Extraterrestrial life at the Terminator?

Life-friendly strip? Most Earth-like exoplanets have a locked rotation—they always face the same side to their star. But what does this mean for their habitability and the chance for water and life? Astronomers have now investigated this in more detail. The result: The chance of a habitable transition zone between the hot day side and the cold night side is relatively high for such planets – especially if they are not pure ocean planets.

In recent years, astronomers have discovered countless Earth twins and Earth-like super-Earths in the habitable zone around alien stars. Most of these potentially life-friendly exoplanets orbit red dwarf stars—stars smaller and cooler than our Sun. Also among such nearby worlds with a reddish sun are an Earth twin around our neighboring star Proxima Centauri and the seven Earth-like planets around the red dwarf TRAPPIST-1 .

The problem of bound rotation

But as promising as these nearby worlds may appear at first glance, they have one major drawback: Because the habitable zone of red dwarfs is relatively close to the star, many of these Earth twins and super-Earths orbit in a locked rotation. As a result, they always turn the same side to their star. “On such planets, the day side is therefore scorching hot and the night side is freezing cold, perhaps even covered by large glaciers,” explains lead author Ana Lobo from the University of California at Irvine.

But that means: Even if the radiation and temperatures on such planets may have life-friendly values ​​on average – in practice these are planets of extremes. This raises the question of whether habitable conditions and liquid water can exist on such exoplanets at all. There could be a chance for this at the terminator – the twilight zone between the day and night side. Because there the extremes of both sides could balance out so that there are local places with habitable conditions.

Bound land and water worlds in the test

Lobo and her team have now investigated in more detail whether this is the case and what such an exoplanet must be like. “We wanted to find out whether there can be a habitable band in this transition zone or whether such terminator habitability becomes impossible once the star-facing region crosses habitable limits,” the astronomers explain. In their model simulation, they analyzed water worlds and land-dominated exoplanets that orbit in a fixed rotation in the habitable zone of a red dwarf.

The surprising result: Contrary to what was thought, ocean worlds do not offer the best chances of a habitable terminator zone under such conditions. Because if their day side exceeds a certain temperature, there is a strong evaporation of water and thus a self-reinforcing greenhouse effect. Even in the twilight zone, such a planet would therefore heat up quickly. In the ocean itself, however, the conditions could still be life-friendly.

Land planet cheaper than ocean planet

However, the conditions would be more favorable on an exoplanet whose surface consists largely of land. “Our global climate model shows that such a planet can have scorching hot temperatures on the sun’s side and freezing temperatures on the nightside. But the terminator zone may have a temperate climate because there is little atmospheric energy transport,” Lobo and her colleagues report.

The twilight zone of exoplanets such as Proxima Centauri b or the Earth twins around TRAPIST-1 could therefore have suitable conditions to allow for lakes and other bodies of liquid water. Sources of the water could include nightside glaciers that melt near the twilight zone, feeding rivers and lakes.

Habitable conditions would be more stable

The simulations also suggest that the twilight strip habitable conditions of land-dominated planets may be more stable and long-lived than those of purely water worlds. “Water-limited atmospheres achieve a more stable configuration than their counterparts on water planets,” the researchers report. “Because they are less prone to losing water vapor to space or freezing water on their nightside.”

“The possibilities of habitable twilight zones uncovered by our team are no longer the stuff of science fiction stories – we have shown that such terminator habitability is real and can be climatically stable,” says Lobo’s colleague Aomawa Shields, who also did the research at NASA’s Virtual Planetary Laboratory in Seattle.

These findings are also important for the search for extraterrestrial life and for biosignatures in the atmospheres of exoplanets. Because they suggest that such spectral signatures could also exist in land-dominated exoplanets in locked rotation. However, it would be possible that the biosignatures would then only be detectable in certain regions of the planet – above the terminator. Searches with the James Webb telescope and other telescopes would have to take this into account. (The Astrophysical Journal, 2023; doi:10.3847/1538-4357/aca970 )