of gases from the interior through volcanic and chemical processes, the absorption and dissolution of gases into molten rock, and the gradual cooling and solidification of the lava surface. Atmospheric escape can occur through several mechanisms. High-energy ultraviolet and X-ray radiation can heat the upper atmosphere, giving molecules enough energy to stream into space. Stellar winds can also erode the atmosphere, while intense irradiation may produce hydrodynamic outflow, in which a rapidly expanding atmosphere carries heavier particles outward with it. The strength of these processes depends on the star, the planet’s mass and gravity, the atmospheric composition, and the evolving temperature of the surface.

On a conventional rocky planet, atmospheric replenishment may come from volcanic outgassing, the release of gases dissolved in magma, or chemical reactions between rocks and the atmosphere. But the Stanford model shows that a lava ocean can change the balance dramatically. Molten rock can dissolve and store large quantities of volatile compounds, including oxygen-bearing molecules, carbon dioxide, water vapor, sulfur-containing gases, and other species. At the same time, gases from the planet’s interior can continue to rise into the atmosphere. As the lava remains hot and mobile, it prevents the planet from sealing away its volatile inventory too quickly. The atmosphere is therefore not replenished in a single catastrophic event; instead, it may be maintained through a long-lived, regulated exchange between the surface and the interior.

This creates a planetary regime in which atmospheric escape and outgassing can approach equilibrium. Radiation removes gas from the upper atmosphere, but the molten surface and interior release additional material at a comparable rate. The atmosphere may still be turbulent, chemically active, and constantly losing particles to space, yet its overall mass can remain substantial over geological timescales. This balance is the defining feature of the cosmic sandbar. According to the researchers, it lies beyond the conventional cosmic shoreline in the region where extreme stellar heating would normally be expected to eliminate an atmosphere. The sandbar is not a physical structure around a star, but a theoretical zone in the relationship between stellar irradiation, planetary mass, surface temperature, atmospheric escape, and interior evolution.

The model also explains why planets at somewhat greater distances from their stars may paradoxically be more vulnerable to becoming airless. These worlds can be cool enough for their magma oceans to solidify relatively quickly, locking gases inside minerals and the solidifying crust. Once the surface hardens, outgassing may slow dramatically. The planet then loses its remaining atmosphere to stellar radiation without an efficient mechanism for replacing it. The researchers describe this region as the “airless valley,” positioned between the cosmic sandbar and the more familiar cosmic shoreline. In this valley, planets are close enough to their stars to experience severe atmospheric erosion but not hot enough to preserve a molten surface capable of sustained replenishment. Mercury may represent a nearby example of the kind of airless world that cooled and solidified before it could maintain a durable atmosphere.

Farther from a star, cooler rocky planets may avoid the most destructive forms of atmospheric escape. Their surfaces can solidify, yet their atmospheres may survive because stellar irradiation is weaker and their volatile compounds are not removed as rapidly. Earth and Venus occupy this broader, more temperate portion of the planetary spectrum, although their histories and present conditions are very different. The new model suggests that the boundary between atmospheric and airless worlds cannot be defined by distance alone. A planet’s mass determines how strongly it holds onto gas, its star determines the intensity and evolution of high-energy radiation, and its interior determines how long it can continue supplying atmospheric material. Surface temperature and the timing of planetary cooling may be equally important, because they determine whether a world remains geologically active or becomes a sealed, volatile-trapping rock.

The concept could reshape how astronomers prioritize planets for future observations. Searches for potentially habitable environments have traditionally focused on the presence of liquid water and the orbital regions where temperatures might permit it. But before scientists can assess habitability, they must determine whether a planet has an atmosphere at all. The cosmic shoreline, amended by the sandbar and airless valley, offers a way to estimate which rocky planets are likely to retain detectable gaseous envelopes. It also provides a framework for interpreting apparently contradictory observations. A thick atmosphere on an ultra-hot super-Earth may not be an anomaly that invalidates the shoreline concept; it may be evidence that the planet occupies a different atmospheric regime, one in which a lava ocean is sustaining the gas layer from below.

