Climates of Terrestrial Exoplanets and Biosignatures

Understanding the climates of terrestrial exoplanets and the detectability of biosignatures is an inherently interdisciplinary challenge, requiring the integration of insights from Solar System exploration, exoplanet observations and climate science. Building from Earth as the only known inhabited planet, NCCR PlanetS has developed models, tools and observational strategies to assess planetary environments far beyond direct reach. Between 2018 and 2025, PlanetS made major contributions across theory, modelling, instrumentation and mission preparation. On the modelling side, the Generic Planetary Climate Model enabled climate studies across a wide range of planetary regimes, from early Venus to temperate terrestrial exoplanets including Proxima b, incorporating advanced developments such as a dynamical slab ocean. In parallel, the THOR global climate model was developed to avoid Earth-centric assumptions and to stably simulate diverse atmospheric regimes. PlanetS has also advanced atmospheric retrieval techniques combining forward modelling, Bayesian inference and machine learning, applied to targets ranging from Solar System bodies to exoplanet phase curves and directly imaged spectra. These efforts have helped assess the scientific return of future missions, notably the Large Interferometer for Exoplanets (LIFE) and to define instrumental requirements for detecting Earth-like atmospheres and biosignatures. Within the Solar System, PlanetS contributed key technologies for biosignature detection, including ORIGIN and SenseLife, enabling in-situ and remote detection of organics, isotopic ratios and microstructures. Finally, PlanetS has played a major role in preparing the next generation of observatories, from JWST, VLT and ELT instruments to LIFE and the Habitable Worlds Observatory. Together, these contributions form an integrated framework advancing the search for life beyond Earth.

💡 Research Summary

The paper provides a comprehensive overview of the interdisciplinary work carried out by the NCCR PlanetS consortium between 2014 and 2025 on the climate of terrestrial exoplanets and the detection of biosignatures. It begins by noting the rapid growth of the exoplanet census—now exceeding 5,800 confirmed planets—and the shift from the discovery of hot Jupiters to the detection of small, temperate worlds around M‑dwarfs such as Proxima b and the TRAPPIST‑1 system. Because these planets differ markedly from Earth in stellar spectrum, activity level, and likely tidal locking, the authors argue that a robust definition of habitability must be grounded in physical climate processes rather than Earth‑centric assumptions.

Habitability is broken down into five essential criteria: liquid water, an energy source, essential nutrients (CHNOPS), protection from harmful radiation, and long‑term stability. The classic Habitable Zone (HZ) is reviewed, with inner limits set by the runaway or moist greenhouse and outer limits by the maximum greenhouse effect. The paper stresses that HZ boundaries evolve with stellar luminosity, atmospheric composition, and planetary rotation, leading to the concept of a Continuous Habitable Zone (CHZ). It also highlights the limitations of a static HZ, especially for tidally locked planets where atmospheric collapse, CO₂ condensation, and heat redistribution become dominant factors.

The core of the work is the development and application of a hierarchy of climate models. Simple 1‑D radiative‑convective equilibrium (RCE) models and 1‑D energy‑balance models (EBM) enable rapid exploration of large parameter spaces, while sophisticated 3‑D General Circulation Models (GCMs) such as the Generic Planetary Climate Model (G‑PCM) and the THOR model capture self‑consistent dynamics, cloud physics, and chemistry. THOR is highlighted for its deliberately non‑Earth‑centric design, allowing arbitrary atmospheric compositions (including hydrogen‑rich or CO₂‑rich cases) and rotation rates, which is crucial for studying low‑mass‑star planets.

Key scientific findings include: (1) the identification of a CO₂ collapse mechanism on the night side of tidally locked planets, establishing a pressure threshold below which CO₂ condenses and dramatically narrows the outer HZ; (2) the quantification of Collision‑Induced Absorption (CIA) in hydrogen‑dominated atmospheres, showing that CIA can extend the outer HZ for M‑dwarf planets by providing additional infrared opacity; (3) the demonstration that tidal locking can actually permit liquid water at higher stellar irradiance than rapidly rotating planets because of efficient day‑night heat transport; (4) a statistical analysis of atmospheric escape that defines a “cosmic shoreline” where low‑mass planets lose their atmospheres, while larger rocky worlds retain them more readily. These results refine the location of habitable zones and inform target selection for future observations.

On the observational side, PlanetS built an end‑to‑end atmospheric retrieval framework that combines forward modeling, Bayesian inference, and machine‑learning emulators. Applied to simulated JWST, ELT, and LIFE data, the framework predicts the signal‑to‑noise ratios required to detect water vapor, CO₂, methane, and other potential biosignatures in transmission, emission, and direct‑imaging spectra. The consortium also contributed hardware for in‑situ and remote biosignature detection, notably the ORIGIN and SenseLife instruments, which can measure organics, isotopic ratios, and microstructures on planetary surfaces and in plume environments.

Finally, the paper discusses the implications for upcoming missions. It evaluates the scientific return of the Large Interferometer For Exoplanets (LIFE) and the Habitable Worlds Observatory (HWO), defining instrumental specifications such as spectral resolution, contrast, and temporal coverage needed to discriminate true biosignatures from false positives. It also outlines how current facilities (JWST, VLT, ELT) can be leveraged to characterize the atmospheres of Proxima b and TRAPPIST‑1 planets, thereby building a legacy dataset that will guide the design of next‑generation observatories.

In summary, the article presents a unified framework that links climate theory, numerical modeling, atmospheric retrieval, biosignature detection technology, and mission concept studies. By integrating Solar System analog research with exoplanet simulations and instrument development, PlanetS provides a roadmap for assessing planetary habitability and for the eventual discovery of life beyond Earth.