I want to play with the idea of a sapient species evolving on a world where at least part of the world experiences a permanent day. The obvious way to doing this would be where part of the world faces the sun always and part of the world phases away, though I'm not limited to that approach so long as a reasonable portion of the world will always have day-light.

I would like to imagine a world as similar to earth as possible given this criteria; or to be more specific a world that applies the most similar evolutionary pressures that earth placed on evolving creatures; other then the obvious no-day part.

In reality I can't just say earth, but without a night. The physical world would be different. Most obvious the constant sunlight being absorbed all day would have an impact on temperature, to keep the world the same temperature the sun would have to have lower radiance or the worlds atmosphere less likely to trap heat. I think this would also encourage more severe weather patterns, particularly near the border between day and night?

Thus I'm wondering how close could a planet get to earth-like, or encouraging of earth-like creatures evolving, while having a permanent day? What are the most significant differences such a planet would have compared to earth, beyond the obvious day itself.

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    $\begingroup$ I'll remind all potential answerers that the hard-science tag has stringent criteria. I don't see that being fulfilled so far. $\endgroup$
    – HDE 226868
    Dec 18, 2015 at 21:16
  • $\begingroup$ Do you know whether you are preferring a rotation which keeps a smaller area always towards the sun, or whether you want the entire face to always be towards the sun? So far people seem to assume the second, probably because it's easier to imagine. $\endgroup$ Dec 18, 2015 at 22:16

4 Answers 4


There isn't a lot of hard science for something like this, unfortunately. The nearest exoplanet to us is lightyears away. That said, I am an enthusiast of astronomy, so I definitely enjoyed doing some research.

As Smoj points out in their answer, Tidal Locking is the idea you're looking for. An exoplanet which is (likely) tidally locked that we have (possibly) discovered within the Goldilocks zone of its host star is Gliese-581g. If it had an atmosphere like Earth's, it would be in for a rough time - well over half of the planet would be very, very cold (remember, seasons on Earth are caused by our 23.4 degree tilt, so sunlight would not effectively distribute close to the day/night border).

However, something interesting I found out when researching this is that with a thicker atmosphere (or even a thinner one containing appropriate proportions of CO$_2$ and H$_2$O, and likely less N$_2$), a planet may be able to effectively distribute heat from its lit side to its dark side via longitudinal winds.

There would indeed be severe winds and ocean currents (not necessarily water oceans), which could make the evolution of life difficult, but hey, "life finds a way".

Your sapient species likely could not be one that evolved on Earth, as if they visited this planet with large amounts of CO$_2$ they would be subject to Hypercapnia (CO$_2$ poisoning). If your species evolved on this fictional planet and then traveled to Earth, they would likely suffer from Hypocapnia, or a lack of CO$_2$.

From Jay's answer:

Anyway, if multiple stars were close enough that a planet would receive daylight-level sun from several of them, would such a system be stable? I'm sure someone's worked out the physics of that to say if it's possible.

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Someone has worked out the physics - in fact, Binary and Trinary star systems are incredibly common! The closest star system to us, Alpha Centauri (4.37ly), is a binary system of Alpha Centauri A, Alpha Centauri B, with a third star Proxima Centauri just hanging out in the outskirts. There has been some evidence to indicate the system may harbor at least one exoplanet, but nothing conclusive.

There is also this game that allows you to (attempt) to build a stable solar system with all sorts of objects.

I mentioned that game because you'll quickly find it's incredible difficult to make a solar system where an exoplanet is in a stable orbit with two other stars, such that it lies between the host star and the orbit of another star around the host star - and I'm not aware of any real world examples found to date.

To answer your initial question:

What are the most significant differences such a planet would have compared to earth, beyond the obvious day itself.

From my research, I'd say, more than anything else, the atmosphere. It would need the correct ratio of greenhouse gases, and the correct density, in order to allow heat to propagate about all sides of the planet.

  • $\begingroup$ To clarify my comment that you reference: Yes, I'm aware that multi-star systems are common. And, without working out the math, I'd guess that a planet could have a stable orbit around a pair of such binary stars if it was far enough away that it would be a close approximation to model the two stars as a single body equal to the combined mass at a point between them. But that would be very far away, resulting in a very cold planet. Could it pass between them, like you're figure-8 picture? What would such an orbit be like? That's what I meant I didn't know. $\endgroup$
    – Jay
    Dec 22, 2015 at 7:06
  • $\begingroup$ If I remember right, the figure-8 orbit only works with three stars of equal or similar mass. With a planet taking the place of one of the stars, it would be significantly lighter. You'd end up with a binary orbit for the stars, and trying to plot the orbit of your exoplanet is the heart of the three-body problem. However, I'd say it wouldn't last long in that system - it would essentially be receiving one gravitational assist after another, until it eventually reached escape velocity and became a rogue planet. $\endgroup$ Dec 22, 2015 at 7:15

I don't claim to be expert in the science involved here. (But then, there really is no true "science" here, as we don't have any examples of such a world available to study. There can only be speculation and extrapolation based on science.)

Assuming one side of the world always faces the sun and the other side always faces away:

Most plants as we know them could not survive on the night side, as they would never get sunlight to power photosynthesis. That would make it tough for animals to live either, as they wouldn't have plants to eat to make a foundation for the food chain. Presumably animals living near the boundary could travel back and forth.

Presumably the day side would get much hotter than the night side. But if the night side is mostly dead anyway, that's not much of a direct problem from a world-building point of view: You just posit distance from the sun, atmospheric composition et al sufficient to make the day side a reasonable temperature for life. Then the dark side is very cold. How much colder depends on thickness of the atmosphere, weather patters, whether there are moons, etc.

