In 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.
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.
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.