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I have a planet where I would like the distinction between night and day to be minimal. It is usually bright and it is usually hot.

The best I can come up with is a tidally locked planet, where people inhabit parts of the terminator zone closest to the star-facing side.

But in my story it would be illogical for such a planet to be the target of a terraforming mission.

I've also thought of just having a really long period of rotation - but that would bring just as much cold/darkness during the night.

Any suggestions?


P.S. - I'm not sure if it's relevant but there is pre-existing planet-wide life (not particularly intelligent - maybe on the order of simple fish).

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  • $\begingroup$ Why would it be illogical to be a terraforming target? terraforming.fandom.com/wiki/Tidally-locked_planet $\endgroup$
    – HSharp
    Jan 31 at 12:01
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    $\begingroup$ What do you mean with "day and night very similar "? Duration? Environmental condition? $\endgroup$
    – L.Dutch
    Jan 31 at 12:08
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    $\begingroup$ I think the most promising terraforming candidates for any body is how easy/quick it would be, not necessarily about the size. A tidally locked planet could have a lot going for it if it has magentosphere to retain an atmosphere, even if it only has a narrow ring of habitabliity it could still potentially sustain a large population. Australia is roughly moon sized and has a narrow band of habitability but seems like a popular destination. $\endgroup$
    – HSharp
    Jan 31 at 13:58
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    $\begingroup$ You could do a binary star where the planet is exactly at the center between the stars. That wouldnt be a stable configuration though. Maybe it can somehow be made stable with additional resonances from other planets? $\endgroup$
    – LazyLizard
    Jan 31 at 14:00
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    $\begingroup$ if planets with a pre-existing non-poisonous atmosphere in the goldilocks zone are rare (not that far-fetched), then the fact that there IS a band where the temperature is right for liquid water might make a tidally locked planet with a workable existing atmosphere a really nice target for terraforming, actually. No atmosphere? gotta import lots of gasses. Poisonous atmosphere? yuck. Too cold? need lots of greenhouse gasses, and even that might not be enough. Too hot? live underground I guess. 100 km by 40000 km of livable conditions, just add oxygen? hell yeah! $\endgroup$
    – Syndic
    Feb 1 at 8:07

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Your atmosphere needs to meet two requirements.

One is that it has an extremely high specific heat, so that it will warm up slowly and hold onto heat for long times. It would help if there were other things, such as large bodies of water, that would also have high specific heat.

The other is that it would need to diffuse the light, like daylight on a cloudy day, only much more. Light would have be scattered from the day side and into the night.

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    $\begingroup$ I don't think this is possible. The atmosphere would have to be insanely large and have a very low density for light to diffuse that much. You would need a very cold and low mass planet to be able to retain an atmosphere like that, at which point you'd just have a planet without any appreciable day time. $\endgroup$
    – Nosajimiki
    Jan 31 at 22:18
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    $\begingroup$ @Mithoron Photos of Venus clearly show a dark night side and a bright day side. The night side does radiate more infrared than one would expect from a planet with a lower atmospheric density, yes, but it does not glow in the visible spectrum. On the surface, Venus only averages as bright as a well lit room, but its nights are every bit as dark as a moonless night on Earth. $\endgroup$
    – Nosajimiki
    Feb 1 at 16:06
  • $\begingroup$ How bright could a high albedo moon potentially make the nighttime on this world? $\endgroup$
    – Kyyshak
    Feb 2 at 12:43
  • $\begingroup$ @Nosajimiki if the inhabitants of this planet see in a more infrared-ish range, that would work pretty well. Only problem is that the Venus atmosphere is inhabitable. $\endgroup$
    – Beefster
    Feb 2 at 18:44
  • $\begingroup$ @Beefster The OP says that this is the target of a terraforming mission (presumably human) meaning you can't rely of the biology of the native life to justify the perception of day/night. In fact, you can't rely on there being a very different atmosphere at all because even if the planet starts that way, the atmosphere has to be reduced to be compatible with human biology. $\endgroup$
    – Nosajimiki
    Feb 5 at 16:16
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Your Planet is near the Galactic Center

Near the center of the galaxy, the density of stars is about 1,000,000 times that of our own local cluster. This means that there would be billions of stars that would qualify as "visible" if seen against a night time backdrop, and that individual stars would average many times brighter than the stars we can see from Earth. That would make the sum of all starlight somewhere in the range of daylight here on Earth give or take an order of magnitude.

