Life on the (Inner) Edge: lessons from Venus, the Habitable Zones, and Exoplanets like the SPECULOOS-2c, and the TRAPPIST-1 system

The characterization of rocky exoplanets in the Habitable Zone (HZ) of their stars has entered a new era of knowledge, with upcoming JWST observation advancing our understanding of the evolution of potentially habitable terrestrial worlds towards the inner edge of the HZ. The evolution of such worlds near the inner edge of their HZ is dominated by the effect of a higher solar flux, but this has been largely in order to establish the limits of the HZ (e.g. Kasting, 1988; Kopparapu et al., 1993) and has rarely been explored for exoplanets. Observations of our Solar System indicate that increasing stellar irradiation transforms Earth-like planets into Venus-like worlds, with uncertainties about whether water would condense at all on a Young Venus (Kasting, 1988; Turbet et al, 2022). As temperatures rise during moist and full runaway greenhouse stages, oceans would evaporate and create steam atmospheres before the water would be lost via rapid photodissociation. With upcoming observations, JWST will be able to distinguish between hot Earths, worlds caught in a moist and full runaway greenhouse stage, and Venus-analogs (i.e. Kaltenegger et al., 2023; Gomez Barrientos et al., 2023; Payne & Kaltenegger, 2024 [accepted]) because these changes in the atmosphere dominate the observable spectra of terrestrial planets. We simulate the changes of a rocky world with increasing stellar irradiation, using known exoplanets such as the Earth-sized planets in the LP 890-9 (Speculoos-2) and Trappist-1 systems. We explore the progression of an Earth-to-Venus atmospheric evolution, examining the key spectral features of the transition from a habitable exo-Earth to a moist and then full runaway greenhouse before ultimately becoming an exo-Venus atmosphere. With NASA’s new focus on exploring and understanding Venus, the history of our hotter neighboring twin planet will become a strong focus of observations. Rocky exoplanets near the inner edge of their star’s HZ may share considerable commonality with the evolutionary stages of a young Venus. Their evolution can shed light on the history of our own Solar System, the environment on a young Venus, and the future of our own world under a brightening Sun.