My previous question ruled out having a moon orbiting a gas giant. I've got an interesting ecosystem (my bad, biome was the wrong word) that evolved based upon the idea that the world is eclipsed for a 6 month period and enters a deep ice age once every couple of years. Is there anything that could cause such a situation?

Perhaps an asteroid belt so thick it darkens the planet as the planet orbits beyond the belt passing above then below the plane of the belt? Is that kind of orbit possible?

Per Morris suggestion below, if the planet orbited a dim binary, could I forego the gas giant entirely? Can a star be so dim that having it come between the planet and the brighter star be like nightfall?

  • $\begingroup$ Supervolcano erupts every few years and the ash blocks out the sun. $\endgroup$ Aug 1, 2019 at 5:57

2 Answers 2


Our asteroid belt is so thin that we send spacecraft through it withiut fear of a collision. It won't overshadow anything.

An orbital arrangement where a planet is eclipsed for a quarter of its orbit is very unlikely. I can think of two situations in which it can work:

  1. If your planet is orbiting a relatively dark body, such as a black hole or a pulsar, and gets its light from a nearby cluster of stars. But for the nearby cluster to provide enough light for life as we know it, you'd be close to stars that don't live long. Such stars would live for just a few hundred million years before going nova, and novas are very bad for life in a range of many dozens of light years from them.

  2. It may be that your planet has a very eccentric orbit, which will give it a wide variation in temperature for a fixed part of its year. It may also be that the atmosphere has something in it that causes the upper layer to go opaque when below a certain threshold temperature. That would cause the temperature variation between summer and winter to be even greater. I think this is much more likely than the first option.

But truth be told, you mentioned a biome, not a full planetary ecossystem. The Esrth's subpolar regions almost fit your bill. A planet with a wider orbit (think Mars's) and possibly a slightly greater axial tilt would have its subpolar zones freezing for six months every couple Earth years.

  • $\begingroup$ The key element to the question is that the planet needs to be in darkness 6 earth months out of every 2 earth years. $\endgroup$
    – Pentallion
    Aug 1, 2019 at 23:44
  • $\begingroup$ @Pentallion - Doesn't "eclipsed for a quarter of its orbit" (assuming the orbital period is 2 earth years) cover that element? If it's orbiting a supermassive black hole once every 2 years, and the black hole is eclipsing the light of a more distant bright year for a fourth of that period, that puts the planet in darkness for 6 months. $\endgroup$
    – Hypnosifl
    Aug 2, 2019 at 12:18

The way I would do it is have your moon orbiting a gas giant which itself is in a P-type orbit in a binary star system. Your gas giant orbits a small, dim star which is a binary companion with a much larger, much brighter star. The primary energy source for your moon is the more distant of the two. The shadow period would occur when the bright companion is on the other side of BOTH of the gas giant itself and the dim star the gas giant orbits. During this period your moon would still periodically get light from the dimmer, closer companion, but depending on how you modeled the system, it would generate the periodicity and the temperature variation pattern you're looking for.

  • $\begingroup$ Would such an occultation last 6 months though? $\endgroup$
    – Innovine
    Aug 2, 2019 at 9:22

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