Extraterrestrial life could take hold in the 'terminator' zones - the line dividing night and daytime - on planets beyond our Solar System, according to a new study.


Astronomers at the University of California, Irvine have been studying exoplanets with one side that always faces the host star, just like how the same side of our Moon always faces Earth.

And, just like on our Moon, the line dividing the lit section and the unlit section of the orbiting body is known as the terminator.

The Moon by William Ian Hamilton, Gloucestershire, UK. Equipment: Canon EOS100D, Mead LX90.
The line dividing the lit and unlit side on our Moon is known as the 'terminator'. Credit: William Ian Hamilton

The team behind the study say exoplanets with a permanent day side and a permanent night side are common because they orbit stars that make up about 70% of the stars in the night sky.

These are so-called M-dwarf stars, and are dimmer than our Sun.

These terminator zones could be just right for the emergence of life on distant exoplanets because they would offer a region that's not too hot and not too cold.

This could be one of key conditions that makes a planet habitable.

"You want a planet that’s in the sweet spot of just the right temperature for having liquid water," says Ana Lobo, a researcher at the UCI Department of Physics & Astronomy who led the study, published in The Astrophysical Journal.

Any planet on which water can pool as a liquid is considered to be potentially habitable for life as we know it, and is key in the search for life beyond Earth.

While a planet's dark side might see temperatures plummet and water freeze, a planet's permanently-illuminated side might see temperatures reach scorching levels, too hot for water to remain on the surface.

An artist's impression of a hot Jupiter; a gas giant similar to Jupiter but orbiting much closer to its host star. Credit: NASA/Ames/JPL-Caltech
An artist's impression of a hot Jupiter; a gas giant similar to Jupiter but orbiting much closer to its host star.
Credit: NASA/Ames/JPL-Caltech

"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," Lobo says.

Lobo and Aomawa Shields, UCI associate professor of physics & astronomy, worked on models of the climates of these 'terminator planets' using computer software that's more commonly used to model planet Earth's climate.

They modified the software to make it suited to analysing the conditions on exoplanets that always have one side facing the host star.

The team say this is a key development in the study of planets beyond the Solar System that could potentially host life, which concentrates largely on water-rich planets.

Did Venus once have oceans? What can Venus today tell us about climate change on Earth? Credit: NASA
Artist's impression of an ocean-rich planet. Credit: NASA

"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," Lobo says.

More like this

The study focussed on pinpointing what sort of planets and what sort of terminator zones could retain liquid water.

The team found that on a water-rich planet, the side facing the star would see all its water evaporated, covering the planet in a thick layer of vapour.

But if the planet has a lot of land, this effect wouldn't occur.

"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 Shields.

"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."

Read the full paper at the Astrophysical Journal.


Read more about this story at news.uci.edu.


Iain Todd BBC Sky at Night Magazine
Iain ToddScience journalist

Iain Todd is BBC Sky at Night Magazine's Content Editor. He fell in love with the night sky when he caught his first glimpse of Orion, aged 10.