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Red Giant
2009-Feb-20, 01:22 AM
How could I get seasons that last for thousands of years on a planet life could have evolved on?

AndrewJ
2009-Feb-20, 01:33 AM
The idea has been done before by Brian Aldiss in his Helliconia Trilogy in the 1980's. His idea involved a binary star system. So, you could check that out for inspiration. Also, a good read.

Ronald Brak
2009-Feb-20, 01:50 AM
Four seasons of two thousand years each comes to 8,000 years. That's a long cycle. You could have a planet orbiting a small star or brown dwarf that provides enough heat for life to survive while in an eliptical and very very long orbit around another star, but that would be a heck of a big orbit. Or there could be a single star which varies its output over thousands of years, which is probably the simplist option. Another possibility is you could have a planet, perhaps without tentonic plates, which follows a regular cycle of climate affecting volcanic activity. also you could have life causing long seasons. Perhaps large amounts of carbon are somehow locked up and then released over a cycle of thousands of years.

Pickled Tink
2009-Feb-20, 02:04 AM
I see two ways to get (Assuming you mean earth years by years) a long long set of seasons.

The first would be to have the planet in the habitable zone of a large star. The obvious problems being that these stars do not live very long, at least not on the scales that we have evidence to suggest are needed for the evolution of sentient life. Also, when they die, they would generally tend to kill everything within a goodly distance of them. Add onto that radiation difficulties and i don't see this being particularly viable, unless you want to have the thing terraformed with some radiation shield thing encasing it.

The other one, which is more plausible, would be a large rocky planet on more eccentric orbit, much like a comet, only nowhere near as extreme. It would have a really long winter, at the least.

Its been a really long time since i last looked at the fine details surrounding these, but we have detected at least one extrasolar planet whose orbit shares more in common with a comet than an ordinary planet. Id be especially leery of using anything i say here without checking it against other sources.

Van Rijn
2009-Feb-20, 07:11 AM
There was the Onoff star in Vinge's A Deepness in the Sky (http://en.wikipedia.org/wiki/A_Deepness_in_the_Sky). That star was highly variable, dim for a couple hundred years, then bright for a few decades. There was a planet orbiting the star that massively freezed (A Pail of Air (http://en.wikipedia.org/wiki/A_Pail_of_Air) level freezing) but when things warmed up, the inhabitants would thaw out.

Anyway, he used a bit of a cheat for the star. This is a minor spoiler, so consider yourself warned . . .







it's very strongly hinted that the reason for the "OnOff" star's variation is radically advanced technology. So, that's another option for long period seasons - artificial solutions.

tony873004
2009-Feb-20, 08:14 AM
In a sense, this describes Earth, as we enter and come out of ice ages in cycles of tens to hundreds of thousands of years.

The Milankovich cycle controls these ice ages and is driven primarily by Earth's changing eccentricity. Jupiter is the primary cause of this. If a planet orbited Alpha Centauri A in its habital zone, Alpha Centauri B would cause a Milankovich cycle with a period measured in decades. So each generation (assuming human life expectancies) would have significantly different weather than the prior generation.

Adjust your star system accordingly and you can get any period you want.

timb
2009-Feb-20, 12:17 PM
As Brak points out, a planet orbiting a small star that orbits a larger one in an eccentric orbit could experience a substantial periodic variation in irradiance. Suppose the primary star is a Sun-like star then an 8,000 year orbit would be 400 AU (let's ignore the mass of the secondary star.... it's small like proxima or something), and for the planet's climate to be substantially affected you'd want the irradiance at periastron to be at least... come to think of it, this model sucks. The "season" when the climate was substantially affected by the brighter star would be very short compared to the period when its effect would be negligible, so it wouldn't be like terrestrial seasons at all. I don't know of any sort of long-lived star that has regular periodic brightness variations with a period ~10 ka either.

A better idea might be based on the fact that a planet without a large moon, such as Mars, can experience quasi-periodic obliquity variation that would have a profound effect on the climate. In the case of Mars the obliquity varies between 0 and 60 over about 125,000 (?) years. This variation would be superimposed on annual seasonal variation of course.