In this alternate scenario, Earth is the third planet in a binary star system. One star is a K-type main sequence star--or "orange dwarf"--that has 80% the mass of a G-type main sequence star--or "yellow dwarf". Orbiting this star from a distance of 45 million miles is another orange dwarf, one that has 45% of our sun's mass.

Now, onto Earth. For simplicity's sake, let us focus on only one specific latitude--one degree. This is well within the tropical zone, where sunlight is 100% direct and daylight is 12 hours per day every day.

Using these specifics listed above, if we're standing at latitude one degree, what would a binary K-type skyscape look like at 6 AM, 12 PM and 6 PM?

  • $\begingroup$ One aspect you may need to rethink is the fact that both orange dwarf stars will be orbiting a common barycentre. I thought it might inside inside Big Orange, but considering their distance of separation it could well be outside it. If so, both stars will orbit a common centre of mass. $\endgroup$
    – a4android
    Apr 28, 2018 at 4:27
  • $\begingroup$ This is a totally trivial point. There is always with which AM or PM should either midnight or noon be. Considering the PM of noon's 12 PM means post Meridian or after 12 noon, then 12 noon cannot be PM, i.e., it cannot be after itself. It might be safer to make the mid-day time either noon or 12 noon and avoid the tedious nitpicking. Something I've done on behalf of everybody else. The armed forces avoid record receiving or transmitting messages at 12 AM or 12 PM because of this confusion. They are recorded at times of 12:01 AM or 12:01 PM when there can be absolutely no confusion. $\endgroup$
    – a4android
    Apr 28, 2018 at 4:34
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    $\begingroup$ In order to have a stable orbit, the planet would need to be pretty far from the barycenter. Probably too far to support a complex form of life, if that is a concern. $\endgroup$
    – Vincent
    Apr 28, 2018 at 5:22
  • $\begingroup$ To over simplify, you've slapped a small sun in the orbit of Venus. The whole orbital mechanics needs to be worked out, and the implications of varying distance from the two suns taken into account. What it would look like depends on the overall state of the system at that moment. Check out the orbital mechanics of the Pluto system for something comparably complex. $\endgroup$
    – pojo-guy
    Apr 28, 2018 at 13:49
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    $\begingroup$ @pojo-guy; sadly, this is a fairly common problem. There is an excellent conversation at almost all levels of SE about negativity, acceptance and a spectrum of other issues, mostly of human origin. Essentially, argument is pointless. $\endgroup$
    – Joe
    Apr 30, 2018 at 16:31

1 Answer 1


Addressing the skyscape question: (In generic terms for a body orbiting a binary pair)

Start by only visualizing the two stars (no planets for now). With the numbers given, the stars will have an orbital period of 0.3 years. That means the K star and G star orbit each other about every 4 months. Let’s imagine from our perspective that K star is directly in front of G star and it’s Day 0. Over the course of 30 days they will move apart, with G moving left and K moving right. At day 30 they are at maximum visual separation with G on our left. Days 30 - 60 they move closer to each other. By Day 60 G star is now directly in front of K star. Days 60 - 90 they move apart with K moving left and G moving right. At day 90 they are at maximum visual separation, but this time with K to the left. Days 90 - 120 they move closer and the cycle continues. Day 0 and day 120 look the same.

As viewed from a planet orbiting the binary pair... (in general terms) This pair of lights will move across the sky as a unit, sometimes closer to each other, sometimes farther apart and sometimes touching. The exact times of dusk and dawn will vary based on the cycle of separation.

The closer the planet is to the binary barycenter, the more pronounced the visual separations will be.

Hmm... I wonder if quadruple rainbows would be a thing.


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