The Earth, currently, is our only model of planetary habitability. There may be life elsewhere in the great, vast galaxy, but ours is the only world on which we know, for certain, it emerged.
The problem is that we haven’t found anything there that looks exactly like our own planet: of the same size and composition, occupying a similar place in its planetary system, just the right “Goldilocks” distance from its star for temperatures conducive to life. as we know it.
Most 5,300 worlds that we have found so far are, in fact, much closer to their host stars than Earth is to the Sun. Thanks to this proximity, they are not only sizzling, but also locked in place. This means that one side is always facing the star, baked in the permanent daylight, and the other is always facing out, in the freezing and perpetual night.
A new paper has discovered that there’s a spot on close-orbiting dual-personality exoplanets that could be habitable: the thin twilight zone where day meets night, known as the terminator.
“You want a planet that’s just the right temperature to have liquid water,” says geophysicist Ana Lobo from the University of California at Irvine.
“This is a planet where the day side can be scorching, 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.”
Our search for Earth-like exoplanets is currently somewhat hampered by the limitations of our technology. Our most useful techniques are best for finding worlds that orbit close enough to their stars, circling in less than 100 days.
If we only looked at stars like the Sun, this could pose a potential habitability problem. Yet most stars in the galaxy are red dwarfs; smaller, fainter and much cooler than our own star.
Although this means that the habitable zone may be a little closer, it also introduces the problem of tidal lock. It happens when the gravitational interaction between two bodies “locks” the rotation of the smaller body over the same period as its orbit, so that one side always faces the larger body. This especially happens in exoplanets with close orbits, because the star’s gravity stretches the exoplanet in such a way that the distortion applies a braking effect. We see it with the Earth and the moonAlso.
For exoplanets, sometimes called “eyeball planets“, this means that both the dayside and the nightside are experiencing climate extremes that might not be the most hospitable. To determine if it’s possible that such worlds are habitable, Lobo and his colleagues used climate modeling software modified generally used for Earth.
Previous attempts to determine the potential habitability of exoplanets have focused much more on water-rich worlds, since life on earth demands it. The team hoped to expand the range of worlds in which we should search for signs of extraterrestrial life.
“We’re trying to draw attention to more water-limited planets, which, although they don’t have extensive oceans, might have lakes or other small bodies of liquid water, and those climates could actually be very promising,” Lobo explains.
Interestingly, the team’s work showed that more water was likely to make eyeball planets less habitable. If the dayside of such a world had liquid oceans, interaction with the star would fill the atmosphere with vapor that could shroud the entire exoplanet, inducing suffocating greenhouse effects.
However, if the exoplanet has a lot of land, then the terminator becomes more habitable. There, ice from nighttime glaciers could melt as temperatures rise above freezing, turning the terminator into a habitable belt surrounding the exoplanet.
This is similar to the findings of a 2013 article published in the journal Astrobiology. Together they suggest it would be worth it add eyeball exoplanets mixing in future searches for signs of life in the atmospheres of planets outside the solar system.
“By exploring these exotic climate states, we increase our chances of finding and correctly identifying a habitable planet in the near future,” says Lobo.
The team’s research has been published in The Astrophysical Journal.