enter image description hereI've been thinking of using a binary model (a planet that rotates around two suns) for a possible story.

The specific model is a S-type planetary system, where the planet orbits the the primary star A (0.8 M☉), while the secondary B (0.6 M☉) orbits the primary at a distance of 385 AU. the planet orbits around primary A at a distance of 0.85 AU.

The question is: what would the day and night look like in a circumbinary system of this type, and how could its configuration significantly affect the climate and seasons of this planet? enter image description here

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    $\begingroup$ At 385 AU the other binary star will look like a bright star in the night sky. It would be 40 times further than Pluto is from the Sun. $\endgroup$
    – Samuel
    Commented Mar 23, 2018 at 2:35
  • $\begingroup$ This isn't circumbinary, this is [the other kind]. Source $\endgroup$ Commented Mar 23, 2018 at 3:08
  • $\begingroup$ @RonJohn It's an S-type. That means the planet orbits around one star that is itself in orbit around a barycenter with another star. $\endgroup$
    – Samuel
    Commented Mar 23, 2018 at 3:10
  • $\begingroup$ Can you draw us a diagram? $\endgroup$ Commented Mar 23, 2018 at 3:15
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    $\begingroup$ @RonJohn, I looked into this a while ago. I can't be sure without doing the math, but a ballpark estimate is that the orbit is stable -- it's close enough to the primary and the secondary is far enough away. $\endgroup$
    – Mark
    Commented Mar 23, 2018 at 5:33

1 Answer 1


If the secondary B (0.6 M☉) is 60% mass of Sun, we can assume it is a large red dwarf with luminosity about 7.2% of Sun's luminosity. Its absolute magnitude will be about 7.68. The distance is 385 AU, which gives us apparent magnitude of about -11. for comparison, full Moon's magnitude is -12.90, top brightness Venus' magnitude is −4.89 and Sirius (the brightest star) is −1.47.

The nighttime effect on the planet would be like from another Moon, which appears seasonally and does not have phases.

Gravitational pull of the star B at this distance will be extremely weak and detectable only with scientific instruments.

  • $\begingroup$ Thank you! Your answer has been very useful! By the way, the primary star due to the proximity would look bigger or a size similar to the sun? $\endgroup$
    – JAMS
    Commented Mar 24, 2018 at 21:29

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