Post by Nic Quattromani:
Tidal locking is one of the more interesting phenomena in the realm of speculative fiction, partly because it clashes with our terracentric ideas of what a planet should look like. While our comfortable, spinning Earth has two icecaps sandwiching a hot equatorial region, its tidally locked counterpart, called an “eyeball world” for the concentric shape of its features, would be oriented with permanent hot and cold poles, with the local star either fixed in the sky or absent entirely. North and south as we know them would have little meaning. Solar power would be as simple as pointing a panel at the sun and leaving it there. And, as strange as these ideas seems to us, such planets are not a rarity in the universe—they may even be more common than the rotating variety.
Here’s how tidal locking works, in brief: if a planet or moon orbits close enough to its parent body, the gravitational forces exerted on the near side and those on the far side won’t be the same. This creates a tidal force that will steadily rob the planet of its angular momentum until it is oriented straight towards the sun. Half of it becomes a scorched desert, half becomes an icy wasteland of perpetual night, and a narrow band around the equator, bathed in an endless sunset, plays host to the only region we’d consider habitable.
The Moon is an everyday example of this. Since the Earth isn’t a star, its satellite doesn’t experience any of the dramatic effects I outlined above, but nevertheless we can only see one side of it—the other remained unknown until 1959.
Tidal locking also occurs with the large moons of Jupiter and Saturn. Pluto and Charon experience it, too, being locked to each other, which interestingly means that one could build an elevator running between them. Also of note is Proxima Centauri b, one of the recent rocky exoplanet finds to take the media by storm. Since it orbits a red dwarf, its semi-major axis is so small that it is almost locked, and while liquid water could well exist on the surface, it would follow the desert hemisphere, habitable ring, icy hemisphere pattern discussed earlier.
Probably, at least. We won’t know what’s going on there until we visit. The thing is, the study of exoplanets is a nascent field, and we’re just barely dipping our toes in the water—in the past decade there have been a few theories calling into question whether tidally locked worlds will have the diametric climate extremes I described above, or even have stable climates at all.
First there is the possibility that a thick atmosphere would even out temperature extremes on an eyeball world. Instead of a desert half and an ice half, we might have a subsolar desert and an antisolar icecap, with several relatively clement gradations between. The exact distribution could vary greatly with distance to the local sun: on a colder world, liquid water might only exist at the subsolar point, while a hotter planet might have its habitable belt in the sunless zone, kept toasty by warm winds blowing over the horizon.
There’s also a less optimistic possibility, raised by Edwin Kite of the University of California in 2011. At the substellar point of a tidally locked world, the combination of heat and rainfall—induced by heat—could massively increase weathering, uncovering mineral deposits (like calcium silicate) capable of removing carbon dioxide and other gases from the atmosphere. If a solar minimum, asteroid impact, or volcanic eruption were to decrease temperatures at that substellar point, the climate system would no longer have its primary means of regulating greenhouse gases. The planet might then head straight for a Venus-like hothouse scenario, which would make life almost impossible, or undertake wild swings in weather, which would at least make life’s job difficult.
So, are the numerous “eyeball worlds” out there likely to be more habitable than we thought, or less? The answer is, yes. It’s really too early to tell, and it’s all so very speculative. But if I were a betting man, my money would be on most tidally locked planets being strange and volatile as suggested by Kite: perhaps not hothouses, but certainly temperamental worlds with many challenges to offer would-be colonists.