The Habitable Zones of Pre-Main-Sequence Stars


Tuesday, January 06 2015 - 12:00 pm, PST
Ramses Ramirez
Cornell University

The habitable zone (HZ) is the region around a star in which liquid water could exist on a planetary surface. Although most HZ studies have focused on the main-sequence period, here we argue that the pre-main-sequence HZ likely provides additional targets for observers. The spatial distribution of liquid water and its change during the pre-main-sequence phase of protoplanetary systems is important in understanding how planets become habitable.

Such worlds are interesting targets for future missions because the coolest stars could provide habitable conditions for up to 2.5 billion years post-accretion. Moreover, for a given star type, planetary systems are more easily resolved because of higher pre-main-sequence stellar luminosities, resulting in larger planet-star separation for cool stars than is the case for the traditional main-sequence (MS) habitable zone (HZ). Using 1-D radiative-convective climate and stellar evolutionary models, we calculate pre-main-sequence HZ distances for F1-M8 stellar types. We also show that accreting planets that are later located in the traditional MS HZ orbiting stars cooler than a K5 (including the full range of M-stars) receive stellar fluxes that exceed the runaway greenhouse threshold, and thus may lose substantial amounts of water initially delivered to them.

We predict that M-star planets need to initially accrete more water than Earth did or, alternatively, have additional water delivered afterwards to remain habitable. Our findings are also consistent with recent claims that Venus lost its water during accretion.

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