Also of note: Indoor lighting averages about 30 times darker than daylight. So, while you could put down on a planet where it's as bright as day at night, this would surely cook you, but if you settled a planet a bit further out from the galactic center, there should be plenty of Earth like planets where night time is 10-50 time darker than day time so that the planet can still sufficiently cool down at night making human life possible, while still being bright enough outside from all of the star light to be able to see things as clearly as if you were standing in a well lit room.

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    $\begingroup$ I think this is the most viable answer if we're looking for realism. Add to this answer that the proposed planet doesn't orbit anywhere near close to its star to reduce the star's impact on the planet's day. Or could be a rogue planet orbiting a cluster of stars. $\endgroup$ Feb 1 at 12:09
  • $\begingroup$ @AhmedTawfik That is why I pointed out the indoor lighting brightness thing, because the sun makes WAY more light than we need to comfortably see, you don't actually need to be that much farther from the sun to maintain a livable environment and still put enough stars in the sky to get "bright" nights. Put an Earth like planet at 1.1AU from the sun, and the right distance from the galactic center and night time will just feel like a cloudy day. $\endgroup$
    – Nosajimiki
    Feb 1 at 16:11
  • $\begingroup$ +1, but the problem here is that the absurd star cluster density means any planet would probably be flinged out of the galactic core in geological time. $\endgroup$ Feb 1 at 19:19
  • $\begingroup$ do you have link for that luminosity from the surrounding stars? And on that, if it is 50x darker, and we assume a similar spectrum, wouldn't that also mean 50x less warmth incoming? Not that that would be a dealbreaker, we might have a younger planet, still retaining more of it's initial heat, just FMI $\endgroup$
    – bukwyrm
    Feb 1 at 20:52
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    $\begingroup$ @bukwyrm "Tod Lauer of the National Optical Astronomy Observatory calculated that observers at the center of M32 would find a 'night sky' as bright as twilight on Earth." (ref) $\endgroup$
    – Ray
    Feb 2 at 15:29
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Your planet has an enormous and incredibly reflective moon.

Earths moon, while large, is not the largest it could possibly appear in the sky according to real-world physics, and it only has an albedo (reflectivity) of 0.12 (12%).

Your planet could have a moon that is as large and close-orbiting as physically possible, and has a surface of a highly reflective material.

This would explain having very bright nights (other than during the new moon), although I couldn't say what effect it would have on nighttime temperatures.

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  • $\begingroup$ That's a good idea. I don't think that it would effect temperature though. Since reflected light wouldn't have much heat. $\endgroup$ Jan 31 at 13:37
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    $\begingroup$ @trubliphone remembering how much I hate them, the answer might be in overcast summer days and nights. When the sunlight during the day hits the clouds instead of the ground so it doesn't heat up as much as on a clear day, but in return those same clouds prevent the heat from escaping during the night. So if that planet often has high humidity and lots of clouds, it would seem plausible - even more so if large bodies of water further serve as temperature buffers. $\endgroup$
    – Syndic
    Feb 1 at 7:57
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    $\begingroup$ @trubliphone "Since reflected light wouldn't have much heat". That only depends on how much is reflected. If it's literally a mirror that reflects everything it would provide similar heat as if it was direct light $\endgroup$
    – Ivo
    Feb 1 at 8:42
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    $\begingroup$ I think a system of warped rings made of diamond asteroids would do to? Also if this "shell" of diamond debris also shades the planet from some light, it can be much closer to the sun. $\endgroup$
    – Pica
    Feb 1 at 15:09
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    $\begingroup$ Instead of just the one moon, perhaps a ring like Saturn's could work too. Or an asteroid belt, but it would have to be much much much denser than the extremely sparse asteroid belts in our real solar system. $\endgroup$
    – Stef
    Feb 1 at 17:27
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Binary/Trinary star system

