Unlike Earth, there are millions of planets that have permanent day sides bathed in the light of their star, and permanent night sides cloaked in darkness. The area between these two sides is called the “terminator zone” and is a logical place to search for other life in the galaxy, says a new study.

Much like the Moon is to Earth, many planets are tidally locked to their suns, meaning that the planet takes the same amount of time to spin on its axis as it does to orbit the sun. This results in the planet always showing the same face to its sun, which in turn results in a permanent light side and a permanent dark side. Neither side would be great for life, as the daylight side is likely to be scorching hot with no liquid water remaining, and the nighttime side would be unimaginably cold, with liquid water being frozen solid.

However, between these two extremes is a land bathed in eternal twilight known as the terminator zone, where the temperatures and conditions could be just right for liquid water – and, therefore life – to exist. Or so say astronomers from the University of California, Irvine (UCI).

In reaching their conclusion, the researchers modified software typically used to study Earth’s climate – this included slowing down planetary rotation to mimic tidally locked planets. They believe their findings could open an entirely new grouping of planets to scrutiny, as astronomers look for those with the right conditions to harbor life. Many of these are found around relatively dim M-dwarf stars, which comprise about 70% of the stars we can see from Earth, so there is a plethora of planets to study.

“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,” said lead scientist Ana Lobo, a postdoctoral researcher in the UCI Department of Physics and Astronomy.

Lobo and her colleagues feel that the most promising terminator zone targets would be those found on land-based exoplanets rather than those covered in water, which would likely be obscured by vapor.

“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,” said Aomawa Shields, UCI associate professor of physics and astronomy and co-author of the study. “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.”

The findings could impact the way in which astronomers look for the biosignatures of life, as they might not be present in a planet’s entire atmosphere, but in a rather thin slice of it. The research also dovetails with an earlier study showing that air circulation between the cold and hot sides of tidally locked planets could create atmospheres conducive to life.

A paper on the study has been published in The Astrophysical Journal.

Source: UCI News