A tidally locked planet does not rotate in relation to its sun. One side is always day, one, always night. This is caused by tidal forces pulling all planets towards this same equilibrium, so it’s completely stable once it does occur…a tidally locked planet at an earthlike distance from the sun would be scorching heat on one side, freezing ice on the other.
What about at different distances? Is there a band of orbital distance where the night side of a tidally locked planet is warm enough for liquid water? Or one far away enough that the day side can have oceans?
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Given the crazy temperature differences you get on Mercury I don’t imagine the dark side of a tidally-locked planet would ever be habitable, though maybe a proper atmosphere would change that. There would be a habitable ring around the planet that’s partly-lit; seems like the kind of thing explored in sci-fi a lot (though the only story I can think of is The Last Day by QI/NSTAAF’s Andy Hunter Murray).
Interesting note: Mercury isn’t tidally locked, but it sure looks so. Its average day is slightly longer than its year. Learned that a few months ago
I learned it when fact-checking my original comment!
Who knows, maybe life is possible on the ring of twilight where it’s not too hot and not too cold. The planet where the Twi’leks in Star Wars are from is that way. Always sounded plausible to me.
Been doing some googling. Most stars are dwarf stars, and the habitable band of a red dwarf is so close the tidal forces are way stronger. It might turn out that most habitable planets are tidally locked.
There’s a book called The City In The Middle Of The Night that takes place on a tidally locked planet. It has politics, science fiction, and lesbians.
My favorite things!
It also kinda makes you feel like shit, but in a really good way.
Even better
Temperatures would be okay in the twilight zone, not sure about water, but winds would blow you out of your shoes.
What about a planet so distant from its star that the day side gets as much light as earth at twilight? 2AU from a sol size star, there should be a quarter as much light during the day. What about there?
It’s worth noting that the human eye has a very wide range that it can adjust for brightness. Full daylight is 10000 lux, a room where visual tasks aren’t being performed can easily be 100 lux. 2500 lux would be very bright light in an indoor environment. From a societal perspective, this likely wouldn’t be enough to have an impact. From an ecological perspective, it could have a significant impact.
I think they would be hard for life to naturally develop on this kind of planet, the winds would be very strong and the water would be always running away from the day zone to precipitate on the night as ice, so even the twilight zone would be hostile to life.
But colonization is a possibility.
