The terminator – no, not that one – is the divide between day and night on a planet.
According to new research in The Astrophysical Journal, the terminator could be a good spot for hosting extra-terrestrial life.
On Earth, the terminator moves around over the course of a day, but on some planets it’s fixed in place: called “tidally locked” planets, the same side always faces its star.
Tidally locked planets have a half where it’s always day, a half where it’s always night, and a “terminator zone” where it’s always twilight.
Such planets are quite common: the habitable zones of M-dwarf stars, which make up 70% of all stars, frequently host tidally locked planets.
“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,” says lead author Dr Ana Lobo, a researcher at the University of California, Irvine, US.
According to the researchers’ models, it’s possible for the temperature to be just right, and therefore support liquid water, in the terminator zone.
The researchers found that ocean-covered planets, which are often the focus of the search for extra-terrestrial life, weren’t good candidates for hosting life in the terminator zone.
“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,” says Lobo.
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If the planet is mostly covered in water, the models suggested that it would evaporate on the day-side and form a planet-wide cloud, making it hard to sustain life.
But more land can mitigate this.
“Ana has shown if there’s a lot of land on the planet, the scenario we call ‘terminator habitability’ can exist a lot more easily,” says co-author Associate Professor Aomawa Shields, also at the University of California, Irvine.
“These new and exotic habitability states our team is uncovering are no longer the stuff of science fiction – Ana has done the work to show that such states can be climatically stable.”