How rare is it to have 2 stars or 2 celestial bodies at least as big as our moon and sun in the sky of a habitable planet? What are the chances of both?

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    $\begingroup$ Isn't every star a sun? Do moons of other planets count as moon in our sky? $\endgroup$ Feb 2 at 14:39
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    $\begingroup$ planetmaker: OP probably wants to know about binary star systems, and number of moons around a habitual planet itself. Otherwise the number 2 would be ridiculously low. $\endgroup$
    – DarkDust
    Feb 2 at 14:52
  • $\begingroup$ @DarkDust is it obviously low? Of the 4 terrestrials planets in our solar system, one of them has two moons, and one has one moon, two have no moons. That’s the entirety of our sample size, we can’t really make a big judgement based on that. $\endgroup$
    – Topcode
    Feb 2 at 16:04
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    $\begingroup$ Like, Mars's Deimos looks like a double-size Venus from the surface, not like a big full moon, and it's booking -- just under one orbit per day. Meanwhile a distant binary could have an orbital distance between the stars that exceeds a lightyear, so you'd have one sun and one very bright star that is sometimes visible during the day, but you won't get a binary sunset out of it. Meanwhile a close binary makes the orbital physics a little hinky (technical term) and you may not be able to get a stable orbit within the habitable zone. $\endgroup$ Feb 2 at 18:14
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    $\begingroup$ To our current understanding, most stars are in a binary system. However, we cannot reliably estimate the rarity of twin moons, since we only have our Solar System to go off of. We can barley image exoplanets, let alone their satellites. Regardless, based on our current information, 1 out of 8 planets (Mars) have this trait, if you want to count dwarf moons. If you're talking about full-sized moons like our own, no planet in the solar system has exactly two, they either have more or fewer. According to National Geographic, 16 out of 1780 exoplanets are in a habitable zone. So pretty rare. $\endgroup$
    – 4NT4R3S
    Feb 3 at 0:12


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