I'm looking to build a "twilight" world, where days are like twilight even when the sky is clear and nights are...very dark. Cloud cover might be able to turn day to an early night. I do envision a day/night cycle and capability of supporting advanced life. Ideally, I'd like the planet to have seasons, though I don't care if it overall skews colder or warmer than earth.

So far, my research for how to achieve such a world would be to have a planet orbiting a red dwarf star. Or rather, a moon orbiting a planet orbiting a red dwarf star since I'm trying to avoid tidal locking. Gas giants are suggested, but I fear they may be too bright.

I have a basic idea of what I need (axial tilt, rotations, etc), but I'm lost in the hard planetary astronomy, such as how a gas giant might affect the brightness of the sky or might help heat a dimmmer world, or even how to keep the sky largely dark. Or if I'm barking up the wrong tree altogether.

Side note: Yes I am assuming the planet can withstand the solar flares.


3 Answers 3


So, what you want is a planet where there's very little light, but it isn't terribly cold. There's an easier way to get this, and it's more interesting.

Take a normal F- or G-type star, pretty much like the Sun. Put your planet in orbit round it. Don't make it a moon of a gas giant, that gets complicated. Have an unusual feature: a large belt of fine dust orbiting closer in to the star than your planet. This intercepts most of the light from the star, which is dimly visible. The dust warms up, and re-radiates the energy as heat. Light comes from the star; heat comes from a large zone in the sky with blurred edges, which has the sun in its centre.

Now, how do we get that dust zone? It's like the interplanetary dust cloud in our solar system, but very much denser. I can't see how it could happen naturally. Does it work for your story for it to be artificial?

  • $\begingroup$ Wouldn't the dust zone radiate most of the heat away into space and on average act to cool the planet as opposed to heating it? For instance an impact winter would be analogous to your space dust cloud and well, would cause a winter. $\endgroup$ Sep 22, 2016 at 20:24
  • $\begingroup$ That's why I didn't specify an orbital radius. Yes, this planet will be cooler than if the dust cloud wasn't there. This trick gives a way for it to be dark without being freezing. $\endgroup$ Sep 22, 2016 at 20:29
  • $\begingroup$ I see what you mean, so you could use any opaque atmosphere with dust, or maybe water vapor to achieve the same effect couldn't you? The dust wouldn't have to be in space. $\endgroup$ Sep 22, 2016 at 20:34
  • $\begingroup$ You could, but dust in an atmosphere will settle out fairly quickly - decades, probably - whereas dust in space can last much longer. I've also been reading Infrared Astronomy - Seeing the Heat, by David L Clements, which caused me to think of this idea. $\endgroup$ Sep 22, 2016 at 20:41
  • $\begingroup$ It doesn't really work for it to be artificial since the civilization on the planet isn't yet all that advanced. An artificial dust cloud would open a rabbit trail of questions I don't really need in the story. I had actually considered this and I really like the idea, but maybe not for this particular world. $\endgroup$
    – Firelocke
    Sep 23, 2016 at 3:24

Make it a nuclear powered twilight planet

If the heat doesn't come from the star, it has to come from somewhere. Why not the planet itself?

Let's assume the following:

  1. This is a roughly Earth like planet. Same size, same atmosphere, same orbit.

  2. It orbits a dim star, red dwarf or some such. This star has very high metalicity, indicating an advanced age of the surrounding stellar neighborhood.

  3. Due to the high metalicity of the solar system, there is an unusually high quantity of uranium in the core and mantle of this planet.

Due to Decay Heat, the uranium will heat up. If this decay heat is high enough, it will heat the crust sufficient to replace the heat that would have come from the Sun on Earth.

Decay Heat decreases with time so either handwaving or careful calculation will be required to figure out how much uranium it will take to keep the planet warm and how long it will be able to do so.

Interesting to note that about half of Earth's heat come from Decay Heat. In this twilight world, maybe 75% or 90% of the planets heat comes from decay heat.

Heating this way should give you the clear skies and warm weather you want while keeping a dim sun.


Unless the gas giant is large enough to be luminous, as opposed to merely reflecting light, I don't see why this is a concern.

The brightness of the sky is going to be a function of four factors split into two categories: cat 1- amount of incoming light 1) Solar illumination, a function of the number of proximate stars, their brightness and distance 2) reflected illumination, a function of both the solar illumination and the number of proximate reflecting bodies, their albedo, and their distance 3) galactic illumination, a function of the solar system's position in the galaxy and the nature of the galaxy

cat 2- the amount of light impinging on the atmosphere that makes it to the surface: 4) atmospheric albedo of your planet, a function of the composition of the atmosphere

by using a fairly dim sun you minimize 1&2. If you want thing really dark at nights you can specify their galaxy such that their night sky has many fewer stars than ours, perhaps an older galaxy and the population of stars is mostly along the red giant branch. If you want both day and night to be darker then you can ramp up 4, meaning most of the light that reaches the planet is reflected back into space.

If you are thinking of a native ecology I would assume it would be based off some form of thermosynthesis (https://en.wikipedia.org/wiki/Thermosynthesis) given the relative lack of visible light while the relative abundance of Infrared.

  • $\begingroup$ So the gas giant would essentially act like our moon? Then the light it reflects could also be dim since their sun is already dim. Could the color light it reflects be different depending on the color of the planet, or will it always basically reflect as white? $\endgroup$
    – Firelocke
    Sep 23, 2016 at 4:08

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