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In this alternate Earth, the axial tilt is still leaning to the extent of the temperate and polar zones having four seasons--spring, summer, autumn and winter. The one key difference is that each season lasts, not for three months, but for three years. This is because it orbits a trinary system--a binary of G-type main-sequence stars orbited by a third, solitary G-type main-sequence star--which widens the habitable zone (the point where liquid water can form) but pushes it a lot further. And as any astronomer knows, the farther a planet is from its parent star(s), the slower the revolution, and therefore the longer the year. So here, we have an alternate Earth with a 12-year revolution. I've been told elsewhere that, with the right amount of starlight, a planet with a longer revolution would result in greater seasonal extremes, which makes sense.

So the question is--if each of Earth's four seasons lasts three years long, would Earth still hold the great diversity of habitats, or would we be looking at some overgrown desert like Tatooine or Arrakis?

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    $\begingroup$ So... each season is 3 Earth years long, but is still a quarter of the planet's own year? $\endgroup$ – Arkenstein XII Aug 22 at 2:06
  • $\begingroup$ @ArkensteinXII No. Each season is 3 Earth years long, totaling up to a revolution of 12 years. $\endgroup$ – JohnWDailey Aug 22 at 2:51
  • $\begingroup$ Yes. 12 Earth years, which is one local year? $\endgroup$ – Arkenstein XII Aug 22 at 2:57
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    $\begingroup$ @ArkensteinXII Yes... $\endgroup$ – JohnWDailey Aug 22 at 3:04
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    $\begingroup$ @Rekesoft This question isn't asking (directly) about evolution, only about the plausibility of an Earth-like biodiversity. That is an answerable question because its answer is either "Yes" or "No" (with some explanation). Asking what the biodiversity would look like would be too broad. Voting to leave open. $\endgroup$ – Frostfyre Aug 22 at 12:52
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Your extended seasons are going have complications to animal and planet lifecycles.

First is winter. The creepy crawls and witty bitty bugs survive over winter by eating the roots of sleeping plants. If winter lasts for years, then the bugs will eat everything and the perennials will all be dead. These plants anchor soil in early spring and the fall while the annuals, sprouting from seeds are just getting going. Animals that hibernate will be very challenged to store up enough food for 3 years.

The long springs and fall will mean, in general, longer and colder periods of rain. This suppresses the microbial mechanisms of decomposition returning nutrients to the soil, leading to soil degradation. It might be offset by insects munching on the stalks and dead leaves from winter, but that will mean essential elements needed for life nitrogen, manganese, etc will be depleted since the bugs will ingest them and use them for their muscles, sinews and nerves

The long summer will impact how trees and bushes that only flower once per year survive. Things like apples and nuts will only be produced one every three years. This means the trees and bushes are putting all that solar energy into their root balls, cause thats what they do when they are making new seeds. This will lead to faster soil depletion. There are tropical trees and stuff that bloom and fruit more than once per year but they won’t survive winters in temperate climates

So, i think your world will be populated by scrubby trees like sage, and not support large fauna. Lots of worms, and bugs, and mammals and lizards that hunt bugs and worms.

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    $\begingroup$ Bugs and cold-blooded animals can just freeze, if not in adult form, then as eggs. It's very hard, if not impossible, to dig food from frozen soil anyway. Thus perenial plants can survive and sprout from roots. $\endgroup$ – Juraj Aug 23 at 22:59
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A very long season would have extreme conditions if the tilt were as high as Earth's. This can be seen in Earth's weather in that the temperature continues to increase after days begin to grow shorter, and the temperature continues to decrease after days begin to grow longer.

One adjustment to reign in the supreme conditions is to make the axial tilt be small so that the temperature is closer to equilibrium with the insolation.

I don't quite understand how you will get three stars in a trinary system with one planet orbiting outside of those three stars, and still have only a 12-Earth-year revolution period. I've assumed that you can, and that the three stars can be considered to be a single star with respect to the stability of the insolation. This is reasonable because the three stars will be closer to each other than the planet is to the stars, so their orbital periods will be faster. If there are resonances between the star orbits and the planet orbit, the insolation variation over the season will be more complex.

