The region that straddles the dividing line between the day and night sides of star-locked exoplanets has unique potential to support life, a new study shows.

Such planets are particularly common because they exist around stars that make up about 70 percent of the stars seen in the night sky, so-called M dwarf stars, which are relatively dimmer than our sun. Terminator zones, the transition between day and night sides, could harbor the “perfect” temperature between too hot and too cold.

“You want a planet that is at the right point of the right temperature to have liquid water,” says Ana Lobo, a postdoctoral researcher in the UCI Department of Physics and Astronomy who led the research, which has just been published in The Astrophysical Journal. Liquid water, as far as scientists know, is an essential ingredient for life. On the dark sides of planets of this class, the perpetual night would produce plummeting temperatures that could cause water to freeze into ice. The side of the planet that always faces its star might be too hot for water to stay in the open air for long.

“This is a planet where the dayside can be very hot, well beyond habitability, and the nightside will freeze over, potentially covered in ice. You could have large glaciers on the nightside,” Lobo said. Lobo, along with Aomawa Shields, UCI associate professor of physics and astronomy, modeled the climate of the ‘terminator’ planets using software normally used to model the climate of our own planet, but with some adjustments, including the slowdown of planetary rotation.

It is believed to be the first time that astronomers have been able to show that such planets can sustain habitable climates confined to this terminal region. Historically, researchers have primarily studied exoplanets covered by oceans in their search for candidates for habitability. But now that Lobo and his team have shown that planets with termination zones are also viable havens for life, the options that life-hunting astronomers can choose from are increasing. “We’re trying to draw attention to more water-limited planets, which despite not having vast oceans, could have lakes or other smaller bodies of liquid water, and these climates could actually be very promising,” Lobo said.

A key to the find, Lobo added, was identifying exactly what type of terminator-zone planet can hold liquid water. If the planet is mostly covered in water, then the water in front of the star, the team found, would likely evaporate and cover the entire planet in a thick layer of steam. But if there is land, this effect should not occur. Recognizing termination zones as potential ports for life also means that astronomers will need to adjust how they study exoplanet climates for signs of life, because the biosignatures that life creates may be present only in specific parts of the planet’s atmosphere.