The atmospheric characterization of sub-Neptune-size exoplanets has experienced a rapid development in recent years. This progress has been enabled by observations made with JWST and other telescopes on the ground. The near-future perspective is similarly optimistic with, for example, ARIEL and the E-ELT becoming active before the end of the decade. These observations are helping us determine which planets were able to retain an atmosphere, whether primary or secondary, and what they are actually composed of. Equally important, significant work is underway to trace the evolution of the planets under the evolving influence of their host stars. In this talk, I will review the current understanding of the processes that drive the mass loss from and compositional evolution of Sub-Neptune-size exoplanets. I will highlight recent advances in the chemical-collisional-radiative modelling of their atmospheres, and identify areas where additional work is needed to produce reliable predictions. Lastly, I will connect with observations when possible and emphasize the significance of using multiple diagnostics to test the available theories.

How to cite: García Muñoz, A.: Modelling the long-term stability of high-metallicity atmospheres. Where we stand and what to do next., EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-37, https://doi.org/10.5194/epsc-dps2025-37, 2025.

The TRAPPIST-1 system, comprising seven Earth-sized planets orbiting an ultra-cool M8 dwarf, offers a unique laboratory for studying atmospheric retention on temperate rocky exoplanets. I will present the inaugural 0.6–5.2 microns JWST/NIRSpec PRISM transmission spectrum of TRAPPIST-1 d, a 0.8 Rearth planet situated at the inner edge of the habitable zone (Teq ~ 262 K). Our observations reveal significant spectral slopes (500–1,000 ppm) attributable to unocculted stellar heterogeneities. After correcting for these effects, the resulting transmission spectrum is flat within 100–150 ppm, showing no detectable molecular features.

We can exclude, with high confidence, clear 1-bar atmospheres dominated by CH4 or CO, as well as high mean molecular weight atmospheres analogous to those of a clear Titan, a clear Venus, early Mars, and both Archean and modern Earth. If TRAPPIST-1 d retains an atmosphere, it is likely either extremely tenuous or obscured by high-altitude aerosols, such as nightside water clouds predicted by 3D general circulation models. Alternatively, the planet may be airless, implying that the inner TRAPPIST-1 planets formed with <4 Earth oceans of water.

This study provides empirical constraints on atmospheric loss processes for terrestrial exoplanets orbiting M dwarfs, contributing to our understanding of the cosmic shoreline and informing models of atmospheric evolution and retention in similar systems.

How to cite: Piaulet-Ghorayeb, C. and the NEAT team and collaborators: TRAPPIST-1 d: A Case Study in Atmospheric Loss at the Inner Edge of the Habitable Zone, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-178, https://doi.org/10.5194/epsc-dps2025-178, 2025.

The “cosmic shoreline”, a semi-empirical relation that separates airless worlds from worlds with atmospheres as proposed by Zahnle & Catling (2017), is now guiding large-scale JWST surveys aimed at detecting rocky exoplanet atmospheres. We expand upon this framework by revisiting the shorelines using existing hydrodynamic escape models applied to Earth-like, Venus-like, and steam atmospheres for rocky exoplanets, and we estimate energy-limited escape rates for CH4 atmospheres. We determine the critical instellation required for atmospheric retention by calculating time-integrated atmospheric mass loss. Our analysis introduces a new metric for target selection in the Rocky Worlds DDT and refines expectations for rocky planet atmosphere searches in Cycle 4. Exploring initial volatile inventory ranging from 0.01% to 1% of planetary mass, we find that its variation prevents the definition of a unique clear-cut shoreline, though non-linear escape physics can reduce this sensitivity to initial conditions. Additionally, uncertain distributions of high-energy stellar evolution and planet age further blur the critical instellations for atmospheric retention, yielding broad shorelines. Hydrodynamic escape models find atmospheric retention is markedly more favorable for higher-mass planets orbiting higher-mass stars, with carbon-rich atmospheres remaining plausible for 55 Cancri e despite its extreme instellation. Dedicated modelling efforts are needed to better constrain the escape dynamics of secondary atmospheres, such as the role of atomic line cooling, especially for Earth-sized planets. Finally, we illustrate how density measurements can be used to statistically test the existence of the cosmic shorelines, emphasizing the need for more precise mass and radius measurements.

