Earth is currently our only blueprint for planetary habitability. There may be life elsewhere in the great, wide galaxy, but ours is the only world we know for certain that it originated.
The problem is that we haven’t found anything out there that exactly resembles our own planet: of the same size and composition, in a similar spot in its planetary system, at just the right “Goldilocks” distance from its star for temperatures suitable for life. . as we know it.
In fact, most of the 5,300 worlds we’ve found so far are much closer to their host stars than Earth is to the sun. Thanks to this closeness, they are not only sizzling, but also sit in place. That means one side is always facing the star, cooked in permanent daylight, and the other side is always facing away, into the icy eternal night.
A new paper has discovered that there is a place on exoplanets with a dual personality orbiting close to each other that may be habitable: the thin twilight zone where day and night meet, known as the terminator.
“You want a planet that is just the right temperature to have liquid water,” says geophysicist Ana Lobo of the University of California Irvine.
“This is a planet where the day side could be blazing hot, well beyond habitability, and the night side will be frigid, possibly covered in ice. You could have big 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 at finding worlds that orbit their stars quite closely and orbit in less than 100 days.
If we only looked at stars like the sun, this could pose a problem for potential habitability. Still, 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 could be a lot closer, it also introduces the problem of tidal locking. This happens when the gravitational interaction between two bodies “locks” the smaller body’s rotation to the same period as its orbit, so that one side always faces the larger body. It is especially common in close-orbiting exoplanets, because the star’s gravity stretches the exoplanet in such a way that the deformation has an inhibiting effect. We also see this with the Earth and the Moon.
For exoplanets, also known as “eyeball planets,” that means the dayside and nightside experience climate extremes that may not be the most hospitable. To determine if such worlds could somehow be habitable, Lobo and her colleagues used custom climate modeling software commonly used for Earth.
Previous efforts 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 in which we should search for signs of extraterrestrial life.
“We’re trying to draw attention to more water-restricted planets, which, despite not having vast oceans, could have lakes or other smaller amounts of liquid water, and these climates could actually be promising,” Lobo explained.
Interestingly, the team’s work showed that more water would likely make eyeball planets less habitable. If the day side of such a world had liquid oceans, the interaction with the star would fill the atmosphere with vapor that could envelop the entire exoplanet, causing suffocating greenhouse effects.
However, if the exoplanet has a lot of land, the terminator becomes more habitable. There, ice from nighttime glaciers could melt as temperatures rise above freezing, turning the terminator into a habitable belt orbiting the exoplanet.
This is similar to the findings of a 2013 article published in the journal Astrobiology. Together, they suggest it would be worth adding eyeball exoplanets to the mix in future searches for signs of life in the atmospheres of extrasolar planets.
“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 is published in The Astrophysical Journal.