The problem is that we haven’t found anything out there exactly like our own planet: of the same size and composition, occupying a similar place in its planetary system, at just the right “Goldilocks” distance from its star for temperatures that are susceptible to life as we know it.
Most of the 5,300 worlds we’ve found to date are actually much closer to their host stars than Earth is to the Sun. Thanks to this proximity, they are not only sizzling, but tidally locked in place. This means that one side always faces the star, prepared in permanent daylight, and the other always faces away, in freezing, eternal night.
A new paper has found that there is a place on close-orbiting, duel-personality exoplanets that could be habitable: the thin twilight zone where day meets night, known as the terminator.
“You want a planet that’s in the sweet spot of just the right temperature to have liquid water,” says geophysicist Ana Lobo of the University of California Irvine.
“This is a planet where the dayside can be scorching hot, way beyond habitability, and the nightside is going to be freezing, potentially covered in ice. You can have big glaciers on the nightside.”
Our search for Earth-like exoplanets is currently somewhat hampered by the limitations of our technology. Our most useful techniques are best at finding worlds that orbit their stars quite closely and whip around in less than 100 days.
If we only looked at stars like the Sun, this could pose a problem for potential habitability. Yet most of the stars in the galaxy are red dwarfs; smaller, fainter and much cooler than our own star.
While this means the habitable zone may be quite a bit closer, it also introduces the problem of tidal locking. This happens when the gravitational interaction between two bodies “locks” the rotation of the smaller body to the same period as its orbit, so that one side always faces the larger body. It occurs especially in exoplanets with close orbits because the star’s gravity stretches the exoplanet in such a way that the distortion activates a braking effect. We also see that with the Earth and the Moon.
For exoplanets, sometimes known as “eyeball planets,” this means that day and night experience extreme climates that may not be the most hospitable. To determine whether there is any way such worlds could be habitable, Lobo and her colleagues used modified climate modeling software normally used for Earth.
Previous attempts to determine the potential habitability of exoplanets have focused much more on worlds rich in water, as life on Earth requires it. The team hoped to expand the range of worlds that we should search for signs of extraterrestrial life.
“We’re trying to draw attention to more water-limited planets that, despite not having widespread oceans, could have lakes or other smaller bodies of liquid water, and those climates could actually be very promising,” explains Lobo.
Interestingly, the team’s work showed that more water would likely make eyeball planets less habitable. If the dayside of such a world had liquid oceans, the interaction with the star would fill the atmosphere with vapors that could envelop the entire exoplanet and induce suffocating greenhouse effects.
But if the exoplanet has a lot of land, then the terminator becomes more habitable. Ice from nighttime glaciers could melt as temperatures rise above freezing, making the terminator a habitable belt orbiting the exoplanet.
This is similar to the findings of a 2013 paper published in the journal Astrobiology. Together, they suggest that it would be worth adding eyeball exoplanets to the mix in future searches for signs of life in the atmospheres of planets outside the solar system.
“By exploring these exotic climates, 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.