# What would day and night look like in a circumbinary system?

I'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?

• 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. – Samuel Mar 23 '18 at 2:35
• This isn't circumbinary, this is [the other kind]. Source – SIGSTACKFAULT Mar 23 '18 at 3:08
• @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. – Samuel Mar 23 '18 at 3:10
• Can you draw us a diagram? – SIGSTACKFAULT Mar 23 '18 at 3:15
• @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. – Mark Mar 23 '18 at 5:33

## 1 Answer

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.

• 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? – JAMS Mar 24 '18 at 21:29