The goal of this session is to establish synergy between the theoretical and observational aspects of the search for habitable exoplanet atmospheres. We welcome contributions related to:
- Which rocky planets can retain observable atmospheres?
- Which factors can influence observations (e.g., clouds, hazes, surface conditions…)?
- Under which combinations of planetary and stellar parameters could worlds be habitable?
- Which biomarkers are observable (with current or upcoming instruments)?
- How can biosignatures be distinguished from false positives?
- What instrumental capabilities are needed to make these observations?
Rocky planets hosted by M-dwarf stars represent the most abundant and accessible class of potentially habitable targets for atmospheric characterisation with current and planned observatories. Owing to the close proximity of the habitable zone around these cooler stars, such planets are expected to be tidally locked, giving rise to a set of atmospheric circulation regimes determined primarily by planetary rotation rate and incident stellar flux. Previous climate modelling studies have commonly identified a characteristic 'eyeball' habitable climate for these worlds, illustrative of the approximately circular area surrounding the sub-stellar point where surface temperatures rise above freezing. Using an ensemble of three general circulation models (ExoCAM, LFRic, and ROCKE-3D), we examine the influence of circulation regime on surface habitability across the inner edge of the M-dwarf habitable zone, simulating Earth-like aquaplanets with rotation periods spanning the ‘fast’, ‘Rhines’, and ‘slow’ regimes (4.25–44.33 days). We make use of a new metric of surface habitability which has been previously validated against past and present habitability on Earth, and extends beyond the traditionally-used 'liquid water' temperature condition to define two habitable temperature ranges for each microbial and complex life, as well as using surface water fluxes as a proxy for water and nutrient availability. This additional constraint produces spatial patterns of habitability that differ from those defined by temperature alone, whereby large areas surrounding the substellar point with habitable temperatures but negative net precipitation (P - E < 0) are now designated as ‘limited’ habitability. Furthermore, distinct spatial patterns of habitability emerge across the ensemble for each regime, indicating a dependence on the atmospheric circulation and associated transport of heat and moisture. For slower-rotators, habitable area is substantially reduced as surface moisture is largely confined to the day-side, while faster rotators show a more extensive habitable area but greater variation between the models in global habitable fraction.
How to cite: Woodward, H., Rushby, A., Komacek, T., Sergeev, D., and Mayne, N.: More than meets the eye(ball) for tidally-locked habitability: dependence on atmospheric circulation regime, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13757, https://doi.org/10.5194/egusphere-egu26-13757, 2026.
The probability of long-term survival of putative life on exoplanets has direct implications for the prevalence of extant life elsewhere. Environmental stability can be greatly attributed to abiotic features of a planetary body. However, we know that Earth’s current state is largely the result of life. Untangling biotic and abiotic influence, though, from Earth's deep history is difficult. To study these phenomena, we turn to computer simulation. We utilize, modify, and, in some cases, combine Planets Model Code (Tyrrell 2020), Tangled Nature Model (Christensen et al. 2002), and Daisy World (Watson & Lovelock 1983) to conduct a series of computer experiments. First, we modify and utilize Planets Model Code (Tyrrell 2020) to investigate worlds that harbor passive biota, which can only affect the environment in a random and unchanging manner over time. In this model, findings from a moderate sample study suggest that the probability of survival ( ps ) of life grows considerably with the increase in life's viable temperature range ( ΔT ) and follows the power law: ps ∝ ΔT 4. Also, we find that the chances of survival of any life on a given planet decrease linearly with time. Finally, we discern that the chances of survival of eukaryotic analogues remain low regardless of their emergence time in a planet's history. We complement these findings with two additional studies. Our current endeavor is to create a new model that adds an active set of evolving and competing species which can affect temperature only on a local scale and temporary basis. To build this adaptive ecology simulation, we modify and merge Planets Model Code (Tyrrell 2020) and Tangled Nature Model (Christensen et al. 2002). Planets Model Code (Tyrrell 2020) is utilized to simulate the climactic characteristics of the exoplanet. Tangled Nature Model (Christensen et al. 2002), which is utilized to run the ecological evolutionary model, operates in the form as modified by Arthur and Nicholson (2023), but with a few additional modifications of our own. Findings from this effort are soon forthcoming. Finally, we comment on plans for a future study, in which we propose a separate model wherein an active ecosystem is the dominant driving force in the stability, or lack thereof, of its home planet. By assessing ps in these limiting cases, we seek to understand if life can be a driver of planetary environmental stability.
References:
Arthur, Rudy and Arwen Nicholson (2023). “A Gaian Habitable Zone”. In: Monthly Notices of the Royal Astronomical Society 521.1. Publisher: Oxford University Press, pp. 690–707.
Christensen, Kim et al. (2002). “Tangled Nature: a Model of Evolutionary Ecology”. In: Journal of Theoretical Biology 216.1. Publisher: Elsevier, pp. 73–84.
Tyrrell, Toby (Oct. 2020). Planets Model code. DOI: 10.5281/zenodo.4081451.