How to cite: Ji, X., Chatterjee, R., Park Coy, B., and Kite, E.: The Cosmic Shoreline Revisited: A Metric for Atmospheric Retention Informed by Hydrodynamic Escape, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1043, https://doi.org/10.5194/epsc-dps2025-1043, 2025.

Survive or not survive, that is the question of the 500-hour JWST Rocky Worlds DDT Program. Whether a terrestrial planets’ atmosphere can suffer under the intense XUV of its host, or if it completely escapes, these are the questions we explore. Zahnle & Catling (2017) defined the Cosmic Shoreline, but recent observations from JWST reveal airless worlds around M-stars, calling for a refinement of this “receding” shoreline (Pass et al. 2025). M-stars spend a longer time in pre-main sequence, subjecting their orbiting worlds to some higher intensity XUV activity. This complicates our present understanding of this shoreline. Investigating chemical effects of planet-star interactions could be the key to a more complete picture of this shoreline. 

We investigate the interplay between photochemistry, mixing, and escape of carbon dioxide atmospheres under intense and mild XUV fluxes as follow on work to both Johnstone et al. (2018) and Nakayama et al. (2022). We expand on this work by adopting thermal structure models from Nakayama et al. (2022) and apply them to identify key chemical pathways for escape. We create a reduced C-O chemical network including neutral and ionic species to identify these pathways. As photochemistry simulations take into account many reactions, these 1D calculations are too computationally expensive to be done in 3D. Although rudimentary at best, the mixing parameter– eddy diffusion term, K_zz, comprises the dynamical element of 1D photochemical simulations. Here, we consider the mixing of photochemical products in competition with escape to explore the chemical pathways of retention and loss. We compare the photochemical model results for active and inactive cases for the Trappist-1 system planets. Then, using the resulting composition-dependent heating and cooling rates for Trappist-1 planets, we assess their propensity for efficient atomic line cooling versus escape. We follow the work of Chatterjee & Pierrehumbert (2024) in this assessment.  Finally, using our pathway analysis, we find an analytical formula for calculating an energy-limited escape boundary for these planets based on composition. 

It is important here to note the limitations of 1D work. First, there exists an exchange of rigor between modelling chemistry and dynamics. Insights from this work are ripe for implementation into 3D GCMs, especially in response to incorporating UV-driven processes for thermospheric modelling mentioned in Ding and Wordsworth (2019). Second, interaction with the interior is important in the early phase of planetary formation, i.e., the magma ocean phase. Due to exchange between atmosphere and magma early in the planet’s formation, incorporation with an interior-atmosphere model would better constrain higher pressure chemical abundances. Although this work focuses on the upper atmosphere, extrapolation to the surface environment is a key goal for understanding a planet. 

Considering planet-star interaction is imperative for the selection of targets for observation. However, it is also important when considering anomalous detections of atmospheres around planets predicted to not have an atmosphere. This could be a first step in determining an atmosphere as non-primary and/or distinguishing between an airless planet and one with high altitude haze. 

How to cite: Blumenthal, S., Chatterjee, R., Nicholls, H., Amard, L., Tsai, S.-M., Komacek, T., and Pierrehumbert, R.: Photochemistry versus Escape in the Trappist-1 planets. , EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-952, https://doi.org/10.5194/epsc-dps2025-952, 2025.

Finding an Earth-like planet in the habitable zone of a star has proven exceedingly difficult, as it pushes the detection limits of current instruments. Up to present day, no conclusive detection of a secondary atmosphere around an Earth-sized planet has been made, whilst thick Venus-like atmospheres have been excluded in many observations. This leaves us wondering if atmospheres around rocky planets are rare.

