In a new study from the University of California, Irvine astronomers describe how extraterrestrial life has the potential to exist on distant exoplanets in a special area called the “terminator zone,” a ring on planets that has one side always facing its star a side that is always dark.
“These planets have a permanent dayside and a permanent nightside,” said Ana Lobo, a postdoctoral fellow at the UCI Department of Physics & Astronomy who led the new work just published in The Astrophysical Journal. Lobo added that such planets are particularly common because they exist around stars that make up about 70 percent of the stars visible in the night sky — 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 could 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 is, as far as scientists know, an essential part of life.
On the dark sides of the terminator planets, perpetual night would result in plummeting temperatures that could cause water to freeze to ice. The side of the planet that always faces its star might be too hot for water 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 be freezing cold and possibly covered in ice. There could be large glaciers on the night side,” Lobo said.
Lobo, along with Aomawa Shields, UCI associate professor of physics and astronomy, modeled the climate of terminator planets using software normally used to model our own planet’s climate, but with some adjustments, including slowing down the planet’s rotation.
It is believed that astronomers were able to show for the first time that such planets can sustain a habitable climate confined to this terminator region. In the past, researchers have primarily looked at 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, the options for life-hunting astronomers to choose from are expanding.
“We’re trying to draw attention to more water-limited planets that, while not having widespread oceans, might have lakes or other smaller bodies of liquid water, and those climates might actually be very promising,” Lobo said.
A key to the result, Lobo added, was determining exactly what type of planet in the terminator zone can retain liquid water. If the planet is mostly covered with water, then the water facing the star would likely evaporate, covering the entire planet in a thick layer of vapor, the team said.
But if there is land, then this effect should not occur.
“Ana showed that the scenario we call ‘Terminator habitability’ can exist much more easily when there’s a lot of land on the planet,” Shields said. “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.”
Recognizing terminator zones as potential havens for life also means that astronomers must adjust the way they probe the exoplanet climate for signs of life, since the biosignatures that life generates may only be present in certain 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 while searching for planets bearing extraterrestrial life could accommodate.
“By exploring these exotic climate states, we increase our chances of finding and correctly identifying a habitable planet in the near future,” Lobo said.
Ana H. Lobo et al, Terminator Habitability: The Case for Limited Water Availability on M-dwarf Planets, The Astrophysical Journal (2023). DOI: 10.3847/1538-4357/aca970