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Basically, a sun-synchronous orbit is where one side of an object is always facing a star as it revolves around it (much like our Moon is, which is where the phrase 'dark side of the moon' originates'). For a planet, this means one side of its hemisphere is locked to face the Sun while the opposite hemisphere will face away from it. This means the side facing the Sun will be in constant sunlight while the opposite side is in complete darkness, with a habitable zone will be possible in-between these opposing hemispheres.

If the habitable zone of a sun-synchronous planet existed, where this area is always in twilight, what life forms would be possible to evolve?

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What you seem to be describing is a tidally locked planet. These types of planets are invariably located extremely close to their parent star. They are not very good candidates for life to develop on, as it will be to hot on the day side and to cold on the night side for liquid water to exist. There is however a bit of a sweet spot called the day/night terminator in which life could theoretically exist. A life form evolving in such a location would likely either develop very good eyesight to mitigate the lack of light, or forge seeing in favor of other senses like hearing. Think of a bat like creature.

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I'd imagine there'd be organisms of various types which change as you move closer toward the sunlit area.

  • In the areas of the habitable zone closest to the sun, it's likely there'd be desert with burrowing animals, maybe some savannas and grasslands.
  • In the more twilight-y areas, grassland would slowly fade to rough rock, perhaps with some mosses.
  • In the darkest areas, hearing/smelling based creatures would live, feeding on moss or scavenging for other dead organisms. Burrowing animals would also likely live in that region, assuming there was enough plant life like moss to support them.
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Some initial thoughts: I doubt little could live permanently where there was no sunlight at all. At least not any major ecosystems. Everything has to have an energy source. An ecosystem usually starts with sunlight. Without it, there would be no plants. Without plants, no herbivores, and on up. However, there could be animals that go into the dark to hide for rest (perhaps burrowing or nesting) and provide a source for some predators. Or animals go there to die. You could possibly have a river that meanders in and out of the dark side that brings nutrients to feed a small ecology in small zones.

Once into the light of the twilight ring, I would bet there would be great ecological diversity in short distance with each type of plant life best suited to that amount of light and animals for consuming those types of plants.

Predators might be best suited to hunting with a certain amount of light. Ambush predators more likely where there is less light. Possibly even relying on IR light. Whereas, in the brighter light, predators might need to be fast creatures that can outrun their prey.

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