Unless you have your mind set on a single star, that could probably be an answer. "Day" is when the closer of the two stars is overhead (or both / all three of them are), "Night" is when you have only (one of) the further away one(s).

Such systems are quite fascinating, and discussions about stable orbits in the goldilocks zone are easy to find on the internet.

If that solves only the "light" and not the "temperature" part of your question, add a lot of water to the planet. If you ever experienced an overcast summer day and night, you know that those are less hot during the day (clouds partially blocking the sunlight from hitting the ground and heating it up) but also don't cool down during the night as much (those same clouds blocking the heat from escaping). Large bodies of water also serve as temperature buffers (takes a while to heat them up / cool them down, so they stabilize temperature around them).

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  • $\begingroup$ This sort of thing is strongly reminiscent of the story/novel Nightfall, in which a planet has six suns (arranged as multiple groups of binary/trinary systems) and while they have a nominal day/night cycle based on the largest and brightest sun, they only experience "true" night once per 2000 years or so (aided by an eclipse) -- and even that is brighter than our nights due to being located in a crowded star cluster. $\endgroup$
    – Miral
    Feb 2 at 3:38
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The temperature requirement is super easy. The brightness one is mind-boggling to do with any level of realism, if you want it to be natural.

The light source for the night side would have to be as bright as the star that the planet orbits. So it would likely have to be another star that is far enough that the planet is not directly orbiting it, big and hot enough to compensate for that, and need an incredibly complicated orbital setting that surely wouldn't last long.

So to be realistic, the light source could be artificial - a network of thousands or even millions of reflector satellites, leftovers from a previous civilization that either lived in planetary conditions too different from what is comfortable to humans or who really messed up the planet into not being inhabitable.

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Ringworld

It's not a planet but a construct, a ring world or a dyson sphere could potentially meet your conditions.

Ringworld from the 1970 novel has a structure that goes around a star, it's immensely huge but by having a star directly in the middle it is always in light.

There could be atmospheric differences but because it's equi-distant to it's Sun at all areas the climate could be similar.

It will always be bright, it will always be hot.

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    $\begingroup$ Note that in Ringworld (great series by the way) the day/night cycle was formed by shadow-squares orbiting the sun closer in, collecting power for the Ring structure and the defense system. $\endgroup$
    – Corey
    Feb 2 at 1:09
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What is your definition of "very similar"?

If it's light-wise sufficient that you can read / work outside during the night, then you're in luck: With a full moon, you can already do that on bright earth nights, so modifying the moon(s) size, reflectivity and/or orbit a bit should completely suffice.

Heat wise large bodies of water combined with good cloud coverage (which could conflict with the light bit but these clouds only need to reflect invisible infared heat/light) could yield amenable night temperatures.

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A moon tidally locked to a gas giant.

If your world is orbitting and tidally locked to a gas giant, it can take care of the brightness requirement on the side facing the gas giant. Due to it orbitting around the gas giant, you still have a day-night cycle with respect to the sun, but at "night" you will have light reflected off the gas giant. Unless the orbit of the world is at a significant angle to the solar plane, you may get a solar eclipse every orbit, but that's a fairly brief interuption of the brightness.

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this is mostly an "out there" idea because normal ones have already been stated

Topology anomaly

The planet literally has only the dayside - whenever you leave the horizon boundary, you appear on the other side

Sky would double-expose, coinciding opposite points. Sunset is is sunrise at the very same time

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