Rocky exoplanets orbiting red dwarf (M-dwarf) stars are among the most promising and abundant astronomical targets for detecting habitable climates.
Planets in the red dwarf habitable zone are likely synchronously rotating, such that astronomers expect significant day-night temperature differences and potentially limited habitability.
Previous studies have focused on scenarios where fractional habitability is confined to the substellar or ‘eye’ region, but in a new paper, University of California, Irvine astronomer Ana Lobo and colleagues explore the possibility of planets with terminator habitability, defined by the existence of a habitable band at the transition between a scorching dayside and a glacial nightside.
Using a global climate model, they show that for water-limited exoplanets it is possible to have scorching temperatures in the ‘eye’ and freezing temperatures on the nightside, while maintaining a temperate climate in the terminator region, due to reduced atmospheric energy transport.
“These planets have a permanent day side and a permanent night side,” Dr. Lobo said.
“Such planets are particularly common because they exist around stars that make up about 70% of the stars seen in the night sky — so-called M-dwarf stars, which are relatively dimmer than our Sun.”
“The terminator is the dividing line between the day and night sides of the planet. Terminator zones could exist in that ‘just right’ temperature zone between too hot and too cold.”
“You want a planet that’s in the sweet spot of just the right temperature for having liquid water, because liquid water, as far as scientists know, is an essential ingredient for life.”
On the dark sides of terminator exoplanets, perpetual night would yield plummeting temperatures that could cause any water to be frozen in ice.
The side of the planet always facing its star could be too hot for water to remain in the open for long.
“This is a planet where the dayside can be scorching hot, well beyond habitability, and the night side is going to be freezing, potentially covered in ice. You could have large glaciers on the night side,” Dr. Lobo said.
Dr. Lobo and colleagues modeled the climate of terminator planets using software typically used to model our own planet’s climate, but with a few adjustments, including slowing down planetary rotation.
It’s believed to be the first time astronomers have been able to show that such planets can sustain habitable climates confined to this terminator region.
Historically, researchers have mostly studied ocean-covered exoplanets in their search for candidates for habitability.
But now that the authors have shown that terminator planets are also viable refuges for life, it increases the options life-hunting astronomers have to choose from.
“We are trying to draw attention to more water-limited planets, which despite not having widespread oceans, could have lakes or other smaller bodies of liquid water, and these climates could actually be very promising,” Dr. Lobo said.
“One key to the finding was pinpointing exactly what kind of terminator zone planet can retain liquid water.”
“If the planet is mostly covered in water, then the water facing the star, we found, would likely evaporate and cover the entire planet in a thick layer of vapor. But if there’s land, this effect shouldn’t occur.”
Sources:
Ana H. Lobo et al. 2023. Terminator Habitability: The Case for Limited Water Availability on M-dwarf Planets. ApJ 945, 161; doi: 10.3847/1538-4357/aca970