We are really lucky with our Sun. At this point, it’s generally not too active, and it’s evolving at a nice pace to give us humans time to adapt to how it is changing and how it will change our environment someday.

Other worlds have very different relationships with their star, relationships that are more akin to what the Apollo astronauts experienced as they looked at the Earth. From their vantage on a tidally locked Moon, they always saw the Earth in the sky. It appeared to rotate, to slowly change phases, but its position was constant. Here on Earth, we always see the exact same face of the Moon, and, weirdly, that means the Earth is pretty much stationary in the lunar sky.

We ended up in this configuration because the Moon is close enough to Earth that our planet’s gravity was able to slow the Moon’s past rotation with torque. A mountain would try and swing past, adding its bulk to the Moon, and gravity would say “no, mine” and pull on that extra mass, slowing the world’s rotation.

What our Earth does to the Moon, other stars are able to do to their close-in planets. This can lead to weird situations where small stars with large planets can end up with tidally locked planets sitting in their habitable zones, one side always facing the sun and one side always facing away. This means a life form on these worlds would have to travel around the globe to experience a sunrise, sunset, and all the parts of a day in between.

Which is weird to imagine. And wreaks havoc on the weather.

The question has become “Can these otherwise good-for-life worlds support life or do the winds and storms and temperatures created in this weird situation make life as we know it impossible?” An article in EOS reviews the literature on this matter and finds that life is possible, but we are likely looking at extreme life – life that in one instance is designed for the high temps of ever-lasting noon or the gale winds of a forever-setting sun. 

This constancy actually may cause some of its own difficulties. Here on Earth, life often cycles with our seasons, but a tidally locked world will have none of that. The amount of light a place gets is just the amount of light it gets… always. We know there is subsurface and deep-sea life here on Earth that experiences this kind of constancy, but even scientists struggle to imagine how you grow a complete civilization from a place where each places climate is distinct, and spreading around the world to fill a niche in the ecosystem just isn’t possible.

The TRAPPIST-1 System is the nearest system to have a planet in the habitable zone that is tidally locked, and as we develop bigger and bigger telescopes, I look forward to new data that will allow us to better imagine this truly bizarre – to humans – kind of a situation.

More Information

Tidally Locked and Loaded with Questions (Eos)