Is it physically possible for a planet to have seasons of different lengths?

Question

Those of you who have read Game Of Thrones will know that in the GOT world seasons do not have a fixed duration, e.g., a winter might last three times longer than the previous one.

I find most interesting how can this be possible (if it can), from a physical point of view. I have thought about it, and I find it difficult to happen. Maybe a certain orbital configuration may make the season's duration, if not totally unpredictable, quite difficult to guess.

The only possible cases I can imagine are a planet constantly approaching its sun (the orbital velocity will increase and thus the seasons' length will decrease constantly) or the opposite case. However, in both cases the length would always increase or always decrease, and not simply vary.

Answer 1

Yes and no.

This NASA page has a good summary of why we have seasons (emphasis mine):

It is true that Earth’s orbit is not a perfect circle. It is a bit lop-sided. During part of the year, Earth is closer to the Sun than at other times. However, in the Northern Hemisphere, we are having winter when Earth is closest to the Sun and summer when it is farthest away! Compared with how far away the Sun is, this change in Earth's distance throughout the year does not make much difference to our weather.

There is a different reason for Earth's seasons.

Earth's axis is an imaginary pole going right through the center of Earth from "top" to "bottom." Earth spins around this pole, making one complete turn each day. That is why we have day and night, and why every part of Earth's surface gets some of each.

Earth has seasons because its axis doesn't stand up straight. Long, long ago, when Earth was young, it is thought that something big hit Earth and knocked it off-kilter. So instead of rotating with its axis straight up and down, it leans over a bit.

Earth's seasons are roughly the same length because the axis constantly* points in the same direction over the course of a year, so the "side" of it that faces the sun changes. In order to generate more variable seasons, you'd need either the distance to the sun to vary significantly more or for the "side" of the axis to rotate fast enough that it's noticeable over a human lifespan. In combination with the normal rotation around the sun, these will produce three different "season cycles", which will, in turn, combine to produce relatively unpredictable seasons over the course of several years.

You can look at the waveform combinations here to get some idea of how quickly these would combine into unrecognizability (just think of the waveforms as temperature graphs and derive seasons from that).

It's worth noting that each of these three cycles comes with their own odd astronomical effects.

Varying distance aka "highly elliptical orbit" and/or "off center orbit": The size of the sun will change over the course of a year, appearing smaller at the far/colder end of the orbit and larger at the near/warmer end. This would probably have to be fairly noticeable in order to have enough effect on the seasons, but it doesn't have to be huge. (The sun is ~0.5 degrees large, so varying ~0.4-~0.6 might be sufficient, but I haven't done the math.)

Moving axis aka "precession": This should be a much slower cycle than the others, but 10 years instead of 26,000 would make a big difference. This would cause the "north" star to change in time with the cycle, with corresponding effects on the rest of the constellations. Navigation by the stars would be much more complicated.

Facing aka "normal seasons": This needs to be a faster cycle than precession, and is related to both of the others, but it should be possible to have a cycle where each close-to-sun approach has a different side of the planet facing the sun, regardless of where the axis is pointing, by varying where the far point is. The "weird" astronomical affect for this one is that the place where the sun rises and sets changes over time, thereby changing the length of a day.

_{* Earth's axis does actually move, but it's so slowly that it takes 26,000 years to complete a cycle}

Answer 2

I'm surprised no one mentioned Sitnikov planets yet.

^{Image found on Wikipedia.}

There is a very unlikely constellation in which a planet moves along the axis through the centre of gravity of a binary star system. In such a system, the planet can oscillate chaotically which, given a tilted rotation axis of the planet, would lead to seasons of varying length. I'm sure this would have other funny effects, and it would be even more unlikely to set up this system with the planet being habitable, but I think it's a very neat explanation for varying seasons.

If you don't want your planets to have two suns, one of the stars could have burned out before the planet was captured (I'm pretty sure the planet would have to have been a rogue planet that was captured by an existing system to set this up).

I'm not an expert in this topic, so I can't provide you with much quantitative information, but I hope this gives you a starting point for further search. For those who understand German, there is a German science blogger who has written at length about these systems, especially in relation to A Song of Ice and Fire.

