Terminator Habitability on M-Dwarf Planets

Rocky planets orbiting M-dwarf stars are among the most promising and abundant astronomical targets for detecting habitable climates. Planets in the M-dwarf habitable zone are likely synchronously rotating, such that we expect significant day-night temperature differences, and potentially limited fractional habitability. Many studies have explored mechanisms that allow, in cases of limited fractional habitability, for temperate climates to occur in the substellar or “eye” region. Here, we explore an alternate climate configuration to determine the viability of “terminator habitability”, defined as the existence of a habitable band at the transition between a scorching day side and a glacial night side.

Using the ExoCAM climate model, we show that terminator habitability does not occur in water-rich aquaplanets, but is easily achievable for water-limited land planets. On water-rich planets, increased stellar flux leads to enhanced net atmospheric energy transport to the night side, reducing the day-night temperature differences. Near the inner edge of the habitable zone, the planetary surface temperature differences become small, such that the terminator does not remain habitable once the day-side temperatures approach runaway or moist greenhouse limits. But on water-limited land planets, the day-night temperature contrasts are more than doubled. Their reduced cloud coverage and lower top-of-atmosphere albedo allows the planetary surface to receive more day-side radiation for the same stellar flux. The increased radiation facilitates high temperatures in the “eye”, while a reduction in the net atmospheric energy transport results in even colder night-side temperatures.

We also show that, while some water-abundant simulations can result in larger fractional habitability, they are highly vulnerable to water loss through cold-trapping on the night-side surface or atmospheric water vapor escape. This result suggests that even if most planets were formed with abundant water, many of their climates could become water-limited and subject to terminator habitability.