Climates of TRAPPIST-1d and e with various surface water distribution

Over the past decade, various astronomical observations have revealed the extreme diversity of exoplanets. Among them, some exoplanets are thought to be rocky planets within the habitable zone. Most studies that investigated their habitability have made the assumption that their water reservoir was large enough that it would fully cover the planetary surface. However, recent theoretical studies show that the amount of water can in fact vary over a wide range (Kimura & Ikoma, 2022). A planet with a relatively small amount of water on its surface is called a land planet. Climate of land planets is controlled by the atmospheric circulation and has been shown to be more resilient to the runaway greenhouse of water vapor, which in turns extends the inner edge of the habitable zone (Kodama et al., 2019). For tidally-locked exo-terrestrial planets, their climates have been investigated using 3D GCM. The relationship between climate and the surface water distribution has been gradually studied in recent years (Lobo et al., 2022; Macdonald et al., 2022). Here, we investigate climates of TRAPPIST-1d and 1e assuming they are land planets (i.e., with small surface water reservoirs) and discuss whether their climates can maintain liquid water on their surface or not. We found that TRAPPIST-1d cannot maintain a stable climate whatever the initial surface water distribution assumed, even when the dry dayside emits a strong thermal radiation to space. For TRAPPIST-1e, we recover surface liquid water with limited boundary between dry and icy regions, in agreement with Leconte et al. (2013). Our work confirms that the inner edge of the habitable zone around TRAPPIST-1 is located between the orbits of TRAPPIST-1d and e (Wolf 2017, Turbet et al. 2018), whatever the initial water reservoir assumed. Thus, our results highlight the importance of studying TRAPPIST-1d as a natural laboratory to test the concept of the habitable zone in JWST era.