I know nothing about what sort of orbit such a planet would have or if this has an impact on other factors such as mass/gravity. I wanted the gravity to be near Earth's, if possible. Would this planet have to orbit a binary star to have this profile? I don't know.

Is there anyone who knows about such things or can direct me to somewhere (a program, perhaps) where I can find out.

As for the seasons, well, it's a bit about the distance between the planet and the sun but more importantly about the tilt of the planet's axis against the ecliptic (the plane on which the planets circle the sun).

If the planet's axis is tilted 90 degrees (Uranus has that) then the planet is practically rolling along on its path, making one half of the planet constantly illuminated, the other constantly in the dark. I confess I've given some thought about using this in a story myself.

A binary star system gives you options such as:
- if the planet is far away from both stars, then it's circling them like any single star system. You can have 'Tattooine' sunsets but that's just about it.
- if the planet is close to one star but far from the other, then the other star can be a moon-like object in the sky but a lot brighter.
- if the distance is just right, the planet can circle BOTH stars but each at a time. Meaning, your planet is making a double loop around them both, drawing a number 8 with the stars in the loops. That would cause some pretty exotic seasons.

None of these can give you 20-year winters though. So my suggestion is to give your planet a very eccentric orbit, sort of like asteroids that are usually very far from the sun but come very close every so often. The sun would be far away most of the time but when the planet comes closer it would get quite warmer.

One final note: if you indeed create a planet like this, it would probably not be from the same solar system but would need some celestial event to bring it to the current star system. Real planets (except Pluto) were formed by the condensing gas which also formed the sun (at least according to some theories). That is why the planets are all on the same plane (ecliptic).

Here is a link to a graphic of how this works for Uranus.

The problem with Uranus is that it gets far too little heat from the Sun. It's not in the habitable zone. Raising the sun's temperature or mass changes things. For starters, the light would have a different color, meaning chlorophyll would have problems. The sun/planets mass and momentum ratio would be different and that would cause complications.

You can always do what GRR Martin did in A song of Ice and Fire: don't give any explanation, just use it.

How would Uranus have a long winter AND a short summer? I thought this was the case here. I'm not trying to be pain, I'm honestly asking your opinion to see an alternative way of thinking.

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So I am looking for 20 year winters, followed by a thaw and a summer (it can be brief, perhaps a few years or equally as long as winter--doesn't matter.)

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...so I think equally long summer/winter cycles would work for him.

We can certainly be inventive, yes. But I was just trying to come up with the simplest option. For example, I would think that a planet could orbit around a large F-class star (with maybe 1.5 times the Sun's mass and five times its luminosity) on a 20-year orbital period, with an 80+ degree axial tilt and a precession roughly equal to the orbital period, and you'd get your 20-year winters without having to be so far away that you're out of the star's habitable zone.

1. An axis of rotation that is tilted to near 90 degrees, so that the planet's rotation is largely irrelevant in terms of its effect on what faces the sun. In other words, the planet is rotating like a bead on an abacus.

2. A very slow orbital rotation that causes a pole to face the sun every 40 years. In other words, the orbital rotation is nearly perpendicular to the planetary rotation.

If these two criteria were met, I would think the mass, orbit, and star type could be identical to Earth. Turn the Earth on its side and get it spinning very slowly perpendicular to its axis of rotation (a 40-year precession), and you'd have it.

This would look something like the 8th, 9th, and 10th graphics here.

So that would be like a twenty-year night? You would only see the sun when spring arrived and it just stuck it first stuck head above the horizon....a little more each day.

If I cut the orbital period down to 30 years, you would still get an effective winter of 20 years (it would take some time for the melt to happen) followed by a warmer period lasting 10 years or so.

At that distance in order to make life sustainable you'd have to have a much thicker atmosphere so the greenhouse effect would keep the temperature livable. A side effect of that is that a thick atmosphere keeps the global temperature fairly constant regardless of which side is facing the sun (think of Venus). This, of course, effectively eliminates seasons caused by axis tilt, as well as temperature disparity between night and day. So the only way you could have season changes would be globally, caused by distance from the sun.

Another alternative would be to make the sun have a higher energy output. The only real way to do that is to make it more massive, which makes it so the distance to have a stable orbit increases even more. There might be a point where the mass/energy output ratio provides a planet where life is sustainable, but I don't know the math for it.

