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Eyeball Worlds: The Most Common Habitable Planets May Be Apocalyptic Hellholes

"Those aren't mountains... it's a wave."
​The dark side of a hypothetical exoplanet. Image: Michael Stern / Flickr

Future humans might find themselves living in all sorts of weird places, from underwater cities to floating cloud abodes. But of all the Earth-like worlds we may one day terraform, few are as hazardous as the so-called "eyeball" exoplanets—frozen rocks with a permanent patch of open ocean.

Of the five mass extinctions in Earth's history, climate change is strongly suspect in at least three. The climate swings expected to occur on eyeball worlds make these events look like bouts of inclement weather.

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Though they haven't earned themselves a tourism poster yet, eyeball planets are shaping up to be one of the most common types of Earth-like worlds in our cosmic neighborhood. That's a conclusion supported by a new Icarus paper, wherein NASA's Anthony Dobrovolskis shows how certain orbital patterns may lock a planet into a specific orientation with respect to its star, causing one of its faces to cook while the other remains stuck in permanent, wintry darkness. What's more, this situation may cause serious climatic instability, leaving eyeball worlds prone to regular and apocalyptic extinctions.

As astronomers scan the sky with ever more advanced telescopes, we've started to build a detailed picture of the stars and planets that surround us. We've learned, for instance, that red dwarfs—stars that are both smaller and dimmer than our sun—are quite common, and that we may find a plethora of Earth-like worlds orbiting them quite closely. But planets in tight orbits are also subject to powerful tidal forces, which can dramatically slow their rotation. While the Earth makes roughly 365 rotations for each trip about the sun, an Earth-like exoplanet orbiting a red dwarf might only make one or two.

"If a planet only makes one rotation for each trip around its star, then it's always going to show the same face to that star, the same way we always see the same face of the moon from Earth," Dobrovolskis told me over the phone. The result—a planet with one hemisphere in permanent day and the other in endless night—gives rise to the so-called eyeball pattern.

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"That's a fairly common phenomenon that's been studied quite a bit," said Dobrovolskis.

What's less well studied—but perhaps just as common—is what happens when you've got a planet in "half-odd resonance," for instance, one that makes three rotations for every two trips around its star. Such is the case on Mercury, as well as the countless exoplanets whose orbits, instead of being perfect circles, are somewhat elliptical.

Using mathematical models, Dobrovolskis shows that planets in half-odd resonance take on a "double eyeball" pattern, meaning that an iceworld would have mirror pupils of water in its eastern and western hemispheres. And Earth-like planets in either configuration—a single or a double eyeball—may be prone to climatic swings that make our Anthropocene woes seem perfectly laughable.

"You can imagine a situation where water is evaporating from the planet's ocean and causing precipitation to occur on the fringes—near the terminator line between day and night," Dobrovolskis said. "Mass will start to build up on the terminator, and that could change the tidal properties of the planet."

Specifically, it could cause the planet's long and short axes to switch places, and the frozen and ice-free portions to rapidly follow suit. You can imagine the apocalyptic effect this would have on, say, marine organisms that are happily enjoying the warmth, sunlight, and liquid water afforded by their planet's eyeball.

While we shouldn't discount the possibility of life on these strange worlds, it seems any organisms that do set up shop are probably going to have to learn to live with sudden and recurrent environmental devastation.

I'm not packing my bags just yet.