‘Terminator zones’ on distant planets could harbor life, astronomers say

In a new study from the University of California, Irvine-based astronomers describe how extraterrestrial life has the potential to exist on distant exoplanets in a special region called the “terminator zone.” a side that is always dark.

“These planets have a permanent day side and a permanent night side,” said Ana Lobo, a postdoctoral researcher in the UCI Department of Physics & Astronomy who led the new work, which was just published in The Astrophysical Journal. Lobo added that such planets are especially common because they exist around stars that make up about 70 percent of the stars seen in the night sky — the so-called M dwarf stars, which are relatively fainter than our sun.

The terminator is the dividing line between the day and night sides of the planet. Terminator zones can exist in that “just right” temperature zone between too hot and too cold.

“You want a planet that’s just the right temperature to have liquid water,” Lobo said, because liquid water, as far as scientists know, is an essential ingredient for life.

On the dark sides of terminator planets, perpetual night would lead to plummeting temperatures that could cause all water to freeze into ice. The side of the planet that always faces the star may be too hot to stay out in the open for long.

“This is a planet where the day side can be scorching hot, well beyond habitability, and the night side will freeze, possibly covered in ice. You could have big glaciers on the night side,” Lobo said.

Lobo, along with Aomawa Shields, UCI associate professor of physics and astronomy, has modeled the climate of terminator planets using software typically used to model our own planet’s climate, but with a few tweaks, including slowing down the rotation of the planet.

This is believed to be the first time astronomers have been able to demonstrate that such planets can maintain a habitable climate limited to this terminator region. Historically, researchers have primarily studied ocean-covered exoplanets in their search for habitability candidates. But now that Lobo and her team have shown that terminator planets are also viable havens for life, this expands the options for life-seeking astronomers to choose from.

“We’re trying to draw attention to more water-restricted planets, which, despite not having vast oceans, could have lakes or other smaller amounts of liquid water, and these climates could actually be promising,” Lobo said.

A key to the finding, Lobo added, was pinpointing exactly what kind of planet in the terminator zone can hold liquid water. If the planet is mostly covered in water, then the water directed at the star, the team found, would likely evaporate and cover the entire planet in a thick layer of vapor.

But if there is land, this effect should not occur.

“Ana has shown that if there is a lot of land on the planet, the scenario we call ‘terminator habitability’ can exist much more easily,” Shields said. “These new and exotic habitability states that our team is discovering are no longer the stuff of science fiction — Ana has done the work to show that such states can be climatically stable.”

Recognizing terminator zones as potential harbors for life also means that astronomers will have to adjust the way they study exoplanet climates for signs of life, because the biosignatures that create life may only be present in specific parts of the planet’s atmosphere.

The work will also help inform future efforts by teams using telescopes such as the James Webb Space Telescope or the Large Ultraviolet Optical Infrared Surveyor telescope currently under development at NASA as they search for planets that may harbor extraterrestrial life.

“By exploring these exotic climate states, we increase our chances of finding and correctly identifying a habitable planet in the near future,” Lobo said.