For generations, science fiction writers have speculated about living on tidally locked planets — worlds that, like Earth’s moon, have a rotation period equal to their orbital period, and thus always have one side facing their parent body.
On the surface of such a planet, there would be no alternation of day and night. Instead, the side facing the parent star would be in perpetual daylight, while the side facing away would experience perpetual night. Only in a narrow (and literal!) twilight zone might the sun appear to bob up and down slightly, rising slightly above the horizon, then dipping slightly below.
(This doesn’t happen on Earth’s moon: One side always faces Earth, but the sun rises and sets every month for most of the lunar surface.)
Worlds of Endless Day?
While tidally locked planets may fire authors’ imaginations, says Space.com, astronomers formerly regarded them as unlikely abodes for life. Tidal locking develops when massive objects orbit close to each other. All large moons in the solar system are tidally locked to the planets they orbit, but no solar system planet is tidally locked to the sun.
In recent decades, however, astronomers have begun to have second thoughts about tidally locked planets. Exoplanet discoveries revealed that many worlds orbit much closer to their parent stars than anyone had previously expected. Some stars have entire planetary systems orbiting closer than Mercury orbits the sun. Many, if not most, of these close-orbiting planets must be tidally locked, with one side perpetually facing the nearby star.
Red Dwarf Stars and the Goldilocks Zone
If all stars were as bright as the sun, these close-orbiting planets would still be ruled out — the entire planet, even the perpetual night side, would grow baking hot.
But most stars aren’t as bright as the sun. Most of the stars in our galaxy are so-called red dwarfs, hundreds of times fainter than the sun. And many close-orbiting, tidally locked planets have been found orbiting red dwarfs right in the stars’ habitable zones — the “Goldilocks” zone, as NASA puts it, where liquid water can exist without either freezing or boiling away.
This discovery could revolutionize our search for habitable exoplanets and life in the universe, but there’s still a hitch: Tidal locking would leave most of the perpetual dayside too hot for liquid water, while the perpetual nightside would be too cold. Any surface water would be lost, either boiling away and being lost to space, or freezing to form a permanent icecap on the night side.
But recent research and modeling, reported in the Astrophysical Journal, suggests that natural processes on some tidally locked planets could allow surface water to remain liquid without either boiling away or freezing.
Atmospheres and oceans don’t simply stay in one place, waiting to be boiled or frozen. They develop winds and currents. And if these are strong enough, in the right place, and blowing or flowing in the right direction, they can transfer heat from the dayside to the nightside efficiently enough to preserve liquid water.
The key is super-rotation — a steady wind or current strong enough to go around the planet faster than the solid planet’s rotation speed. This sounds vaguely like magic, but it isn’t: It just takes energy, ultimately derived from starlight.
In our own solar system, a notable example of super-rotation is the atmosphere of Venus, as Space.com points out. Winds in Venus’ upper atmosphere circle the planet in a few days, while its surface rotation takes months. (Venus is nearly, but not quite, tidally locked to the sun.) Even here on Earth, eastbound jet stream winds outpace Earth’s rotation speed, though our wind patterns are so complex that full super-rotation doesn’t develop.
Ocean currents can also potentially super-rotate, if the ocean is deep enough along the equator and not blocked by continents. This matters because liquid water is extremely good at trapping and storing heat — so much the better for cooling off the dayside of a tidally locked planet and keeping the nightside warm.
If super-rotation proves to be common in the universe, the search for habitable exoplanets will be revolutionized. Our exoplanet search technology is already able to detect winds on exoplanets, and our capabilities continue to grow. If planets orbiting in the habitable zones of red dwarf stars can be candidates for habitability, examining them could be a very busy research program in decades to come.
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