Questions about tidally locking a fictional planet to its star

Factual Questions
1

Suppose I’m writing a fictional story set on a hypothetical planet that becomes tidally locked with its star. This planet therefore has a side which always faces toward the star, and a side which always faces away from the star.

The resulting changes would be pretty devastating for anything living on that planet, so we wouldn’t have much of a story unless it was a particularly short one.

However, I was thinking about using a kind of lunar eclipse to provide temporary shelter from the star’s light; from a moon in particularly close orbit to the planet, moving relatively slowly, such that it blocks out a large portion of the sky for a long while, and shields the planet’s sunny side from light for perhaps for 10 hours or so, continuing around the planet. The star would also be smaller and dimmer than our Sun, so that the extremely long non-eclipse period (what ends up being analogous to “day” on this planet) would heat up the surface of the planet rather slowly, and not be as intense as the sunlight one would expect at noon in the summer, and the heat would dissipate quickly during the eclipse period, resulting in a habitable planet with day-like and night-like periods, despite the tidal locking.

I’d imagine the day period would be extremely long compared to the night (eclipse) period.

With that in mind, how feasible is all of this?

Would the proximity of the moon prevent the tidal locking between the planet and the star in the first place? Surely the gravitational effects of the moon would be significant here.

Would a satellite covering a significant portion of the sky due to its proximity not also generate significant problems for the planet, such as quakes and extreme flooding due to tides?

My guess is that the moon would have to be less massive in relation to the planet than our moon is to the Earth, to reduce its gravitational footprint and avoid significant quaking and rising tides. and therefore, it would have to be much closer to the planet than the moon is to the Earth, to provide the eclipse effect I’m looking for.

If you have some expertise in astronomy, physics, and so forth, and are qualified to provide an intelligent and accurate response to this hypothetical, I’d be interested to know your opinion here.

I’d allow a little leeway for feasibility as it would be in a fantasy setting, but I don’t want to explain everything away by invoking magic. Something realistic would be helpful.

2

Yes. For the eclipses to be frequent and long, your moon will need to have a much larger angular size than your sun. The tidal effect from a body is proportional to its density times the cube of its angular size. So to make the sun the dominant tidal influence, there will have to be a huge difference in density between the sun and the moon.

I suppose you might be able to do it with a planet very close to neutron star sun, but then you’re going to have to explain how a planet (with a large, low-density satellite, even) ended up so close to a neutron star. It can’t have been there when the original star went supernova, and there’s a fairly short time window after that for it to migrate in, before the neutron star gets too cold to be worth anything.

3

If the moon is close enough to create tides and quakes, I would assume that the planet would tend to become tidal-locked with the moon. Hench, the tidal lock of the sun would be broken. To put another way, the planet would be tidally-locked with the moon long before it were tidally-locked with the sun.

If the earth were tidal-locked with the sun, some ocean evaporation would drift to the dark side, condense, and fall as snow on the far side. Eventually, all the water would be in the form of glaciers. Whadda ya gonna drink?

4

A low orbit for the moon is also a fast orbit.

5

Perhaps a better solution would be cloud formation in the mesosphere rather than an eclipsing moon. This would reflect solar energy and keep temperatures more moderate on your planet. Certainly this is unrealistic but still better than magic or some strange and unusual form of gravity.

6

You could finesse the problem by just having life (and certain other interesting features) exist only in the narrow band between the too-hot day and too-cold night.

I suppose the temperature gradient there would provide a huge source of free energy that would have directed evolution along some interesting paths.

7

How about a coalescing moon? A moon that is forming from debris around the planet that is in the form of a debris cloud–kind of a localized asteroid belt. The cloud covers some angular distance but allows some sun to filter through. When the cloud passes full sun hits the sun facing side.

8

I have a book, whose title i don’t remember, in which there is a planet that is being farmed for the benefit of earth, with an annual visit by freighters. The planet has an orbit/rotation the makes periods of hot dry climate and extremely cold periods. The whole population gets on a land train to move through the excessively fertile and hostile central area to the hot part.

9

It’s worth noting that a planet tidally locked to the Sun would have a very slow rotation rate, which means that the moon’s orbit will be unstable over geological time, with the ultimate result that it will hit the planet (or be disrupted by tidal effects) Tidal acceleration - Wikipedia

10

Or a close to 100% water moon.

