It is not an unknown scenario in science fiction; a sun that stays stable in the sky of a planet. Humans (or other sapient species) without space suits battling hot deserts, cold dark night sides, or the lush land at the never moving border of night and day.

Tidal locking (or captured rotation) occurs when the gravitational gradient makes one side of an astronomical body always face another, an effect known as synchronous rotation. For example, the same side of the Moon always faces the Earth.

Is life for humans and our supporting species possible on a planet where the sun does not move across the sky?

Looking for answers with credible science, best case scenario, with an oxygen generating biosphere evolving naturally. Is it possible or does some science rule out the possibility?

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    $\begingroup$ Not what you asked for, but it might interest you: Tidal locking doesn't necessarily mean that the Sun is fixed in the planet's sky, if the planet's orbit is eccentric enough, as is the case for a large fraction of known exoplanets, and for Mercury! Here's a SETI Talk by Tony Dobrovolskis about the insolation patterns of such planets: seti.org/weeky-lecture/patterns-sunlight-extra-solar-planets $\endgroup$
    – LocalFluff
    Jul 22, 2014 at 10:33
  • $\begingroup$ Try asking this on the Worldbuilding stack (if it’s not already there!) $\endgroup$
    – JDługosz
    Oct 9, 2016 at 11:59
  • $\begingroup$ @LocalFluff mind that Mercury isn't tidally locked :) $\endgroup$
    – jwenting
    Mar 12, 2020 at 5:02

1 Answer 1


LocalFluff's comment is the key to how such a planet could be habitable.

A one-face planet will be uninhabitable as the atmosphere will freeze out on the dark side unless there's something major that prevents this (and that something major probably is pretty dangerous.)

However, a tidally locked but eccentric planet isn't inherently uninhabitable. You can get enough wobble to keep it from freezing out. The thermal swings will be major but in a sheltered location and with a sufficient atmosphere it might be possible, especially if they retreat to caves at the point of close approach.

  • $\begingroup$ I find it difficult to image a planet with it's atmosphere frozen on one side an not the other. It seems more likely that the thermal difference (assuming atmosphere) would support trade winds that would transfer thermal energy around the planet. $\endgroup$ Jul 28, 2014 at 10:37
  • $\begingroup$ @JamesJenkins Sure the winds bring energy but not enough to stop the freezing. It used to be thought that the dark side of Mercury was one of the coldest spots in the solar system. $\endgroup$ Jul 28, 2014 at 22:05
  • $\begingroup$ At these pressures air would liquefy. For a planet to be stably tidally locked (and not just rotating slowly) it needs to have density irregularity with more dense part on the "bright" side, meaning it's "downhill" from the "dark" side. Rivers of frozen air flowing back and evaporating near the terminator? - BTW, Mercury is in a 2:3 tidal lock, 1 sidereal day lasting 1.5 orbital period. It was long believed to be in 1:1 tidal lock because the period when it's observable (neither eclipsed by Sun nor eclipsing it) was giving similar observations thanks to that tidal lock. $\endgroup$
    – SF.
    Dec 16, 2015 at 7:50
  • $\begingroup$ On the other hand, a Cytherian (Venus) day is 117 Earth days, pretty long. Its wind speeds of I think up to 700 km/h might help it. Maybe it wouldn't work with a habitable atmosphere about 1% the thickness of Venus at the surface. @JamesJenkins $\endgroup$
    – LocalFluff
    Oct 9, 2016 at 15:34

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