Without tidal locking, would it be possible to have a planet with a region roughly the shape and size of the USA and Canada in which it is always "day"? (By day I mean at least as bright as a clear day immediately after the sun has set). I would like the planet to have roughly the gravity of Earth, and to not be tidally locked to a star. This is because I want it to be able to generate a magnetic field. However, the surface conditions of the planet do not necessarily have to be conducive to life. I'd also prefer it if the configuration could plausibly form naturally.

If it is possible, what kind of conditions would it require?

  • $\begingroup$ Density enough to match Earth. Rotating. Small enough that light refracts all the way around it... that last requirement necessitates an atmosphere of some sort. Hmmm. $\endgroup$ – SRM Sep 2 '18 at 21:38
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    $\begingroup$ Would a highly reflective moon be ok? $\endgroup$ – SRM Sep 2 '18 at 21:39
  • $\begingroup$ Yes, but I'm going to add to the question that it should be possible to form naturally $\endgroup$ – Xandar The Zenon Sep 2 '18 at 21:44
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    $\begingroup$ Lava planet, where lava lits everything. $\endgroup$ – rus9384 Sep 2 '18 at 22:19
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    $\begingroup$ @RonJohn: I think you and Raditz are talking at cross purposes here. Instead of "area", try "region", as in "a planet with a region roughly the shape and size of the USA and Canada" $\endgroup$ – nzaman Sep 3 '18 at 13:18

This could be achieved on a world with no axial tilt where the continent in question was at the north or south pole.

At the pole the sun would appear to rotate around the horizon giving a dawn or dusk appearance. At lower latitudes in the northern hemisphere (higher latitudes in the southern hemisphere) the sun would appear to tilt up at one side of the horizon and dip down at the other, but the light levels would still be those of early dawn / late dusk for many hundreds of miles.

This could easily form naturally. Although planets usually have some axial tilt this is just hapstance.and not a requirement. 1g gravity would be possible and the planet could be inhabited. The only potential issue would be the size of the area lit by the sun in this manner.

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    $\begingroup$ This does not work as well as you think. Areas at the pole will be occluded by the terrain around them, excluding mountain peaks. en.wikipedia.org/wiki/Peak_of_eternal_light While you can have a few mountain peaks be eternally lit, this is a far cry from a whole continent. $\endgroup$ – Ryan_L Sep 3 '18 at 0:27
  • $\begingroup$ The area does not have to be in full sun light. After the sun has set it remain light for for a considerable time $\endgroup$ – Slarty Sep 3 '18 at 1:34
  • $\begingroup$ You could also have atmospheric effects scattering the light - I think this is a good stab at a solution. $\endgroup$ – Tim B Sep 3 '18 at 15:25
  • $\begingroup$ The sun will be below the horizon for approximately 12 hours a day in such a situation except for a few kilometers around the pole. The reason is the curvature of the planet itself which will occlude the sun. $\endgroup$ – Keith Morrison Sep 4 '18 at 1:48
  • $\begingroup$ Very true, but the sun will not dip that far below the horizon until you are a considerable distance from the pole. As I write this where I am now the sun has set, but it's still daylight out side $\endgroup$ – Slarty Sep 4 '18 at 17:34

It could be a rogue planet in the core of the galaxy. There are so many stars in the core of most galaxies that night is as bright, or even brighter than day on Earth. Now if this planet is a rogue planet and not bound to a star, there will be no side getting any more light than any other. The sky in every direction will be equally bright.

It may not be truly identically bright, but it could be close enough that you could not tell without scientific equipment.

