In the current fantasy novel I'm working on, I decided that I wanted to have a part of the planet where it was always daylight. After doing some research I learnt about the concept of tidally locked planets and further research has led me to consider using a habitable moon tidally locked to a gas giant. Although I still have questions, such as:

Which side would be the hot side and which the cold? I originally thought this would be the side facing the sun but have consumed various conflicting information about this and I'm now confused.

Would it be possible to have desert and savanna biomes on the hot side and could there be significant bodies of water on the hot side?

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    $\begingroup$ Welcome to Worldbuilding Stack Exchange, Habiba! A tidally-locked moon won't have the same sort of climate as a tidally-locked planet - it's tidally locked to the planet, so only one half faces the planet, but as it orbits around that planet, a different sections will face the star - in which case I think tidal locking won't really achieve the effect you're looking. $\endgroup$
    – HDE 226868
    Jun 17 '20 at 21:06
  • $\begingroup$ I would recommend that you check out the Worldbuilding Pasta Blog. It has some really amazing and understandable articles on how to build your world from the ground up. The last article was about realistic climate and how to design it. It won't answer your question, but it will help you to understand what you're looking for and to ask the right questions. $\endgroup$ Jun 17 '20 at 21:25
  • $\begingroup$ The Dying of Light means this one. $\endgroup$ Jun 17 '20 at 21:37
  • $\begingroup$ Thanks, I did go over there, the author of the site says the information mainly refers to planets that are like earth and that there would be separate articles for tidally locked planets which he hasn't put up yet. $\endgroup$ Jun 17 '20 at 21:42
  • $\begingroup$ Thanks @HDE 226868. This would mean it wouldn't have the perpetual day look and the side facing the sun would experience 'night'? $\endgroup$ Jun 17 '20 at 21:56

So, if a planet is tidally locked to its parent star, yes, one side will be in constant daylight and the other in constant night.

This would, logically, lead to very high temperatures on the day side and very cold temperatures on the night side. The most habitable regions (temperature-wise) would be along the border between day and night (called the terminator), where the sun is at or close to the horizon. However, such extremes in temperature would likely lead to extreme weather patterns: as the hot, low-pressure air on the day side and cold, high-pressure air on the night side will generate incredibly strong winds flowing between the two hemispheres.

In order for a planet to become tidally locked with its parent star, it would also have to have a relatively close orbit around the star, with an orbital period (its "year") somewhere on the order of only days or weeks. In order for the planet to still be habitable so close to the star, the star must be very small and cool, something like an M-class red dwarf star.

The TRAPPIST-1 exoplanetary system is a good example of this. The seven(!) rocky planets of the system orbit very close to their star, a cool red dwarf. The fourth planet in the system, the one thought to have the highest potential for habitable conditions, has an orbital period of about 6 days.

Also, a common misconception about tidally-locked planets is that the sub-stellar continents (spot directly under the sun) would be barren deserts. A lot of studies, like this one:


actually show that the amount of precipitation will increase drastically on the daylight side. [Here are a few animated simulations from that same study:


that illustrate as much. This means that you probably won't end up with lots of deserts as much as huge rainforests. Therefore, it is likely, as you asked, that there would be large bodies of water across the daytime side. However, it is surely still possible to form some deserts on the daytime side; notice several areas in the animations where precipitation is still low. This probably has something to do with the prevailing winds and mountains and rain shadows and all of that—but I am an astronomer, after all, not a climate scientist.

A tidally-locked moon, however, is a completely different ball game. The Earth's Moon is tidally locked—it only ever shows us one hemisphere due to its synchronous rotation. But we all know that the moon goes through phases (sometimes it's full, sometimes only half is visible, sometimes a crescent, and sometimes we can't see it at all), which is due to its month-long orbit around the Earth. In other words, takes about two weeks for a single "day" to pass on the Moon, from sunrise to sunset, and then another two weeks for the sun to rise again.

But the important takeaway here is that the Moon experiences night and day just like we do, albeit on much longer timescales. An astronaut living on the moon would even watch the Earth go through similar phases to what we see looking at the moon, except that the Earth wouldn't change its position very much in the sky; it would pretty much stay in the same spot all the time. All of this is simply due to the fact the Moon is tidally locked to the Earth, not the sun.