11

Introduce some manner of harmonic oscillation so the debris field varies in density in a regular fashion.

12

Clarke used the idea of tidal lock in his short story The Wall of Darkness (which, however, is set in an altogether different universe, with a really strange geometry…)

13

It might work with multiple moons.

14

There is really no way the scenario stated by the o.p. be remotely plausible, not just for the reasons stated by Chronos, but because a moon large enough to provide any significant eclipse umbra (the fully shaded aspect behind the moon) and far enough to cover the star’s aspect would be subject to the same gravitational tides that caused the planet to become tidally locked. And tidal locking of a roughly spherical planet is a gradual process that takes tens or hundreds of millions of years at least, requiring the tidal energy to be dispersed as tectonic strain energy (e.g. flexure and release of the crust or asymmetry of the planetary mass). This wouldn’t occur over an organism’s lifetime; this on the order of evolutionary periods.

Stranger

15

Moons around tidally-locked planets are unlikely, as noted above. So no shading.

However, if we were to discover a tidally-locked Earth-like planet at some time in the future, I wouldn’t dismiss it completely as a suitable location for colonisation. A suitably advanced civilisation could build a sunshade between the planet and the star, located at or near the Lagrange L1 point; this could regulate the temperatures on the sunward side. A system of lightweight mirrors behind the planet could illuminate the dark side, if you want to live there as well.

But it would be much easier to ignore the planet and live in space stations of various kinds; by the time we get round to interstellar colonisation (if we ever do) we’ll probably be very good at that.

16

A wiki page about the sunshade concept here

17

Perhaps even better: a ring system.

Planets are often tilted relative to the ecliptic, so the ring could appear to periodically pass in front of the star, filtering sunlight during that period. Rings are not usually very dense, so I’m not sure exactly how much benefit you’d get out of this, but if the goal is to take a moderately hot sun side and give it a moderately cool “night” then a ring might just do the trick.

However… going back to the OP’s issues I don’t think any amount of shade gets you a habitable planet. It isn’t just a question of controlling heat on the sunny side. If you’re maintaining Earth-like temperatures on one side, the back side will be beyond frigid. Anytime water evaporates and moves to the back side, it will turn into ice and never return. For that matter, you’ll probably freeze CO2 and other atmospheric gases. A stable, habitable atmosphere would depend on the dark side being warmer than 32 F at least occasionally. That kind of planet has more in common with Venus than Earth.

18

Rings naturally form in a very thin layer. The main rings of Saturn, for instance, are about 10 m thick. (There are thicker sections which are perturbed by the moons but they are so low density that the don’t block any significant amount of light.) And it is impossible for a stable ring structure for form around a tidally locked world; between the perturbing force of the main body (which again, is strong enough to lock the world’s rotation to face the sun) and mass concentrations of the world which only rotates once in its orbit while the ring constituents have to move at stable orbital speed, any attempt to establish a ring will become destabilized in short order.

Stranger

19

If you just want a very long day and night, you could model it like Mercury. Mercury’s year lasts 88 Earth days. Due to a resonance, its sidereal day is 2/3 as long 58 2/3 Earth days. But this makes its solar day (what common people think of as the day sunrise to sunrise) lasts 176 Earth days. That is, Mercury has a solar daytime and a solar night time each of which last one Mercurian year.

Also Steve Baxter’s Proxima takes place on a planet circling Proxima Centauri which is tidally locked. He has native life there as well as Earth people colonizing it. I haven’t checked this one, but his science is generally pretty reasonable so you probably don’t need your moon to cool things off enough for life.

20

It’s possible that the situation in the OP won’t ever happen. By that, I mean a planet with a significant atmosphere getting into a tidal lock with its sun.

Consider Venus. It actually should be in a tidal lock but instead rotates slowly backward. Why? Well, it could be a collision with another body, but there’s no evidence otherwise for that (and you’d expect there would be for a collision of that magnitude). Some astronomers have figured that there’s a tidal effect in the thick Venusian atmosphere that causes the slow retro-rotation. Other astronomers have concluded that this effect would also happen with an atmosphere as thin as Earth’s.