Decades ago, when I still had a full head of hair, I remember mapping a planet for a science fiction tabletop game that never panned out in the end and, because mapping an entire planet looked like too much work, I had ruled that the planet spun on its axis precisely in synch with its revolutions around the local star.
Which, to me, meant that one half of the planet was purdy much frozen solid, one half was scorched to oblivion by the star, and there was only a narrow ring of habitable land on the “equator”, which in that context was a plane perpendicular to the sun’s rays.
Furthermore, I posited that it would always be twilight on the equator, with the sun always appearing to the locals to be just above the “northern” horizon (where the north pole is, arbitrarily, the one always pointing towards the star).
So, astronomy major Dopers : how utterly wrong was 16 year old me ? Could that kind of solar system exist, and would it really work that way ?
Tidal locking of a planet is entirely possible. The moon, for example, is tidally locked to earth. It could just as easily happen with a planet to a sun.
Assuming the planet would be habitable if it was not tidally locked, I see no reason that your suggestion wouldn’t work. One side would be constantly arid desert and the other would be frozen, with a ring of habitable zone. And yes, it would constantly look like twilight.
For some good reason of sci-fi combat on a world like this, check out the Ciaphus Cain series by Sandy Mitchell. It’s a Warhammer 40k universe book, and I think it’s the 4th in the series (I can’t remember because I have it in an omnibus).
Nitpick: I wouldn’t think the north pole would be the point always pointing towards the star, as it is not on the axis of (slow, tidally locked) spin. I’d put the north and south poles on the axis of spin, relative to the stars, as usual.
You could call the points pointing towards and away from the star the Hot and Cold poles. Or Day pole and Night pole.
I was taught in school that Mercury was tidally locked. I have since learned that they had already worked out the 3:2 ratio in the mid 1960s, a full decade or so earlier than when I went to school and was taught otherwise. I wonder how long they continued to teach that in schools.
The orbit will never be perfectly circular, while the rotation will have a steady pace, so there will be some degree of “libration” with the strip where the sun is right at the horizon moving back and forth a little, depending on the eccentricity of the ellipse.
Since it’s fantasy, you could have fire demons come from the centre of the sunward hemisphere and ice demons come from the centre of the space-ward hemisphere.
Probably until the textbooks wore out and had to be replaced.
Isaac Asimov wrote a children’s novel that I read in the early '70s that was about Mercury and was very similar to the OP’s scenario with life existing in a thin ring between the hot and cold sides.
I bet it pained him to have a novel based on out of date science so soon.
Just the other day I read a short story by Arthur C Clarke called Wall of Darkness, which was set on just such a planet. The eternally cold shadowed part of the planet was blocked off from the habitable zone by a seemingly impenetrable wall, but the story had a twist… well, half a twist. I won’t explain what I mean by that…
I doubt it. Convection currents usually happen because a fluid is denser when it’s colder, and so it “wants” to sink. If the thermal gradient is completely at right angles to the direction of gravity (as it would be on this hypothetical planet), then these currents can’t arise.
In another novel by Asimov, Nemesis, there is a red dwarf star, with a giant gas planet tidally locked. But the planet has an Earth sized satellite, that has all parts of its surface periodically exposed to the star, so it is habitable.
The author presented a lot of math and came to the conclusion that over time, all of the water available loose on the surface of the planet would end up as a vast ring of ice around the dark side of the twilight edge that would periodically release vapor or condensation due to libration. Otherwise, the planet would be arid.
I don’t math, so I can’t vouch one way or the other for the numbers, but it certainly was interesting to read.
Why not? Air on the Sun side heats and rises, then flows to the dark side, where it cools. The cold, heavy air on the dark side falls, then flows to the Sun side because it’s denser than the air that’s already there.
Or Day pole and Night pole.