Another solution is to have a binary system with the planet making a roughly figure 8 orbit around the two suns. It would approach closer to one sun than to the other, allowing for a "summer" at one phase and a "winter" at another.

Or summer when the planet is between the two suns and thus getting the full effect of both their energy outputs, and winter when it gets to the extreme side of either sun at the outside loops of the 8.

A third solution is to have the planet behave like mercury (or the moon), where it rotates at exactly the same rate as it orbits, so one side is always facing the sun. With a thin atmosphere that makes it so that the side facing the sun is very hot, and the side facing away is very cold. If the rotation is slightly off from the orbit then you could have one side facing the sun for long periods of time, ie summer, and when it finally does rotate so that area is on the dark side you have winter. This creates the problem of constant day or constant night.

Fantasy is a lot easier. You can pull a George R. R. Martin and not have to explain why there's long winters and summers, and the duration of them varies unpredictably.

Edit: Corrected a faulty assumption.

Then the only problem left (well, the only one obvious to me, anyway) is that pesky 20-year night, which you solve by either (1) making this a binary system with an outer sun that provides dim "daylight" (but then you have very long "days" and the orbit becomes very complex because of the gravitational interactions between the two suns); or (2) having one or more large, bright moons with orbital planes that take them around the darkside and provide reflected light. You probably need this anyway to get that extreme near-perpendicular wobble to be stable.

Or, um, something else altogether. I don't know.

The only place where it would be a big deal is if the story revolved around the detail of it in some way, like where something the characters do is only possible because the planet is the way it is.

Hardcore science fiction used to be popular, and disciples of that genre would lynch you for trying to just say "this is how it is" explaining it. And heaven help you if it happens to be impossible >.<. Thankfully that's fallen by the wayside.

Another thing to consider on this is how powerful the sun would be, and how big a difference between summer and winter would be.

@Zero: I agree; there's no need to go into boring detail about what the orbit is, what the tilt axis is, etc. That has for sure gone out of fashion in all but the hardest SF.

I do think, though, (just personal opinion) that the resulting world needs to be plausible in terms of things like

* what constitutes a day throughout the seasons, since you may (depending on how you end up "building" the star system) have something like what happens in Alaska in terms of length and intensity of daylight/twilight/night over the course of a year. I actually wonder, to keithjgrant's point, whether an orbital period or the precessional period would be the defining interval for people on such a planet. One orbit wouldn't really mean much unless you use the binary system option.

* what the "in between" seasons are like (esp. if the planet is rotating "sideways" and you live near the equator)

* what the long-term effect is of having solar radiation bombarding one face of a planet for such a long period of time

Besides, that kind of stuff makes for very interesting plotting possibilities, IMO.

Sorry if I'm over-posting here, but I do thank skadder for posting something so interesting! I love to think through problems like this--even when I really don't know what I'm talking about.

Edited to add this: Here I was claiming to be trying to find the simplest solution, but rstegman's suggestion might be the easiest if you can get the right combination of rotation, orbital period, and solar intensity. Might be hard, though, to come up with an orbit with a big enough ellipse to give you a 20-year winter without baking the planet at perigee and turning it into a cold dead rock at apogee. I'd be willing to suspend disbelief at that level, though. As several people have said, it's the more story we tend to care about these days than the specifics of the underlying science.

And now I really will quit. I promise. Really. I mean it.

Orbital period scales as r^(3/2)M^(-1/2), where r is the length of the orbit's semi-major axis and M is the mass of the central body (the sun). (Wikipedia, Orbital period)

Brightness scales as r^(-2)M^3.9 for stars on the main sequence. (Wikipedia, Luminosity)

If we require that the orbital period be greater than that of Earth by a factor of 20 and that the brightness be equal to that of the Earth, this gives two equations in two variables. If we normalize r and M to the semi-major axis of the earth's orbit and the mass of the sun, we can write the equations as

a^(3/2)M^(-1/2)=20

and

a^(-2)M^(3.9)=1

Turning to Wolfram Alpha gives, if I did it correctly, M=3.44, and r=11.1221. In other words, the sun must be 3.44 times more massive than our sun and the semi-major axis must be 11.1221 times larger than our semi-major axis.

