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…
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