Tides Tighten the Hycean Habitable Zone

Abstract

Hycean planets—exoplanets with substantial water ice layers, deep surface oceans, and hydrogen-rich atmospheres—are thought to be favorable environments for life. Due to a relative paucity of atmospheric greenhouse gases, hycean planets have been thought to have wider habitable zones (HZs) than Earth-like planets, extending down to a few times 10−3 au for those orbiting M dwarfs. In this Letter, we reconsider the hycean HZ accounting for star–planet tidal interaction. We show that for a moderately eccentric hycean planet, the surface temperature contribution from tidal heating truncates the HZ at significantly larger orbital radii and that moderate eccentricity is readily obtained from any massive outer companion in the system. Though few current hycean planet candidates orbit stars of low-enough mass for tides to plausibly significantly alter the extent of the HZ, this effect will be important to note as more such candidates are identified orbiting M dwarfs. We suggest that tides are a significant factor both for determining the extent of the hycean HZ around low-mass stars and for the development of a detectable hycean biosphere.