That's very interesting, Doug. Brings up a question I had when I was looking at the need for increased solar intensity in one of these scenarios. Material I was reading today seemed to indicate that a star that much more massive than ours would burn through its main sequence too quickly to support the evolution of complex life forms on any of its planets.

@Doug

So, broadly--an earth-sized planet can orbit a star 3.4 times the mass of our sun with a 20 year orbit if it does so at 11.1 AU from the star? Correct?

Would this not give earth-like seasons spread across the globe? So the equator would be similar to our equatorial regions (apart for the blue shift) and temperate regions would have seasons that were 5 years long? The artic circle would have 10 years of darkness followed by 10 years of day?

The presumes a tilt (not the correct word--axial inclination?) similar to earth's.

I suppose I could have one small continent and have everyone on that (Very long winter), but you say this planet would be hotter than ours...would this not negate the effects of the winter period--in that warm air would circulate from the warmer regions.

You want a planet that experiences a 20 year ice age, a warming period, then balmy summer for a few years, correct? Here's how you do it.

Ice planet Skadder circles a small sun, about the mass of Jupiter (maybe a bit a larger). The orbit is circular with virturally no tilt. One season, winter.

The small star orbits around a large one, probably should be a bit bigger than Sol. The small star orbit is elliptical, commet like (not so drastic though).

Two things happen when the planets small star gets close to the primary one. The dual heat from the two stars makes things warmer. The gravity pull of the large star draws the planet into an elliptical one so when the planets orbit is behind its parent star it is also closer to its own sun. Tidal effects would be drastic, just to let you know.

I would make the small suns comet like orbit run from a range of a 500 million to 100 million. The heat from the dual suns wouldn't be so great to make your planet lifeless. I would make the orbit of the planet around its parent star at about a million miles.

The length of the season is determined by time of orbit. Our orbit takes one year, so our seasonal cycle is one year. If you want a 20 year seasonal cycle, the planet has to orbit its sun once per 20 years. The closer the planet to the sun, the faster it orbits, so you will need to place it farther away than earth. To create a habitable zone farther from earth, you will need a brighter star.

Here's some general links, good luck!

http://en.wikipedia.org/wiki/Season
http://en.wikipedia.org/wiki/Habitable_zone

Why don't you just put a giant volcano (like Yellowstone) on the planet. Once about every 50 years it blows its top and fills the atmosphere with enough dust and debris where it creates a nuclear winter that lasts for approximately 20 years. As the CO2 in the atmosphere begins to deplete, the greenhouse effect takes hold and causes the atmosphere to heat up steadily. The cycle then repeats itself.

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So, broadly--an earth-sized planet can orbit a star 3.4 times the mass of our sun with a 20 year orbit if it does so at 11.1 AU from the star? Correct?

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Would this not give earth-like seasons spread across the globe? So the equator would be similar to our equatorial regions (apart for the blue shift) and temperate regions would have seasons that were 5 years long? The artic circle would have 10 years of darkness followed by 10 years of day?

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The presumes a tilt (not the correct word--axial inclination?) similar to earth's.

I suppose I could have one small continent and have everyone on that (Very long winter), but you say this planet would be hotter than ours...would this not negate the effects of the winter period--in that warm air would circulate from the warmer regions.

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You're right about the tilt. My goal when I made the calculation was to make the planet as Earth-like as possible while still giving it a year that is 20 times as long as ours.

However, I guess I didn't communicate clearly when I said the sun would be hotter. I meant that the sun would be hotter, not the planet. That means that a rainbow from their sun would look brighter on the blue, indigo, violet side than a rainbow from ours would. Brightness gives a measure of the energy that reaches the planet from the sun and that was set to be equal to that of Earth. When I said that the sun might be hotter what I meant was that the colors would change. You can treat stars roughly like black body radiators. When a black body gets hotter, blue light is enhanced faster than red (and ultraviolet faster than blue, etc.) meaning that the sun will have a different color than ours.

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I want ice cold winter (they are aliens) for 20 years, preferably with no where warmer on the planet for them to wander to. They have to stuck in the cold (and dark) until spring comes.

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If you want the whole planet to be affected simultaneously then you might abandon tilt and go completely with distance. An eccentric orbit would be a nice way to make this a repeating, stable cycle.

