Eyeball Earths

Eyeball_EarthIn episode #0 of our brand-spanking new SciFi Ideas podcast we talked about tidally locked worlds (with reference to Alien Profile: The Yzaz by Johnson Borrero). Since my co-host Dave was fairly unfamiliar with the concept of tidal locking and the effect it can have on habitable worlds, I decided it would be a good idea to share some information for those of you who aren’t in the know.

One particularly fascinating result of tidal locking is the potential formation of “eyeball Earths”. If you’ve never heard of “eyeball Earths”, prepare to have your mind blown by this hypothetical but entirely realistic concept of strange new worlds.

First, lets review the basics of tidal locking…

Tidally Locked Worlds

Tidal locking is a process by which an astronomical body attracts the surface of a smaller, orbiting body to the extent that the rotation of the orbiting body becomes synchronous with its orbit around the larger body. This ‘synchronous rotation’ means that one side of the smaller body always faces the larger body, even as it orbits around it. Just as the gravitational pull of the moon attracts water on Earth creating a a tidal effect in our oceans, large bodies can attract the solid surface of smaller planets or moons creating a drag effect and essentially locking them into position (relative position, that is).

Confused? Basically, we’re talking about planets where one side continually faces the sun (like the planet Remus in Star Trek). One side of the planet is continually light, the other is in perpetual darkness.

tidal locking

But tidal locking doesn’t just affect planets; in actual fact, tidal locking most typically occurs in planet-moon systems. Our own moon, for instance, is tidally locked to the Earth, with one side of the dusty old snowball continually smiling down on us regardless of its position in the sky. The time it takes for the moon to complete one revolution (a lunar ‘day’) is the same as the time it takes for the moon to complete one orbit of the Earth.

Most significant moons in the solar system are tidally locked to their parent planet – the Galilean moons, for example – due to their position and relative size. Generally speaking, the closer a planet or moon orbits its primary, the more likely it is to become tidally locked.

Interestingly, some closely orbiting binary stars are thought to be tidally locked with eachother, but that’s boring, let’s get back to talking about planets.

No planets in the solar system are tidally locked to the sun, but many planets in other star systems are likely to have this type of relationship with their own stars. Being baked by eternal light on one side and being perpetually dark and frozen on the other would make these planets particularly harsh environments for life to exist, but that doesn’t mean we should rule out life on tidally locked worlds entirely. In fact, if a tidally locked planet were to become what’s known as an “eyeball Earth”, it could potentially be quite a pleasant vacation spot.

Eyeball Earths

So, tidally locked planets are continually blasted by the sun on one side and perpetually dark on the other causing extremes of hot and cold. But what would happen if a tidally locked planet were also located in a star’s habitable “Goldilocks zone” – the area in which temperatures are just right for water to exist in liquid form? What would the effect be on a planet such as, say, Earth?

Science fiction writers aren’t the only ones to give consideration to the potential habitability of tidally locked worlds, lots of clever science people have also been busily pondering this issue, and what they’ve come up with is rather interesting. It has been speculated that these planets could be habitable afterall, at least in the ring-shaped region between the light side and the dark side (the “twilight zone”, if you like) where liquid water could form into lakes and seas.

While the extreme heat of the sun-baked light side of the planet would cause any and all water to evaporate, creating an enormous desert, and while the perpetual night of the dark side would cause any water to freeze, the “twilight zone” between these two regions could provide a happy medium, allowing liquid water to pool into lakes and seas, and possibly allowing the existence of life as we know it. Here, any ice flowing from the cold side of the planet (or being pushed outward by tectonic forces) would melt, and any water vapour expelled by the desert heat would be allowed to fall as rain.

The result would be a planet speculated to look something like an eyeball, with a large central desert (the pupil) surrounded by a ring-shaped ocean (the iris), hence the nickname “eyeball Earth”.


Extreme weather systems would likely be the main obstacle to life on planets such as these, but that’s also what makes the scenario of life-bearing tidally locked worlds so plausible.¬†You see, as hot air expands on the light side of the planet, it would be forced towards the dark side of the planet, where it would rapidly cool. The cool air – forced outward by an influx of warm air – would then flow back towards the sun-baked desert. This ongoing cycle of heating and cooling would likely produce very strong winds and possibly dangerous cyclones too, but it would also cause a partial cooling of the desert and a warming of the ice sheet.

The movement of hot and cold air would expand the boundaries of the temperate region into both the light and dark sides of the planet making life possible beyond the thin twilight zone. The weather system would also allow for greater movement of water molecules in the form of rain clouds, making the climate considerably more dynamic and conducive to life.

Of course, the “eyeball Earth” model is still only speculation, and much would depend on the composition of the planet and its atmosphere, but with billions of planets out there this strange and fascinating configuration is thought very likely to exist at least somewhere in the galaxy, if not commonly so.

Eyeball Earths are most likely to form in orbit of red giant stars, which are cool enough to have closely-orbiting tidally locked planets fall within the boundaries of their habitable “Goldilocks zone”.