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  • $\begingroup$ While I like this answer, the temperature might not be Earth-like. And even if it was, the gravitational forces from so many Suns might rip the planet apart. $\endgroup$ – Sydney Sleeper Sep 3 '18 at 0:17
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    $\begingroup$ The planet will very likely be hot since its being heated on all sides. Gravity is no issue though because this planet would be further from any of these stars than we are from the sun. It would be well outside the Roche Limit of any of them. en.wikipedia.org/wiki/Roche_limit $\endgroup$ – Ryan_L Sep 3 '18 at 0:21
  • $\begingroup$ Globular cluster can also work. $\endgroup$ – zibadawa timmy Sep 3 '18 at 2:50
  • $\begingroup$ You can fix the temperature just by having it the right overall brightness coming in - long term it's not going to be stable but short to medium term this idea works. $\endgroup$ – Tim B Sep 3 '18 at 15:24

Multiple suns

In the book Nightfall by Issac Asimov, the planet Kalgash is bathed in perpetual sunlight. Kalgash has six suns. With the orbit of the suns, there are always at least two suns in the sky in a single day and there is always at least one sun in the sky at any given time. With multiple suns, you can have perpetual sunlight covering the entire planet.

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    $\begingroup$ Actually it is a problem designing a star system where a planet or any part of it has eternal day. I've tried. It is easy to design a system where a planet or a part has constant daylight for years, or decades, or centuries, or maybe even millennia. But sooner or later the period of eternal day will end and not return again for years, decades, centuries, or millennia. If a planet is billions of years old, like most interesting planets in science fiction, millennia of constant light will alternate with periods of some night millions of times, no matter how many suns it has. continued $\endgroup$ – M. A. Golding Sep 4 '18 at 19:23
  • $\begingroup$ continued. Designing a multi star system that is dynamically stable for millions of years and keeps one planet or part of a planet constantly in sun light is very, very difficult. See a professional astronomer's attempts to design a Kalgash-like system. planetplanet.net/2018/02/02/… planetplanet.net/2018/03/21/asimov-kalgash-take2 $\endgroup$ – M. A. Golding Sep 4 '18 at 19:28
  • $\begingroup$ Wow, I thought that a planet with 4 suns (the paired ones counted as one because they have to be together) could easily be set up to have constant daylight. I guess I forgot that the stars can't just orbit a small planet. $\endgroup$ – John Locke Sep 5 '18 at 1:11

Set it in a long-period binary system where the second star was captured after the planet's solar system formed, such that the second star's orbit is highly inclined with respect to the planet. This can result in periods on the order of thousands of years in which the second star is continually approximately over the poles of the planet, and would just make circles in the sky. You will probably want a red giant to serve as the secondary star, and in any case it would provide enough light to see comfortably by all the time, but not contribute a whole lot of heat at the necessary distances.


This is because I want it to be able to generate a magnetic field.

Just go ahead and make the planet tidally locked to its star, because tidal locking does not rule out the generation of a planetary magnetic field. Make the star small and give the planet a close orbit, and it can still rotate with a period on the order of 1 Earth day.

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The simplest way to have a region of a planet where it is always daytime, as you point out, is to have the planet be tidally locked to its star. However, you've explicitly excluded this in your question.

I can see two general categories of answers: those with one light source that is active all the time (which probably isn't a star, so the phrase "after the sun has set" doesn't make much sense), and those with two or more light sources (one of which may be a star) that take turns illuminating the region.

Category 1

My first thought on reading this question was that there may be some kind of onmidirectional, ambient light source completely surrounding your planet. Maybe the planet is buried somewhere in the depths of a very bright nebula, lit up by newborn stars. Or a dying star, for that matter. Maybe the planet is in the middle of a very dense globular cluster, and the light comes from the millions of stars in the cluster- although you might have to explain why none of those stars are close enough to be considered "the sun". Or perhaps the light comes not from a star, but rather a black hole. A quasar's beam could illuminate a dust cloud floating in intergalactic space, which could in turn contain your planet, just as in the nebula example above.

Another possibility is that the light comes not from a star or galaxy, but rather from the universe itself. Very early in the universe's history, everything was very hot, and radiation- light- was simply present everywhere. This primordial light could be what illuminates your planet, perhaps even before the universe cooled to the point where it became transparent and what existed of that omnipresent primordial light became the Cosmic Microwave Background. It wasn't in the microwave spectrum back then, of course (it was in the visible spectrum- the orange part, to be specific), but in the billions of years since, the expansion of the universe stretched out the waves of light, causing it to redshift and cool.