The moons of Jupiter are also tidally locked to the planet (as most large moons tend to be), so they keep one side constantly facing Jupiter, like our moon does for us. Jupiter's Galilean orbit much faster than our moon, though; a full orbit for Europa is only about 3 and a half days, so there'd only be about 42 hours between sunrise and sunset. But during every orbit, Europa passes behind Jupiter's shadow—essentially a solar eclipse but lasting for hours. This means that there are regular periods where the entire moon experiences a pitch-black night.

Long story short, though: if you want your story to be set on a planet that experiences constant daylight, it must be set on a tidally-locked planet and not a moon. Any moon, even if it is tidally locked to the parent planet, will experience regular cycles of day and night.

But even on a tidally-locked planet, the permanent daylight side would be oppressively hot. The desert regions would be beyond inhospitable, but, thankfully, the high humidity of the rainforest regions would keep the temperatures relatively low. The coastal regions might also be similarly temperate. Regions at higher latitudes and regions near the day-night border would also have much more bearable temperatures.

Whatever life evolved on this world would have to have some sort of mechanism for enduring extreme heat—perhaps something like the long, wide ears of a desert jackrabbit that act as radiators to expel excess heat. Or perhaps they are semi-aquatic and use the oceans to stay cool. But the creative parts get to be your choice :)

Hopefully that answers your question! Let me know if you have more.

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    $\begingroup$ The daylight side of a tidelocked world need not be oppressively hot, if it is merely sufficiently far away from its star. $\endgroup$ Jun 18 '20 at 3:18
  • $\begingroup$ You are correct. I guess I should have specified an Earth-sized planet that receives the same amount of stellar flux as our Earth. If you moved the planet a bit further away, it could have a very temperate climate. This perfectly highlights the difficulty in establishing a "habitable zone" around a star. Too many variables. $\endgroup$
    – Geroditus
    Jun 22 '20 at 18:39

It IS possible to have a tidally locked moon that always has the same side facing the sun, but it would require that the planet it orbits is also tidally locked to the sun (a 1:1 spin-orbit resonance). This would, however, require the moon to orbit its planet at a rather long distance. Let us imagine that we had a Saturn-sized planet orbiting our sun in the Earth's orbit and that an Earth-sized moon orbited that planet, with both being tidally locked to their parent. The moon's orbit around the planet would then have to be one Earth year. Orbital period is proportional to orbital radius to the power of 1½, and comparing with the actual moons of Saturn, that would put our planet at an orbital radius of roughly 10 million km or roughly 25 times our Moon's orbital radius. At such as distance, it is not bound very tightly to the planet - in fact, its orbit would be 5 times farther out than the L1 and L2 Lagrangian points. Moving your planet farther out would require also moving the moon's orbit farther out, so this doesn't solve anything. You could instead place your moon in one of these Lagrangians, but they are not stable, so it would be unlikely to stay there for long.

However, you don't need your moon to be tidally locked to your planet to have it always having the same side facing the sun. It just needs to have a rotation period exactly equal to the planet's year. This may sound quite unlikely, but it is possible. Having the planet in spin-orbit resonance with the sun and the moon in spin-orbit resonance with the planet would help. Say that the planet has a year of 4 revolutions (4:1 resonance). Now say that the moon has 4 orbits a year, giving a geostationary orbit (it is always above the same point on the planet) and has a 1:4 spin-orbit resonance with the planet (it rotates once every 4 orbits), then it would always have the same side facing the sun. This is an unlikely, but possible dynamic. The moon's orbital period and distance would be very similar to that of Saturn's outermost moon Iapetus and should hence be stable.

For the climate, Geroditus has a good answer. I can add that if the planet radiates a lot of heat, it can be farther out and still place the moon in the habitable zone, and in this case, the planet's radiation would make the extremes between day and night less extreme. The planet would lightly heat the moon's night side when it is in the sky, and the day side would receive slightly less heat when the planet isn't in the sky, creating a weak summer/winter cycle.


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