Article written by Mark Ball.

  • John Henry Reiher, Jr

    I once worked out what the atmospheric circulation patterns were for a tidally locked world. Came up with a diagram that shows how air circulates from front to back and front again.


  • Thanks for sharing that, John!
    Does that mean that it would be too hot on the edge of the light side, but mildly warm on the edge of the night side?

  • John Henry Reiher, Jr

    It means that it will be warm along the terminator band of the world. But that will vary with terrain and cloud cover. It will be cold on the midnight side, but not as cold as people make it out to be. The ice will move from the pole to terminator, to melt in the warmer climes. Noonside will be hot as proverbial Hell, even with the cool air moving in.

    Throw in plate tectonics and things get interesting, as continents slide into darkness and back out into the light. I’d imagine that rifts would be very volcanic and spewing lava and ash all day long on the Noonside.

  • Gabriel

    That’s an interesting diagram, John.
    If the oceans were scarce, or shallow, I would expect the sunny-side to be a dry desert due to evaporation. Landlocked seas would inevitably turn into salt pans. Did you try to work-out how a deep-oceaned world (with land) would behave?
    My guess is that the sunny-side will make the rising hot air pick-up lots of humidity, which it would lose gradually as it moves to the dark side. My guess is that the terminator (twilight zone) will receive lots of rain. Am I right?

    A second point to consider: Vegetation is most likely to be concentrated around the terminator, with water availability being a second limiting factor. Given high volcanic activity on the sunny side, and scarcer vegetation than on Earth, Carbon-dioxide greenhouse gas is expected to support higher temperatures. A chance for a Venus-like runaway greenhouse effect seems higher?

    Understanding this is a prelude to a slightly more complex scenario, where the planet is tilted as Uranus is, around 90 degrees. I’m looking forward to see a simulation of that kind.

  • The actual location on the planet of the habitable band would depend strongly on the planet’s insolation. If the planet were closer than Earth, relative to the star’s luminosity, the band of tolerable temperatures may be well onto the planet’s darkside, thus pretty much putting paid to any photosynthetic ecosystem. Cooler planets would tend to have their habitable band more on the lightside. This is generally the preferable state of affairs.

    Potentially, an Eyeball Earth situated toward the outer edge of the HZ might find its habitable zone right on top of the substellar point. We could call that a Pupil Earth :).

    Probably a maximally habitable Eyeball Earth would be one where the band of tolerable temperatures is situated very near the terminator with intolerably frigid temperatures pretty much at the far end of the twilight zone. This would place temperate zone pretty much where most of the useful liquid water is. Tolerable temperatures on a Pupil Earth would likely lie in a very dry location.

    Also, the colder the planet overall, the drier its likely to be(all else being equal) since there is a much larger area for atmospheric moisture to freeze off into. On the other hand, the subsolar convergence zone, being an area of rising air parcels due to global circulation, may provide unexpectedly great rainfall.

    Mr. Reiher’s diagram probably illustrate a planet with relatively fast rotation. Possibly it’s at the inner edge of a K-star’s HZ or orbiting an M-class star. Planets with years and thus rotation periods of close to a day or less might have more convection cells, but I doubt it since that probably requires a day of much less than 24 hours. Look at the banding on Jupiter to get an idea of just how extreme that can get. For rotation periods longer than a few months, circulation would probably settle down into a single cell: rising over the subsolar point and sinking over the antisolar point.

    Best case, you’d want the boundary between the Ferrel Cell and the Polar(or Noctial) Cell somewhere over the habitable band. Since that boundary is going to be at least as much driven by thermal considerations as rotational(especially if the rotation is slow), this is likely to fall pretty close to the terminator. The polar front on our own planet, with relatively fast rotation, waves around quite a lot. This causes midlatitude cyclones that bring precipitation to a pretty wide area. The same should be true for most Eyeball Earths, so figure the habitable band to have interesting weather.

    For a Pupil Earth weather will be a lot more monotonous. If it rains much, it’ll rain all the time. If it isn’t raining all the time, what little moisture comes will rain out quickly and torrentially and when its over things will clear up real quick.

  • For a slightly different take on the same idea, look at Libratia(http://www.worlddreambank.org/L/LIB.HTM) from Chris Wayan’s Planetocopia(http://www.worlddreambank.org/P/PLANETS.HTM).

    It’s not complete and probably most people reading this site are already familiar with Wayan’s work, but it’s definitely worth looking at.

  • What an interesting place to set a story. I was imagining what kind of civilization would exist on a planet like this, and I immediately pictured an alien Christopher Columbus boldly declaring that if you travel far enough across the dessert (or the ice sheet), you would reach the other side of the world.

  • coreyfurman

    I’m surprised no one has mentioned Zarmina’s World. I’ve written a book on it, and its tidally locked nature is crucial to the story. http://coreyfurman.net.