Terminator zones and the search for life
For a terminator zone to be potentially habitable, it needs to be in orbit around the right size of star. Planets only develop tidal locking when they orbit close to their host star. Mercury is the closest thing to a tidally locked planet that we see in our Solar System, but it is too far to have become fully locked; it takes 88 days to orbit the Sun, and 57 days to rotate (a 3:2 spin-orbit resonance). But as we see from Mercury, even its proximity to a star like our Sun creates temperatures that are far too extreme for habitability.
Around cooler, smaller red dwarf (or M-dwarf) stars, the region where planets can become tidally locked aligns with the star’s habitable zone, where temperatures allow for liquid water on a planet’s surface. About 75% of all stars are M-dwarf stars. In comparison, our Sun, a G-type main sequence star that makes up only about 7% of stars in the Universe.
“We suspect many of the planets orbiting M-stars are tidally locked because they are close to their stars, while still being in the habitable zone, which is an interesting scenario,” said Stephen Kane, a professor of planetary astrophysics at the University of California, Riverside who was not affiliated with the Lobo study.
Because life as we know it requires liquid water, the search for life on exoplanets has typically focused on water-abundant worlds. A narrow window of 0-100 degrees Celsius and a consistent and sufficient atmospheric pressure are needed for liquid water to pool. While these conditions are rare to find on entire exoplanets, they might be more common along terminator zones of tidally locked exoplanets, as long as they meet certain other criteria.
One other condition necessary for habitability may be volcanism. Kane believes that volcanism is a “necessary but insufficient requirement for habitability” So, while he expects to see volcanism on an exoplanet with a terminator zone, volcanism is not the only indication that the exoplanet will have one.
“Volcanism could play an interesting and important role in the climate of these planets,” Lobo added.
A highly active volcanic planet, however, may produce too much carbon dioxide and effectively destroy its chances at habitability along the terminator zone. According to Kane, excessive amounts of carbon dioxide could create a runaway greenhouse effect, where thermal radiation is trapped. This would hypothetically prevent liquid water from forming — not ideal conditions for habitability, whether along a terminator zone or across an entire planet.