Terminal Habitability: The Goldilocks Zone of Tidally Locked Exoplanets

Artist’s rendition of a tidally locked Earth-like planet. Credit: NASA/JPL-Caltech.

With the launch of NASA’s JWST, the hunt for near-Earth-sized exoplanets in the Habitable Zones of stars is heating up. The Habitable Zone (HZ) is defined as the zone around a star where it is warm enough for liquid water — and potentially life — to exist on a planet’s surface. Red dwarf stars, making up approximately 70% of all stars in the Milky Way Galaxy (and the odd mining craft), have not been stellar candidates as stars that might have life-supporting planets because red dwarf stars have low luminosity, causing their HZ to be close to their star. Consequently, planets in the HZ are often tidally locked, with one side of the planet in perpetual daylight that is too hot for life and one side in a forever night that is too cold to support life. However, it has been suggested that conditions on such a planet’s day-night terminator zone might be just right to support life. In January, JWST discovered its first rocky exoplanet orbiting a red dwarf star in its HZ.

A research team led by Ana Lobo from Caltech used global climate simulations to explore the potential for habitability in the terminator zone of planets around red dwarf stars. They considered both ocean planets and Dune-like desert planets. This work shows that habitable conditions could be maintained on desert planets but not ocean planets. This is due to the advection of water vapor, a potent greenhouse gas that leads to a steam atmosphere, and the effective transport of heat between hemispheres, homogenizing the (high) surface temperatures between the day side and night side. In contrast, water-poor desert planets do not have sufficient water to cause the same extreme greenhouse conditions and are far less efficient at moving heat between the hemispheres. In some water-rich cases, water vapor can be sequestered as ice in the night side, leading to a drying out of the day side. Over time, some of these cases may allow for terminator zone habitability. Additional observations with JWST will offer tests of these models. READ MORE