Future observations will test whether the cosmic sandbar is a real population of planets or a compelling theoretical possibility. The James Webb Space Telescope and upcoming exoplanet surveys will be able to measure atmospheric compositions, temperature patterns, and signs of chemical disequilibrium on some of these worlds. Detecting molecules associated with volcanic outgassing, identifying differences between the dayside and nightside atmospheres, and comparing planets of different ages could reveal whether molten surfaces are actively regulating atmospheric mass. Such measurements may also show how quickly lava worlds evolve from atmosphere-rich planets into airless bodies as their surfaces cool. “The cosmic shoreline isn’t a lost cause,” Nguyen said. “There’s a broader set of parameters that can enable a planet to generate and maintain an atmosphere.” If the prediction holds, some of the most inhospitable-looking planets in the galaxy may become crucial laboratories for understanding how atmospheres are born, transformed, and preserved.

Subject of Research: Atmospheric retention and evolution on lava-covered rocky exoplanets

Article Title: An Evolving Cosmic Shoreline and Sandbar Bounding the Rocky Airless Valley

News Publication Date: 25-Aug-2026

Web References: https://doi.org/10.3847/2047-8213/ae9743; https://science.nasa.gov/universe/exoplanets/oceans-beaches-cosmic-shorelines-our-changing-views-of-habitable-planets/; https://planets.stanford.edu/people/laura-schaefer

References: The Astrophysical Journal Letters, DOI: 10.3847/2047-8213/ae9743

Keywords

Lava worlds, exoplanets, cosmic shoreline, cosmic sandbar, atmospheric escape, planetary atmospheres, 55 Cancri e, James Webb Space Telescope, outgassing, super-Earths, habitability, planetary evolution

Thermal phase curves represent a powerful observational tool, measuring the planet’s emitted infrared radiation as it orbits its star, thus mapping temperature variations across its surface. For TRAPPIST-1 b, the data paints a vivid picture of an intensely hot dayside with a brightness temperature measured at approximately 490 Kelvin, contrasted by near-zero emission from its nightside. Such an extreme temperature contrast is a hallmark of a body lacking a substantial atmosphere, which would otherwise redistribute heat to the nightside. A lack of phase offset further corroborates the notion of a dark, desolate surface, absorbing stellar radiation without the moderating influence of atmospheric circulation.

In stark contrast, TRAPPIST-1 c displays a somewhat cooler dayside, with temperatures measured around 369 Kelvin alongside an equally frigid nightside. This thermal profile suggests that the planet either possesses a tenuous atmosphere, potentially oxygen-rich but very thin, or alternatively has an airless surface with higher reflectivity. The possibility of a sparse oxygen atmosphere, perhaps a remnant of past atmospheric loss or photodissociation processes, hints at evolutionary divergence from TRAPPIST-1 b, despite their shared stellar environment and similar bulk compositions.

Intriguingly, the data decisively disfavors atmospheric models with surface pressures exceeding approximately one bar for both planets. This threshold, close to Earth’s atmospheric pressure at sea level, is significant, as gases under such conditions would redistribute heat efficiently, smoothing out temperature contrasts and creating measurable phase offsets, which the JWST observations do not detect. This observation forces scientists to reconsider mechanisms by which planets around red dwarfs can lose or fail to accumulate thick atmospheres over geological timescales.

The harsh radiation environment generated by low-mass red dwarfs is well-known to impact planetary atmospheres profoundly. High-energy stellar winds and frequent flares can strip away atmospheric gases, an effect exacerbated by close orbital proximities needed to maintain temperate conditions. TRAPPIST-1 b and c, residing very close to their host star, presumably endure intense stellar activity, which might explain the absence or extreme depletion of robust atmospheres. However, the presence of any oxygen or reflective surfaces on TRAPPIST-1 c opens questions about atmospheric chemistry and planetary surface interactions that merit further exploration.

The implications of these findings ripple across multiple domains of astrophysics and astrobiology. Firstly, the stark contrast with Earth-like thick atmospheres challenges assumptions about habitability around M-dwarf stars. Such stars constitute the majority of stars in the galaxy and are prime targets for searching potentially life-supporting planets. The absence of thick atmospheres on planets like TRAPPIST-1 b and c suggests that habitability criteria must take into account the difficulty of atmospheric retention under intense stellar radiation fluxes, altering priorities for biosignature searches.

Moreover, the data showcases the exceptional capability of JWST’s mid-infrared instruments to probe planetary climates and surface conditions remotely. This method provides not only temperature measurements but also constraints on atmospheric composition and pressure, offering a new lens through which to study exoplanetary atmospheres in exquisite detail. Future observational campaigns will likely exploit these methods for a broader sample of rocky exoplanets, potentially categorizing them by atmospheric presence and evolutionary history.

