Astronomers say ‘terminator zones’ on distant planets could harbor alien life

Some exoplanets have one side permanently facing their star while the other side is in perpetual darkness. The ring-shaped border between these permanent day and night regions is called the “termination zone.” In a new article in The Astrophysical Journal, UC Irvine physics and astronomy researchers say this area has the potential to host extraterrestrial life. Credit: Ana Lobo / UCI

These intermediate regions could be prime sites for liquid water. In a new study, astronomers at the University of California, Irvine (UCI) describe how extraterrestrial life has the potential to exist on distant exoplanets within a special area called the “terminator zone,” which is a ring on planets that have a side that always faces its star and 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 and Astronomy who led the new work, which was just published March 16 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 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. Termination zones could exist in that “perfect” temperature zone between too hot and too cold. “You want a planet that is at the right point of 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 the terminator planets, 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 day side can be very hot, well beyond habitability, and the night side will be icy, potentially covered in ice. You could have big glaciers on the night side,” Lobo said. Lobo, along with Aomawa Shields, a UCI associate professor of physics and astronomy, modeled the climate of the terminal planets using software normally used to model our own planet’s climate, but with a few tweaks, including slowing down the 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 terminator planets 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 large 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. “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 states of habitability 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 termination zones as potential ports for life also means 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.

The work will also help inform future efforts by teams using telescopes such as the James Webb Space Telescope or the Ultraviolet-Optical Infrared Large 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 properly identifying a habitable planet in the near future,” Lobo said. Reference: “Terminator Habitability: The Case for Limited Water Availability on M Dwarf Planets” by Ana H. Lobo, Aomawa L. Shields, Igor Z. Palubski, and Eric Wolf, March 16, 2023, Astrophysical Journal. DOI: 10.3847/1538-4357/aca970