Planetary Climates at the Edge of Habitability: Temperature and Water Cycling on Hot and Arid Terrestrial Worlds

The discovery of over 5,000 extrasolar planets (exoplanets) has raised fundamental questions about planetary habitability, diversity, and evolution. How do planetary atmospheres and surface environments respond to basic parameters such as temperature and composition? How do these climates evolve over time, and what conditions make a terrestrial planet habitable? To address these questions, I use a combination of numerical models and theoretical frameworks to explore climate transitions as a planet deviates from our most familiar reference point: present-day Earth. In the first part of this work, I focus on hot, Venus-like planets. Using a two-column, two-layer energy balance model, I show that climate evolution near the inner edge of the habitable zone is governed by the coupling between the runaway greenhouse effect and cloud feedbacks. In the second part, I turn to arid, Mars-like planets. Using both a 3D General Circulation Model (GCM) and energy balance model, I find the surface temperature distribution — specifically, the surface lapse rate — is primarily controlled by the strength of the greenhouse effect. This temperature structure, in turn, shapes the water cycle and atmospheric circulation on arid planets. For example, in an idealized GCM with suppressed surface evaporation, I identify a transition in the tropical hydrological cycle from rainy to rain-free regimes. This regime transition can be explained by a rising lifting condensation level (LCL) and stronger re-evaporation of raindrops. Together, these results deepen our understanding of terrestrial planetary climates beyond previous object-focused studies of the solar system and aquaplanet-focused studies of terrestrial exoplanets.