Eyeball World

Colloquial term for a habitable planet around a dim star (M- or K-class) orbiting so close it becomes tidally locked to its primary with one hemisphere in perpetual sunlight and the other in eternal darkness. "Eyeball" describes its appearance from space: an ocean or desert bathed in harsh light with a temperate twilight zone and poles. Due to the sheer prevalence of red dwarf suns this is actually the most common type of habitable planet, though species such as humans rarely settle them.

Common Characteristics

Eyeball worlds are categorized along two dimensions: temperature and water content. Hot worlds have barren baked sunward sides with liquid water restricted to twilight regions and the darkside, while cold ones can have seas on the dayside, but the night freezes to ice.
Depending on the eccentricity of its orbit the planet can have seasons and even libration zones which experience long days as the sun rises and then sets down the same horizon. Otherwise, the climate in any given location is largely static and unchanging.

Ecology

On planets with an extreme hot, life and settlement are confined to the annular twilight region. While only hardy lifeforms can survive in the perpetual heat of the sub-stellar regions and nothing can live in the darkness of the far side, the twilight area is just warm enough to support liquid water and a comfortable climate. On colder eyeball worlds, this habitable area extends further out into the sunlit regions.

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