Continuing on a previous post of mine (link: What would my Sun look like to fit the properties of my Earth?) where I wanted my planet to orbit around its Sun in 365 (Earth-)years. However this did not seemed feasible. Especially because there would be extreme (short-waved) UV-light coming from that big, far away, blue Sun that would strip away the Planet's atmosphere.

Could the problem be solved under the following conditions?

  • My planet and its Sun have approximately the same properties as our Earth and Sun with following exceptions:
  • The planet has no axial tilt.
  • The Planet's orbit is somewhere between Earth's and Mars's (this because I want the overal climate to be slightly cooler)
  • The Sun (which I'll call Small Sun from now on) orbits around a Big(ger) Sun in an orbit of around 365 (Earth-)years.
  • The plane of orbit Big Sun-Small Sun is perpendicular to the plane of orbit of Small Sun-Planet. Consequently the Big Sun will only be visible for half of the Big Year (=182.5 of Earth-years).
  • When the Big Sun is not visible for almost two centuries the Planet's climate cools down in such a way that it is noticeable. However the Planet would still be habitable for humans.

Is it possible to have a solar system with aforementioned conditions in which the seasons on my planet are a result of the presence of the Big Sun and in which life for humans would be impossible if that extra heat from the Big Sun wasn't there? What would be approximately a good composition for the solar system (Big Sun - Small Sun - Planet) ? And is the UV-problem solved by this new composition?

  • $\begingroup$ Is this more like a binary star system (two suns in the center) or is the planet-small sun-big sun constellation more like a moon-planet-sun constellation? In other words, is the smaller sun like a planet and the planet like a moon? Either way I don't see why the big sun is not visible half of the big year. Eclipses of any kind are very rare in space. $\endgroup$
    – Matthias
    Commented Dec 9, 2022 at 10:26
  • $\begingroup$ It is more like a binary star system where one of the stars basically orbits the other star because the more-or-less stationary star is much more massive than its binary companion. When the two planes of orbit are perpendicular to each other, you can have a situation where (imagining standing on one of the poles of the Planet) at one moment in the Big Year the Big Sun is right above you in the sky and half a Big Year later the Big Sun is right below you (so not visible). $\endgroup$
    – KjVdB
    Commented Dec 9, 2022 at 13:23

2 Answers 2


There are some problems with your big-sun, small-sun system.

When the two planes of orbit are perpendicular to each other, you can have a situation where (imagining standing on one of the poles of the Planet) at one moment in the Big Year the Big Sun is right above you in the sky and half a Big Year later the Big Sun is right below you (so not visible).

Thing is, the big sun being hidden from one pole does not hide it from the other pole! This means that your 365-year cycle will cause long winters and long summers, but your requirement that the planet as a whole is not made uninhabitable. There's some equivalent of the tropics which will remain basically fine all year round, and the furthest north/south regions will be periodically inhabitable causing long, slow migratory/transhumance cycles as people escape the long winter.

There will be odd spots in the middle when the big sun is on the opposite side of the small sun, when you'll get a dip in global temperature for a little while, but not long enough to freeze everyone to death.

What you might consider is having the little sun's orbit about the big sun be quite elliptical. That means that the big sun may or may not be visible at any given point on the planet, but at some point in the little sun's orbit it will be close (bringing about a short but hot "big summer") and for a longer point in its orbit it will be far away (bringing about a longer "big winter").

There's no particular reason that this could not be made to work, though I'm not going to try and conjure up some parameters for your system at this stage, given that your ideas seem to be somewhat in flux still!


You have an interesting evolution problem.

On one hand, since any endemic life would have evolved for the conditions, a major change in the conditions could be catastrophic for that life.

On the other hand, if your population came from elsewhere, it is certain to have evolved for different conditions, and is actively maintaining the right environment in spite of the local conditions. A change in those conditions may alter the energy balance of the life-support accommodations, but would probably be handled by the population's technology.

If this is a hard-science story, you will need a backstory explaining how the system formed. The story doesn't have to be written in the text, but you should know how the system formed in to provide the right framework. It may alter the more global framework of your star system, such as where in the galaxy it is located, the type of galaxy that contains it, or possibly that it is outside a galaxy.

The curious twist is that the orbital axis for the planet around the small sun is normal to the orbital axis of the small sun around the big sun.

Typically, all bodies condense from the same rotating cloud and the velocity vectors are all in the same plane. Some disruption or capture event may have occurred to bring these bodies together.

This could let you have the suns have different metalicities or ages since they wouldn't need to have come from the same source. You could also have interesting companion bodies, since the Ort cloud objects of one star could now be inner-system objects in the merged system.


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