Watson, Andrew J. and James E. Lovelock (Jan. 1983). “Biological Homeostasis of the Global Environment: the Parable of Daisyworld”. In: Tellus B: Chemical and Physical Meteorology 35.4, p. 284. ISSN: 1600-0889, 0280-6509.
How to cite: Evans, J., Lingam, M., and Riousset, J.: Biotic Factors in Long-Term Planetary Habitability, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14528, https://doi.org/10.5194/egusphere-egu26-14528, 2026.
How to cite: Lloret, Z. and Voigt, A.: Kilometer-scale Climate Modeling of TRAPPIST-1e Using ICON-Sapphire: Peering Through the High Clouds, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16853, https://doi.org/10.5194/egusphere-egu26-16853, 2026.
Planetary habitability is often framed in terms of static boundaries such as the circumstellar habitable zone [1]. However, planetary climates are intrinsically nonlinear and may admit multiple coexisting climate stable states under identical stellar and atmospheric forcing due to feedbacks such as ice–albedo and greenhouse processes [2, 3]. This climate multistability implies that Earth-like planets can occupy fundamentally different climate regimes including temperate, globally glaciated (snowball), and post-runaway greenhouse states, and can undergo abrupt transitions between them at critical forcing thresholds. Such tipping points have profound implications for long-term habitability, as rapid transitions could outpace biological adaptation.
Here, we investigate the multistable climate structure of an Earth-like aquaplanet using a computationally efficient dynamical slab ocean model [4] coupled to the Generic-PCM global climate model (GCM; previously the LMD Generic GCM [5, 6]). The ocean model features sea-ice and snow evolution, wind-driven (Ekman) transport, horizontal eddy diffusion, Gent–McWilliams transport, and convection, while remaining much cheaper than a fully dynamic ocean model. In a modern-Earth configuration, the coupled system reproduces key observed climatic attributes, including the major oceanic heat flows, an annually averaged surface temperature of 13°C, a planetary albedo of 0.32, and sea ice coverage spanning 18 million sq. km [4].
We perform systematic parameter space exploration in stellar forcing to construct bifurcation diagrams and map the stable climate branches of an Earth-like aquaplanet. We identify at least five distinct stable climate regimes, including states previously inaccessible in the Generic-PCM but consistent with results obtained using fully dynamic ocean models [3, 7]. By selectively disabling ocean heat transport at the edge states, we demonstrate which branches are primarily sustained by atmospheric processes (e.g., the post-runaway states seen in [8, 9]), and which rely on ocean dynamics (this work). These results illustrate how climate multistability fundamentally reshapes the mapping between planetary and stellar parameters and the range of conditions under which worlds can be habitable.
Distinct stable climate states imply (radically) different observables, from high-albedo snowball planets to warm, water-rich post-runaway climates with enhanced water vapour columns and cloud cover. We therefore discuss the implications of climate multistability for the interpretation of exoplanet observations, with a particular focus on how combining complementary spectral regimes (reflected light + thermal emission; HWO + LIFE) can provide synergistic constraints on atmospheric and surface properties [e.g., [10]).
References:
[1] Kasting et al. (1993)
[2] Strogatz (2018)
[3] Brunetti et al. (2019)
[4] Bhatnagar et al. (in review)
[5] Hourdin et al. (2006)
[6] Forget et al. (in prep)
[7] Brunetti & Ragon (2023)
[8] Turbet et al. (2021)
[9] Chaverot et al. (2023)
[10] Alei et al. (2024)
How to cite: Bhatnagar, S., Bolmont, E., Brunetti, M., and Kasparian, J.: Climate multistability and the dynamical boundaries of planetary habitability, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18746, https://doi.org/10.5194/egusphere-egu26-18746, 2026.
To this day, there has not yet been any definitive detection of an atmosphere around an Earth-sized planet. Most of these observations are of planets orbiting M-dwarfs very close-in, where conditions are much harsher than at an Earth-like distance from a Sun-like star. Both thermal and non-thermal loss processes are likely much more effective in removing the atmospheres of these M-dwarf Earth-sized planets. However, the existence of Hot-Jupiters demonstrates that large atmospheres can be retained around massive enough close-in planets. Even in our own Solar System we can see the importance of planetary mass to atmospheric retention when we look at Earth and its two neighbours.
In this work we explore how the mass of close-in exoplanets affects the loss of secondary atmospheres. We achieve this by using the Kompot code, a 1D self-consistent thermo-chemical code, to model various CO2/N2 upper atmospheres from first principles. These models allow us to calculate the Jeans escape, a type of thermal escape. We then combine these loss calculations with models of various stellar types to determine which planet-star combination is most likely to retain an atmosphere. These results are particularly useful to select targets for observations with large instruments such as JWST.
How to cite: Van Looveren, G., Kislyakova, K., Raorane, A., Mueller, L., and Macdonald, E.: From Earth-sized to Super-sized, the importance of planetary mass for atmospheric retention, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20532, https://doi.org/10.5194/egusphere-egu26-20532, 2026.