Alternatively, your universe could be heading toward a Big Crunch. When the entire universe contracts, the CMB will heat up again, recreating the conditions in the early universe. Only with more entropy. And more black holes. Probably.

One thing to keep in mind is that in order to function, life (and any interesting thermodynamic process) requires not just energy, but rather an energy gradient. Life on Earth takes in visible light from the Sun, and re-emits infrared back out into space. However, bathing a planet in blackbody radiation from all directions will just heat it up to the same temperature as the radiation, and all thermodynamic processes on it will cease.

This is a problem for the cases where the CMB is the source of light, since that is a blackbody spectrum. However, astrophysics provides a solution: Black holes. Black holes do not heat up when exposed to radiation. In fact, their temperatures depend only on their size- and larger black holes are cooler. A civilization in the early universe could take energy from the CMB and dump its waste heat into a large primordial black hole; whereas one in the Big Crunch era could do the same with either a stellar-mass or supermassive black hole.

The nebula cases don't have this problem, by the way. A planet in an emission nebula is only bathed with a few different wavelengths of light, and could emit waste heat at other wavelengths. For reflection nebulae, it's even simpler: these nebulae mostly scatter blue light, while they're more transparent to red light. And in the globular cluster case, there will be plenty of space between the stars where waste heat could be emitted.

Another, much more mundane possibility, first mentioned in the comments (credit to SRM on this) would be simply be a planet with a very thick atmosphere that scatters light all the way around the planet's surface, causing the entire planet to be in a perpetual state of twilight. It'll be brighter when the star is closer to its zenith, and darker at nadir, but if the atmosphere is thick and dusty enough, there could be enough light to see all day long.

Or, as in Slarty's answer, just put your region at one of the poles of a planet with no axial tilt, and have perpetual dusk.

But those are boring. On to Category 2!

One way to have perpetual daylight on a planet would be to have the planet be illuminated from both sides. Perhaps the planet actually a moon of a large gas giant, and lies at the moon's L1 Lagrange point. After sunset, the gas giant rises, and provides a reasonable amount of light. Or, better yet, the planet could be situated at the L1 point of a binary star system. Perhaps one star is a red dwarf, and the other is a yellow main-sequence star, like our Sun. As the red dwarf is smaller and lighter than the Sun-like star, the L1 point will be closer to it- so the two stars may actually appear to have the same luminosity. In other words, midday when the yellow star is up would be just as bright as midday when the red star is up.

However, the problem with these two scenarios is that the L1 point is a point of unstable equilibrium. In order to actually stay there, your planet will have to actively fire rocket engines to keep from falling into a chaotic orbit. A manmade satellite can do that, for a while, at least, but a naturally-occuring planet will not.

Another way might be to take something like the Earth-Moon system and throw in several more, brighter moons. Earth's moon looks white, but its regolith is actually kind of a dark gray. Changing the material to something more reflective could make moonlit nights considerably brighter. And adding more moons will allow more nights to be moonlit. Now, if the moons are close in to the planet, they will gravitationally interact with each other and potentially destabilize each others' orbits, and there's still the possibility that they might occasionally all be below the horizon at the same time. However, we can solve both of these problems with one stone: Orbital Resonance.

There are several moons in our Solar System whose orbital periods are some simple fraction of that of another moon orbiting the same planet. For instance, three of Jupiter's moons are in a 1:2:4 orbital resonance, meaning for every time that Ganymede completes an orbit around Jupiter, Europa completes exactly two orbits, and Io completes exactly four. This particular configuration doesn't guarantee that there's always a moon above the horizon, but with enough moons, it should be possible to arrange such a configuration. I'm just not sure how.

Alternatively, there could be only one moon in a geostationary orbit. Or, equivalently, the planet could be tidally locked with the moon. This ensures that the moon will always be visible from (and able to provide moonlight to) half of the planet. Your region could be situated directly under the moon.

Another possibility is that the secondary light source is entirely artificial. During the day, the sun shines; during the night, various lampposts and whatnot turn on and provide illumination. Maybe they're solar powered and charge up during the day; maybe the light is actually sunlight, brought over from the other side of the planet by way of fiber optics.

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