main sequence stars recent work shows that understanding the temporal evolution

Main sequence stars recent work shows that

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main-sequence stars, recent work shows that understanding the temporal evolution of the habitable zone is essential to determining a planet’s habitability (e.g., [ 51 , 130 , 131 ]). Hart [ 36 ] was the first to consider the temporal evolution of the habitable zone as a star ages, which he termed the “continuous habitable zone”. Subsequently, Kasting et al. [ 1 ] calculated a continuous habitable zone for stars of 0.5–1.5 solar masses during the main-sequence of stellar evolution. These authors had decided to ignore the temporal evolution of the post-main-sequence and pre-main-sequence habitable zones. However, as Danchi and Lopez [ 130 ] showed, the temporal evolution is important because the chances for life to be detected improves the longer that planets can stay within this extended temporal HZ. Not only is it possible for life to potentially exist during the formative and ending phases of a star’s life (e.g., [ 51 , 130 , 131 ]), but an educated assessment of main-sequence habitability cannot be made without understanding these other phases [ 51 ]. Very little work has been done on these topics, but this is an exciting area with many possibilities. I will summarize recent results. 8.1. Habitability during the Pre-Main-Sequence M-dwarfs are smaller and cooler than other stars, which requires that their HZ planets be on closer-in orbits. Such tightly-packed orbits trigger strong tidal forces that gradually slow rotation rates until synchronous rotation is achieved. Although these worlds may be located within the main-sequence HZ, night-side temperatures are so cold that the major atmospheric constituent (H 2 O near the inner edge or CO 2 near the outer edge) would condense out en masse, triggering atmospheric collapse and rendering them uninhabitable [ 174 ]. However, a ~1–1.5-bar CO 2 atmosphere may be dense enough to efficiently transfer heat between the day- and night-sides and maintain habitable conditions [ 175 , 176 ]. Subsequent work suggests that dense enough atmospheres may also preclude synchronous rotation entirely [ 177 ]. However, all of this assumes the untested premise that life is possible in very dense CO 2 atmospheres (see Section 15.2 ). Also, such tightly-packed orbits around M-stars suggest high impactor velocities, which tends to favor the erosion of planetary atmospheres (e.g., [ 178 , 179 ] and see counterpoints in Section 9 ). Moreover, such proximity to their host stars indicates a high radiation environment exposed to stellar winds and flares [ 180 , 181 ]. For instance, if Proxima Centauri b is assumed to have an Earth-like atmosphere, 1 bar of CO 2 can be lost in under 25 Myr, with much greater losses over geologic timescales [ 181 ]. Plus, even if the stellar radiation does not completely remove the atmosphere, the surface may be sterilized and unable to support life, much like the Martian surface today (e.g., [ 182 ]).
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