There are lots of factors that we believe affect the development of life (plant or animal): distance from sun, mass and type of sun, mass of planet, liquid (water), tilt of planet, rotation of planet, platonic movement, magnetic field, volcanic activity, atmospheric conditions, etc.

When you said that there wouldn't be enough time for life to develop on a planet with elliptical orbits, did you mean that a more massive sun would burn out more quickly? Did you mean that the warm seasons between the long winters would be too short? Did you mean that the orbit would be unstable?

The first two points seem valid but not necessarily definitive. The third could be valid if, as you posit, other planets have orbits that interfere with the orbit of the planet of interest.

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Just to play devil's advocate - the main problem with the eliptical orbit theory (causing 20 year seasons) is that this does not allow enough time for life to develop. The only way this could happen is if another sun or other large gravitational force stretched out the orbit of all of the planets after life had already taken hold. If it's only one planet with this orbit, it will get pounded as it crosses the orbits of other planets.

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Just think of it this way. Earth orbits Jupiter. Jupiter throws off heat, enough to warm Earth to an average temperature -20, -30 C of our mean tempature now. Give Jupiter a comet like orbit. Halleys, but less drastic. When Jupiter swings near the sun, Earth experiences a brief warming peroid.

The physics are there. Jupiter could support a planet of Earths mass. Elipitical orbits exist. Add the two together and you have Skadders world. A hellish frigid existence with a few hot rocky moments spaced apart.

So if you had this elliptical orbit in system, I would presume that collision/intersection of another planetary body's orbit to be most unlikely as a ellipsis capable of bringing about said winter would not need to be enormous--would it?

If you had your original planet, circling a star 3.4. times our solar mass at 11 AU and stuck it in a system without any nearby planets (Mars-Mercury) you would have enough space, wouldn't you?

The planet's orbit could have been disturbed by something large, in the past, brushing close to it before crashing into the sun...other planets could have been on the other side of the sun and unaffected.

@Philo...wouldn't the above have allowed life to develop prior to the near collision and after there would have been a mass extinction, with only those capable of surviving evolving.

I presume this elliptical orbit would allow for winters days with weak sunlight...

@snapper

This Jupiter-size star would not produce light? Just heat? From what I gather a Jupiter-size mass is incapable of igniting as a star due to a lack of mass. The lack of light is not a major issue, but I guess the heat would make it tolerable.

Technically it would be a moon...which i quite like. The summer would need to be longer than a few brief moments...

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So if you had this elliptical orbit in system, I would presume that collision/intersection of another planetary body's orbit to be most unlikely as a ellipsis capable of bringing about said winter would not need to be enormous--would it?

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The size of the orbit is dictated by the length of the year and the mass of the sun. It's a big orbit because the years are long. It's eccentric because the seasons are lop-sided. But as far as I know, we haven't seen enough solar systems to establish any rule about the distribution of planets. And even if there were such a rule it would have exceptions. I don't see any reason for there to be interfering planets if you don't want them.

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If you had your original planet, circling a star 3.4. times our solar mass at 11 AU and stuck it in a system without any nearby planets (Mars-Mercury) you would have enough space, wouldn't you?

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Right. If nearby planets are gone, so is the problem.

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The planet's orbit could have been disturbed by something large, in the past, brushing close to it before crashing into the sun...other planets could have been on the other side of the sun and unaffected.

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Sounds fine to me.

Alpha Centari

http://en.wikipedia.org/wiki/Alpha_Centauri

It is a binary system (with a straggler star(proxima centari) that they are unsure of). One star is a bit bigger than ours, the other a bit smaller. Star A is the center of the system. Star B revolves in an orbit like the one I suggested. Its ellipitical path takes it from a Pluto point at its furtherist to a Saturn one at its closest. (That's 2 billion at its furtherest and 1 billion at its closest)

Gliese 581

This is a red dwarf star. It has a mass, lumination, and size of about 30% of our own. It has a planet that is in an orbit that will support life, less than 20 million miles from the star.

You could take that dwarf star (it is a very stable star, older than ours) shrink it a bit, make the planet orbit it at 15 million miles, then orbit that star around the bigger one. The small star could have an orbit that is 35 of our years long. 20 could be ice age cold, then a five year warming spring, five year scorcing summer, and a five year cooling fall.