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So, imagine an atmosphere-less planet, tidally locked to a sun-like star. How close to the star can the planet be before its dark side becomes too hot?

I imagine that at some point the rocks on its sunlit side will melt and evaporate so that the dark side would experience rocky precipitation, is it true?

Also, at some point the atmosphere of the star itself would engulf the planet.

But at which point these effects will make the conditions on the dark side unbearable?

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Impossible to generalize

You should check out this excellent answer from Astronomy SE. There is no hard and fast rule for how hot a planet will get x AU from the nearest star. Wikipedia has some helpful math that can get you into the right ballpark if you specify things like the planet's albedo, size, and other variables.

Think about our own Solar System as an example. Mercury is closer to the Sun but it has a lower surface temperature than Venus. And unlike Mercury, the "dark side" of Venus isn't much cooler than the side that faces the Sun.

(Edited for clarity.)

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  • $\begingroup$ The question is about a planet with no atmosphere. $\endgroup$
    – Anixx
    May 17, 2021 at 20:23
  • $\begingroup$ @Anixx I edited my answer to clarify that it's also impossible to generalize even without an atmosphere $\endgroup$
    – Andrew Brēza
    May 17, 2021 at 20:27

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