Is it possible that a planet could host human life in a habitable zone with a perpetual twilight without tidal locking?

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    $\begingroup$ Can I ask what technoogy you are imagining they have? And about how many of them you want to have there ? $\endgroup$
    – P Chapman
    Dec 3, 2017 at 2:58
  • $\begingroup$ Are you open to suggestions involving "planets" that are not actually planet shaped? $\endgroup$
    – Ash
    Dec 3, 2017 at 12:57

1 Answer 1



In order to have perpetual twilight, the Sun must constantly appear to be on or near the horizon from at least some point on the planet's surface. One way to to this is for the planet to not not rotate relative to its star- that is, for it to be tidally locked. Another is for the planet to have zero axial tilt.

If a planet has zero axial tilt, it'll have normal days and nights over most of its surface, but perpetual twilight at the poles.

I propose that your planet has zero axial tilt and is at or just inside the inner edge of its sun's habitable zone. Most of the surface is far too hot for humans to colonize comfortably; but at the poles, it's just cool enough. A "day", for your colonists, will be the amount of time it takes for the Sun to complete one circuit all the way around the horizon. Your colonists may construct a clock based on the length of this "day" (perhaps by defining "midnight" to be "when the Sun goes behind that mountain in the distance and casts a shadow over the colony for a few minutes"), or they may eschew locally-derived timekeeping entirely and use 24-hour clocks from Earth.

If I'm not mistaken, the Sun would actually appear to be above the horizon 24/7 at the poles of such a planet, due to how the atmosphere refracts light, so this would technically be more of a perpetual sunset than perpetual twilight. That said, a perpetual sunset may well be close enough to twilight for your purposes. After all, on a tidally locked planets, the inhabited portions are typically on the sunlit side. It's hard to grow crops or power solar panels on the dark side.

  • $\begingroup$ Thanks for your time and disposition. Could the colonists live in an area with tundra or should they inhabit specifically at the poles? $\endgroup$
    – Jano Moore
    Dec 3, 2017 at 13:56
  • $\begingroup$ I schemed about this all night and now here it is. My addition: if the planet as you describe were cylindrical rotating about its long axis the entire flat poles would be in perpetual sunset as you describe. A cylindrical planet would need to be a construct, but if your planet spins fast enough it will bulge at the equator and flatten at the poles - this gives a larger area with the light characteristics you want. Finally you could have a mountain range circumferentially around the polar areas - these would cast long shadows and produce real twilight. $\endgroup$
    – Willk
    Dec 3, 2017 at 15:24
  • $\begingroup$ Poles don't have to be tundra. There have been periods in Earth's history with warm poles. $\endgroup$
    – rek
    Dec 3, 2017 at 17:03
  • $\begingroup$ A cool thing about a very fast rotating, flattened ovoid planet. Gravity would be low at the equator because of centrifugal force and distance from the center of mass. Very large creatures could exist there. $\endgroup$
    – Willk
    Dec 3, 2017 at 17:50

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