I'm just trying to write a RPG adventure for a sci fi company. So when I have to figure out what kind of orbits, masses, major axis, etc are needed for the system I need to design and I see:

$T=2\pi$ times the square root of etc., etc.

Yeah, that's not going to work. I need a tool so that I can design a binary system with a planet orbiting a dim star that in turn orbits a bright star in its habitable zone and this planet is eclipsed by its dim star 6 earth months out of every 24 earth months, approximately. And still be in a habitable zone.

I can see that if the dim star orbits the bright star every two earth years while the planet orbits the dim star every earth year, my eclipse needs will be met. Alternately, if the planets orbital period is two earth years and the dim star orbits the bright star every earth year, my eclipse needs will be met. But having the equations and knowing how to do anything with them, well, as I said, I need a tool.

Anyone have one where I can input masses and distances trial and error until I get the results I need?

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    $\begingroup$ I am extremely dubious that such an arrangement could be made. To keep the planet "safe" from the brighter star's gravitational and thermal influence you would need to move the dim star so far from it that there's no way such a long eclipse could happen (IMO). I do not think the orbital periods could be made to work while keeping the planet in a safe stable orbit. $\endgroup$ Aug 2, 2019 at 3:15
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    $\begingroup$ What exactly is a sci fi company? $\endgroup$
    – L.Dutch
    Aug 2, 2019 at 4:25

2 Answers 2


Better than that; there's a game that will do it.

Steam has a game called Universe Sandbox where you can line up different planets, stars, et al with different masses and trajectories. I've never tried it but as I understand it the game is capable of simulating the orbits of planets, stars and the like in a way that you can graphically see the results.

In point of fact I believe there are a couple of celestial simulation games on Steam but this is just one that appeared in my discovery queue one day and I remembered it. Have a look for yourself but I suspect that this would solve a lot of your issues.

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    $\begingroup$ I have. It won't help the OP because the arrangement described in the question does not seem feasible. $\endgroup$ Aug 2, 2019 at 10:52
  • $\begingroup$ @Renan finding out how/why it's not feasible through experimentation and subsequently having a tool to find a setup that does work for his purposes may still help OP. Wouldn't shut this one down too quickly. $\endgroup$
    – Small Moon
    Aug 2, 2019 at 16:30
  • $\begingroup$ @SmallMoon OP is asking for a tool which does not require knowing rocket science in order to find out. Turns out he will need to know it anyway with any tool that might be useful. In my own answer I try to elaborate on it in a non-arhitmetical way. $\endgroup$ Aug 2, 2019 at 16:43
  • $\begingroup$ beat me to it! The game isn't very fun imo but it will certainly work for this. $\endgroup$ Aug 3, 2019 at 1:43

The arrangement you wish does not seem feasible.

If by "dim" star you mean a star that is dimmer than our sun, you are probably thinking of a red dwarf. Such stars are thought to be far from habitable, because:

  • The habitable zone around them requires a tidal lock;
  • They are usually flare stars, which means the star facing side of planets in the habitable is toast;
  • They vary their luminosity a lot. Even then, it is mostly infrared. Those stars have an average surface temperature of 1700K (compare with ~5600K of the Sun).
  • The proximity to the star means that the star's own magnetic field will probably overcome the planet's, meaning little to no protection from solar wind, meaning no atmosphere.

If that is not discouraging enough: if a binary system has evolved in a way that has caused one star to be much brighter larger than the other, then it is most likely a semidetached system. That's because in general bright stars are young, dim ones are old (this is a very base supergeneralization), and if two stars close to each other have different ages, they most likely have formed in different zones and are probably not orbiting each other. For them to orbit each other the younger, most massive one should capture the older one, and such captures are more likely to happen at the galactic core (because you can't capture alone, you need a third body and that's where you find a lot of stars in close space) - where excess radiation is hazardous to life as we know it. So, if two stars are orbiting each other and one is much brighter than the other, they probably formed together and evolved to be like this:

Semidetached binary star

The dimmer one in this case is the one to the right. The one to the left is colder, but its sheer size gives it more shining area, and therefore more luminosity.

This does not bode well for your planet. If it orbits in a plane that allows for an eclipse, it will cross the mass transfer stream when it forms. That will either decellerate the planet into crashing onto the smaller star, or fling it away from the binary. But not without causing it to be torn apart first. Not from the collision, but due to gravity!

In your previous question about six-month periods of darkness, I gave you an answer which I think is far more feasible and which does not depend on a binary system. I think you should consider that one.

  • $\begingroup$ 1: Planet doesn't have to be in the red dwarfs habitable zone, it needs to be in the larger, brighter stars habitable zone. 2. They stop being flare stars after they cool way down, which means get lots dimmer, which is what we are talking about. So older red dwarf. 3. Since this depends upon the brighter stars habitable zone, the planet need not be in close proximity. Also, the red dwarf would be in a P orbit around the brighter star, not orbiting each other. Now, if there are problems with that situation, then I'm all ears. $\endgroup$
    – Pentallion
    Aug 3, 2019 at 7:44
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    $\begingroup$ @pentallion for a star to be large enough to be orbited by another star within its habitable zone, and then a planet, you will get neither a six-months eclipse, nor even an eclipse at all. It will look more like a transit of Venus, which lasts for a few hours and doesn't dim the parent star in a way you can perceive with the naked eye. $\endgroup$ Aug 3, 2019 at 10:21

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