The hotter day side would mean that air gets heated and expands, and then must move toward the dark side where it cools. Of course air can't continually flow from light to dark so there must be currents bringing it back. In short, I think you have some pretty constant high winds.

Similarly with ocean currents. I think the surface will tend to be moving dark-ward while there's an undercurrent flowing light-ward.

If the people basically live on the light side, the dark side is a barren, unknown, mysterious land. In early days a few brave souls probably venture there. As technology advances eventually they reach the point where they can launch serious expeditions and truly colonize the place.

Another scenario may be for the planet to be in a system with multiple suns, so that all or most of the surface is getting light from at least one of the suns at any given time.

(Isaac Asimov wrote a story about such a world many years ago called "Nightfall", where it is only night for one day every thousand years or some such, and when that day comes people go insane. It was a well-written, entertaining story, but it seemed to me to have a lot of plot holes. Like, given the whole multiple-star premise, any particular spot on the planet might have night only on these rare occasions, when it happens that given the dynamics of the system, that part of the planet is facing away from all the stars, and/or they are eclipsed by moons. But the whole planet wouldn't go dark at the same time, it would be one piece here, one piece there. So even if we accepted that the darkness drives people insane, it wouldn't be the whole planet at once, just part of it, so it's not clear why civilization would collapse. And why would darkness drive people insane? Even if it's never night, people don't normally go insane because they experience a previously unknown natural phenomenon. I didn't go insane the first time I saw a tornado. And even without night, don't people on this planet have caves, windowless basements, shipping boxes, etc, that they would experience darkness now and then? Oh well, whatever.)

Anyway, if multiple stars were close enough that a planet would receive daylight-level sun from several of them, would such a system be stable? I'm sure someone's worked out the physics of that to say if it's possible.

  • $\begingroup$ I think you're right. The convection currents would be constant. There'd be a lot of energy in that wind... $\endgroup$
    – Smoj
    Dec 18, 2015 at 15:44
  • $\begingroup$ hmm, what about a second sun providing some heat to the 'night' side? or even a few moons reflecting sun light back to it? not necessarily enough to keep every side a perfect heat, but potentially enough to keep the winds from being too severe? Or...could enough water handle the energy transfer without winds? possible not, since eventually the water has to freeze. $\endgroup$
    – dsollen
    Dec 18, 2015 at 15:57
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    $\begingroup$ Another sun close enough to provide heat brings up the same questions of stability of orbits as another sun close enough to provide light. RE moons: I was thinking that several large moons could result in perpetual light all over the world. Maybe not daylight, but enough to see by. How much heat do we get from the Moon? I don't know. Interesting question. $\endgroup$
    – Jay
    Dec 18, 2015 at 20:36
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    $\begingroup$ I respectfully disagree that there can be no science used here. It's a question of orbital mechanics, and exoplanets are understood surprisingly well. $\endgroup$
    – HDE 226868
    Dec 18, 2015 at 21:15
  • $\begingroup$ @hde226868 This is a side issue, but: "Science" means "knowledge gained by experimentation and observation". As we have no worlds like the poster describes to use in our experiments, we cannot study the question scientifically. We can extrapolate from existing scientific knowledge, but any conclusions are not science, they are hypotheses. We may put a very high confidence in some of these hypotheses, but until we actually perform the experiment, they are not scientific facts. We can use scientific-sounding language to discuss the question, but all sorts of pseudo-science people do that too. $\endgroup$
    – Jay
    Dec 22, 2015 at 6:59

An example, at least in terms of the mechanics involved, is the moon. Tidal locking explains this.

If you looked at Earth as a star instead of a planet, the moon would always have one side heated, and the other side cooled, as it has one side facing and the other side facing away at all times during its orbit. A night and a day side, with a temperature gradient from one to the other.

For an Earth-like planet orbiting a Sol-like sun, the nature of the day/night hemispheres of the planet would depend on how far away it was from the sun.

The closer it is to the sun, the larger the heated day side would be, and through atmospheric convection forces, the hotter the night side would be too.

At a Goldilocks distance away from the sun (not too hot, not too cold, just right), you'd end up with a band around the planet as a tangent to the orbit, separating the two hemispheres.

The temperature within this band could be suitable for life. The hemisphere closer to the sun would be too hot, cooling as it approaches the band. The hemisphere away from the sun would be too cold.

So you'd have a permanent day side which is within the temperature gradient suitable for life, and part of a night side which is also suitable for life.

Hugh Howey in one of his Molly Fyde books (The Bern Saga series) has a scenario much like this.

Half of the world is scorched, the other half frozen. With habitation in a narrow band.

Actually, from memory the world in the story had two suns. But the principle would work with only one sun.

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    $\begingroup$ if you were in the 'band' of proper heat wouldn't most of your heat energy come from radiation from the hot part of the world, not directly from the sun? I ask because I wonder if you would get sufficent direct sunlight to power photosynthesis this way, or if the amount of sunlight received would need to be much lower to keep proper temperatures due to the extra heat being transferred? Is there a reason for a 'band' between hot and cold instead of simply placing the world further away from the sun so the hot side is just right? $\endgroup$
    – dsollen
    Dec 18, 2015 at 15:55
  • $\begingroup$ @dsollen Good question. I guess yes, it would depend on the distance from the sun. But either way you'd still get a gradient. That gradient could be more like a warmer polar cap on the sun side, with the rest of the planet too cold. $\endgroup$
    – Smoj
    Dec 18, 2015 at 16:07

Explore this. Chris Wayan does an excellent job of exploring what the parameters of a tidally-locked earthlike planet with permanent day/night sides would look like. He takes an imaginative, entertaining and reasonable approach. Hope you find this informative!



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