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  • $\begingroup$ How much smaller are we talking? $\endgroup$ – JohnWDailey Aug 22 at 3:04
  • $\begingroup$ I'm not a climate specialist. On Earth, the input and the response are nearly 90 degrees out of phase. Without understanding all the non-linear or possibly chaotic effects, I'd start with the linear argument and set the tilt to one-twelfth of the Earth's value. $\endgroup$ – cmm Aug 22 at 3:09
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Trying to get your planet to orbit a trinary star system is probably more complicated than it is worth. You seem to want an earth like season system/planet with a stretched out time frame. For that you really do not need any crazy changes to what earth experiences. The only thing you have to have is a longer orbital period, as you need an orbit that takes roughly three times that is the earths. To keep the planet habitable you will need it to be further out, which means you need a hotter star. Move your "sun " a bit up the spectral system, maybe a low class F star, or even a G1, and you have the sort of star that will provide exactly what you are looking for. Keep in mind Mars is nearly habitable and has an orbit of 687 days. So star/ Planet system doesn't need to be much off of what our solar system is like to manage a realistic take on what you are looking for.

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    $\begingroup$ F-class stars are too short-lived for what I've had in mind. $\endgroup$ – JohnWDailey Aug 22 at 2:52
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The habitable Zone of the planet would be mostly the tropical Zone, and Flora and Fauna would spread to the north or south if there is sprint or summer. 1000 Days of Winter would be deadly, while spring with melting ice, much of rain and water would also not be easy, followed by a 1000 days of summer.. So life would be in the tropes and only partly expland to the other areas. On the other side, even in the deserts there is life an we have animals in the artic zones, so life would find a way. Add so streams like the gulf stream and you could have a wider habitable zone (New York ha as climate like in middle or north Europe, but has the same latitude as Rome).

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    $\begingroup$ 1000 days of summer in a tropical zone would be equally deadly $\endgroup$ – Separatrix Aug 22 at 11:14
  • $\begingroup$ AFAIK in the tropical zone there are no seasons. $\endgroup$ – Julian Egner Aug 22 at 11:31
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    $\begingroup$ It's not that simple in practice, the seasons are different from temperate regions, tending to be a dry season and a wet season. A 1000 day dry season isn't going to leave many survivors, but then nor will a 1000 day wet (monsoon) season. $\endgroup$ – Separatrix Aug 22 at 11:37
  • $\begingroup$ Wait a moment - isnt there only one dry and one wet season a year? Then on this plantet there would be a 2000 day (6 Earth-Years) dry season and 2000 day wet/monsoon-season. While I think, that a dry season can be survived whith some preparation, a 2000-day-monsoon could be a problem... $\endgroup$ – Julian Egner Aug 22 at 11:43
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Doubling (forget trippling if you want to be realistic), but creating a binary system could work, but wouldn't give you as much additional distance as you might think.

Two roughly equal roughly sun like stars in a tight (say .5 AU or less) orbit around each other and a distant planet, somewhere between Mars & Jupiter distance - to extend the year.

Two sunlike stars doubles the luminosity, which means you can, using the inverse square rule, move the planet 1.414 times further away and it gets equal luminosity (but the interesting 2 star sky, which is always fun). Presumably that close, everything orbits on the same eliptic, but at different hemispheres, you'd get two sunsets most days. Kinda cool.

1.414 times further gives you an orbital period to the 3/2 power of that, or 2^(3/4) or 1.68 orbital periods, but you also have to adjust for twice the mass in the center, so, that speeds the orbit up by the square root of two.

Long story short, by adding a star, going from 1 sun like star to 2, keeping the luminosity the same, you've only increased the orbital period by the 4th root of 2, or about 19%. Your year is 19% longer.

3 stars becomes unstable for a planetary orbit within a few AU, so I won't even go there.

What you can do is give the planet a stronger greenhouse gas. That'll probably weaken the seasons, but you could move the planet considerably further away if it has a thicker atmosphere with more CO2 and/or Methane or water vapor. You could perhaps even double the distance using the right atmosphere which would make the year 2.828 times longer, add your 19% increase to that and you get 3.36 earth years.

12 Earth years is too difficult to explain unless you go for more massive, hotter, more bluish stars, which have shorter lifespans.

If you have two stars each with 1.5 solar masses their luminosity grows by the 4th power of that or about 5 times. The inverse square rule means, 2.25 times the distance and 3.375 times the orbital period. Now you're getting closer, but 1.5 solar mass stars have shorter lifespans, about 3 billion years.

Adjust accordingly, but 1.19 multiple for 2 stars, 3.37 multiple for 1.5 solar mass per star, maybe as much as double if you give the planet a strong greenhouse gas.

1.19 x 3.37 x 2 = 8 year orbital period. That's about as far as I'd be comfortable pushing it.

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