These discoveries also raise profound questions about the formation and evolution of planets around faint red dwarfs. That two neighboring planets within the same system — possibly having formed from similar primordial material — exhibit such divergent atmospheric states suggests complex evolutionary pathways. Differences could stem from stochastic events such as giant impacts, variable stellar activity phases, or even divergent internal geophysical processes affecting outgassing and atmospheric escape rates.

To understand the absence of thick atmospheres further, scientists rely on atmospheric escape models incorporating hydrodynamic blow-off driven by extreme ultraviolet and X-ray irradiation, combined with photochemical modeling of atmospheric constituents like oxygen, carbon dioxide, and water vapor. The observed constraints on surface pressure imply that either these models need refinement or that other unaccounted-for processes, such as magnetic field shielding, replenishment cycles, or planetary magnetospheres, play significant roles in shaping atmospheric fate.

The lack of observable phase offset in TRAPPIST-1 b’s thermal emission is particularly telling. In planets with substantial atmospheres, especially those with zonal winds and heat transport, the hottest region tends to be displaced eastward of the substellar point, generating a measurable phase shift in the infrared light curve. The zero phase offset measured here reiterates the conclusion of an airless or nearly airless world, casting light on the thermal dynamics of these distant terrains.

For TRAPPIST-1 c, the possibility of a reflective surface or a tenuous atmosphere adds an exciting dimension. Surface reflectivity can be influenced by high-albedo materials such as silicates or ice, which might exist given the planet’s cooler dayside temperature relative to TRAPPIST-1 b. Alternatively, oxygen-rich but rarefied atmospheres may represent a late evolutionary stage, where volatile loss has stripped away lighter species, leaving behind residual oxygen produced by photodissociation, a phenomenon that complicates the interpretation of oxygen as a bioindicator.

The comparison between planets within the same system underlines the dynamic complexity that awaits characterization in exoplanetary science. TRAPPIST-1 remains a cornerstone system for these inquiries, as its proximity, favorable inclination, and the number of Earth-sized planets provide unmatched opportunities for comparative exoplanetology. Upcoming JWST observations and next-generation telescopes will likely deepen this understanding and explore potential atmospheres on its more distant, potentially habitable worlds.

Ultimately, these findings redefine current paradigms about atmospheric survival in extreme environments, pushing the envelope of our understanding of exoplanet habitability. They highlight the necessity to integrate observational data with advanced modeling of stellar-planet interactions, atmospheric chemistry, and planetary interiors. As we accumulate more data from TRAPPIST-1 and similar systems, we continue to refine the conditions under which rocky planets can sustain atmospheres and, by extension, life.

Intriguingly, the study also emphasizes the importance of context when interpreting atmospheric signatures. The mere detection or absence of an atmosphere does not unambiguously indicate habitability or biological potential but must be situated within a framework that considers stellar characteristics, orbital dynamics, and planetary properties. TRAPPIST-1 b and c exemplify worlds where similar starting conditions can yield dramatically different present states, challenging the notion of “Earth-like” planets based solely on size and location.

This pioneering research utilizing JWST thermal phase curves stands as a testament to the transformative power of cutting-edge space telescopes in exoplanetary science. It sets the stage not only for more refined studies of TRAPPIST-1 but also for extensive surveys of rocky planets around nearby red dwarfs, ultimately enhancing our grasp on planet formation, atmospheric evolution, and the potential for life beyond our solar system.

As humanity’s search for understanding moves forward, the barren, dark surface of TRAPPIST-1 b and the enigmatic, cool TRAPPIST-1 c remind us that each exoplanet possesses a unique narrative shaped by complex cosmic forces. Only by unraveling these stories through meticulous observation and innovative analysis can we piece together the intricate puzzle of planetary atmospheres and the prospects for life in the cosmos.

Subject of Research: Atmospheric characterization of rocky exoplanets TRAPPIST-1 b and TRAPPIST-1 c using JWST thermal phase curve observations.

Article Title: No thick atmosphere around TRAPPIST-1 b and c from JWST thermal phase curves.

Article References:
Gillon, M., Ducrot, E., Bell, T.J. et al. No thick atmosphere around TRAPPIST-1 b and c from JWST thermal phase curves. Nat Astron (2026). https://doi.org/10.1038/s41550-026-02806-9

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41550-026-02806-9