Tidally locked exoplanets could be habitable in the ‘terminator zone’

Exoplanets can have all sorts of strange environments, and one feature which is relatively common to find among exoplanets but doesn’t exist among planets in our solar system is tidal locking. This is where one side of the planet always faces its star and the other side always faces out into space, so one side gets incredibly hot while the other side is freezing cold. That doesn’t sound like a comfortable environment for life, but recent research shows that it is possible that these exoplanets could be habitable in the narrow band which separates the two sides.

Known as the “terminator zone,” this is the ring around a planet between the hot side, called the dayside, and the cold side, called the nightside. This zone separates two vastly different climates. “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,” explained the lead researcher, Ana Lobo of the University of California, Irvine, in a statement.

An artist's rendition of the terminator zone of an exoplanet.
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 a “terminator zone.” In a new paper in The Astrophysical Journal, physics and astronomy researchers at UC Irvine say this area has the potential to support extraterrestrial life. Ana Lobo / UCI

The researchers modeled such planets using the same software used to model Earth’s climate but adjusted for factors like planetary rotation. They found that despite the existence of extreme temperatures nearby, the terminator zone could be an appropriate temperature for liquid water to exist on the surface — the essential component for potential habitability.

However, there needs to be a balance of water and land on the planet for this to work. If the planet is mostly covered in water, it would evaporate from the dayside and shroud the planet in vapor. But if enough land is present, this acts as a stabilizing force to allow liquid water to stay on the surface.

“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 said.

This research is exciting because it broadens the range of planets that we could examine when looking for evidence of life beyond Earth. Tidally locked planets often exist around M-dwarf stars, which are dimmer than our sun and are some of the most common stars in the galaxy. If any of these stars host such planets, the pool of exoplanets where we can search for life expands considerably.

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

The research is published in The Astrophysical Journal.

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