The concept of the cosmic shoreline was proposed as a division between planets with atmospheres and those without atmospheres. This division is based on Solar System planets, comparing the escape velocity and the instellation of the planet. But even in our own Solar System, we can see that the structures of atmospheres are more complicated than just these two parameters. For example, even though the instellation at Venus’ orbital distance is higher than at Earth's, the uppermost layers of our sister planet are considerably cooler. This can be attributed to the high CO2 content of the Venusian atmosphere, which has a cooling effect at higher altitudes. This difference demonstrates the importance of the chemical composition.

In this presentation, we will take a closer look at some of the parameters that influence the thermal structure of a secondary atmosphere. These include the planetary mass, instellation, and atmospheric composition. Using self-consistent model atmospheres, we can determine the thermal losses and what set of parameters results in a loss exceeding the rate at which an atmosphere can be replenished by outgassing. We use such a grid of planetary models to determine the highest instellation a planet can endure before reaching this catastrophic mass loss rate.

However, a planetary system does not only consist of a planet, but also a star. The properties of a host star are equally important in our search for secondary atmospheres. Using stellar evolution models, we determine not only the distance where the catastrophic mass loss rate is reached, but also how it evolves throughout the lifetime of the system. Planets closer in than this distance are unlikely to retain their atmospheres.

When we calculate the habitable zone of a system, in addition to our atmospheric retention distance, we can estimate which planets could fulfil both the temperature and pressure requirements to support liquid surface water. Our grid of models shows that for stars at an age of 1 Gyr, the entire habitable zone is closer-in than the atmospheric retention distance for all stars with masses below 0.4 solar masses. These conclusions are further complicated if we take into account the initial rotation rate of the star. Initially rapidly rotating stars remain much more luminous in X-ray and UV wavelengths for much longer, exposing the planet to a higher instellation for much longer. Consequently, the atmospheric retention distance is farther out for initially fast-rotating stars.

In the figure below, we show where planets scheduled for observation fall on this stellar mass vs orbital distance plot. The planets represent scheduled JWST targets (blue circles) and Ariel candidates (red triangles) with masses below 2 Earth masses.  The red line denotes the atmospheric retention distance for a CO2-dominated atmosphere, whilst the green shaded area shows the habitable zone. This figure demonstrates that it is unlikely that any of these planets will hold on to any significant atmosphere.

How to cite: Van Looveren, G., Boro Saikia, S., Herbort, O., Schleich, S., Güdel, M., Johnstone, C., and Kislyakova, K.: Atmospheric retention distances for rocky exoplanets: Are there even atmospheres where we look?, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1265, https://doi.org/10.5194/epsc-dps2025-1265, 2025.

Short-period exoplanets are highly irradiated by their host stars’ X-ray and EUV (XUV)
photoionising radiation which heats the upper atmosphere resulting in atmospheric escape.
Because some metals (e.g., C, N, O, Mg, etc.) have larger X-ray photoionisation cross sections
and because X-rays are energetic enough to induce multiple ionisations per photon, it is
necessary to carefully treat the interaction between X-rays and metals. Our model, Wind-AE, is
a relatively fast, open-source 1D hydrodynamic photoionisation relaxation model that takes into
account X-ray and metal physics. Here, we present the findings of several case studies of
metal-rich outflow structure and energetics across planetary mass, radius, flux, and metallicity
space, as well as grids of mass loss rates for high metallicity planets (Z=1,5,10,100). In the high
metallicity regime, as well as in the high XUV flux regime, we find that C, O, Fe, and Ca line
cooling are significant. We also find that mass loss rates decrease more steeply with increasing
metallicity for planets with high escape velocities than for planets with lower escape velocities.

How to cite: Broome, M., Murray-Clay, R., Tang, Y., Owen, J., and McCann, J.: Modeling Atmospheric Escape from Metal-rich Planets with Wind-AE, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1850, https://doi.org/10.5194/epsc-dps2025-1850, 2025.