Can super-rotating oceans cool off extreme exoplanets?

Super-rotation could help make tidally locked worlds habitable.

Artist’s illustration of the exoplanet Proxima b, the Earth-size world that orbits in the “habitable zone” of the red dwarf star Proxima Centauri. Proxima b is likely tidally locked, always showing the same face to its host star.

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of Ask a Spaceman and Space Radio, and author of How to Die in Space. He contributed this article to’s Expert Voices: Opinions and Insights.

Astronomers continue to find potentially habitable worlds around small, red stars. But those worlds are almost certainly tidally locked, with one side of the planet constantly facing its star.

This poses a severe challenge to the possibility of life on these alien planets, but new research suggests a way to more evenly cool those planets: ocean currents whipping around the worlds faster than they rotate.

Locking it in

We’re finding tons of exoplanets out there. With NASA’s Kepler mission (whose databases are still fruitful after the space telescope’s death), the agency’s Transiting Exoplanet Survey Satellite and scores of ground-based missions, astronomers are discovering world after world orbiting distant stars. The ultimate goal: find an Earth-like planet orbiting a sun-like star at just the right distance so that the heat the planet receives from its star is just enough to melt ice, but not too hot to boil it away.

This is the “habitable zone,” the region around every star where liquid water can stay nice and liquid. And while we haven’t yet found an exact copy of Earth, we have come close: planets roughly the size of our own, orbiting within the habitable zone, but around small, red dwarf stars.

On one hand, this is amazing, because red dwarfs are by far the most common kind of star in the galaxy, and so there must be scores of planets in a multitude of habitable zones. But on the other hand, it’s kind of frustrating, because of something known as tidal locking.

When a small object orbits close to a large object (like, say, the moon around Earth or a planet around a star), the bigger object will raise tides on the smaller one. (Technically, the smaller object also raises tides on the bigger object, but they’re not nearly as major and we don’t need to worry about that now.) With those extra tidal lumps, the smaller object will slowly develop a lopsided preference: instead of getting any old rotation it wants, it will end up “locking.”

This locking forces the rotation of the small object to match its orbit around the larger one. You can see the results of this just by looking at the full moon: since the moon is tidally locked to Earth, it always presents the same face to us, and it wasn’t until the space age that we were able to get a glimpse of its backside.

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