Lastly, to address the problem of "why would such a system even exist?" If there is any (however remote chance) that this system could be set up, then by assuming an infinite universe and appealing to the infinite monkey theorem this system is bound to exist somewhere.

Answer 3

Sure you could have an erratic orbit. This is an orbit that has the planet coming sigifigantly closer to the sun at one point and then much further at its opposite but that the orbit is moving as well. In this way you could have a winter that is much longer than another and then a very long summer. This would be predictable once you understand the physics behind the orbit but I suspect that it would have been at least the 15th century before it would have been even close to understood if it happened on earth.

Answer 4

I'll offer a substantially different approach from what has been said so far. The orbit of your planet doesn't necessarily need to be eccentric or instable and neither does it need a rapidly oscillating axis of rotation.

The cause of these fluctuations in temperature could be similar to our earth's ENSO which is a global phenomenon where the surface temperature on the equatorial pacific ocean fluctuates erraticly over a period of 2-7 years. These differences can easily reach 3°C between warm and cold periods and are extremely hard to predict.

Essentially, the west pacific ocean is generally warmer that the east. Every 2-7 years, this warm spot begins to expand and drifts east. The upward flow of air this causes over the pacific ocean causes more eastward wind and less westward wind, which in turn accelerates the eastward drift of this warm surface water.

How exactly this process then reverses is not entirely clear as is the cause for this phenomenon and what exactly is needed for a cycle like this to exist is also a difficult topic. So I suggest you do some further reading there.

Some prerequisites seem to be a large ocean and not too many temperature fluctuations apart from these effects. (hence why it occurs at the equator).

I can imagine that on a planet where there are two large land masses, seperated by a huge ocean, east to west (and another smaller ocean on the other side of course) that has a small axial tilt (and thus barely noticable traditional climates), these effects could cause what would seem like irregular seasons.

Answer 5

Co-orbital configuration

It is possible for a planet to change its orbit every so often, resulting in the length of a year changing. This is just one more example among the many answers here of something that could cause seasons to lengthen and shorten in a cycle.

An example of a planet that has this property is the Earth. Our planet is in a co-orbital configuration with the asteroid Cruithne. This means that over the course of a 770 year cycle, the Earth's orbit is mostly constant but changes by 1.3 centimetres, then changes back roughly half way through the cycle. The length of Earth's year is near constant for several centuries, and then lengthens for a few centuries, and then returns to the shorter year.

Clearly the difference is negligible for the Earth, but the effect on the much smaller Cruithne is a change in orbit of over half a million kilometres. The length of its year is still only changed by about 2 days (it alternates between a roughly 364 day year and a roughly 366 day year), but this shows that a change of year length that happens consistently every 770 years can be maintained. You may get interesting changes of season if your world is co-orbital with a planet of similar size to itself, or with a planet much larger than itself. The length of the year can change, and also the distance from the sun changes so that winter can take up a different proportion of that year.

Answer 6

There are several options (as detailed by the answers on this page), but one of the most possible with an Earth-like set-up, is an eccentric orbit heavily affected by surrounding planets and the planet's sun. The majority of this post is a reference from Dr. Irv. Boomberg, of the University of Toronto, Canada, from this page, of special interest is this section. The page details that the seasons on Earth actually vary from year to year, but only minorly.

This variation is caused to the most part by a slight eccentricity of Earth's orbit. We all pretty much know that Earth's orbit is elliptical, with the Sun being the one of the center points of the ellipse. However, the exact path of that orbit varies, usually because of the gravitational effects of other planets, and the moon. When Earth passes closer to other planets, they pull it off its orbit a little, and when the Earth passes closet to the Sun changes. When the Earth is closer to the sun, it moves faster, but when it is farther it moves slower. This makes a change in the length of seasons. A more eccentric orbit than Earth's would make this change more obvious.

The change in Earth's orbit is also partially caused by the Sun's movement. The Sun can be "wobbled" or slightly moved out of the orbital center of the solar system by the gravitational force of Jupiter. This doesn't make a huge difference, but it can affect when the Earth moves closest to the sun, effecting seasons. This means that a solar system with a sun that "wobbled" more, there would be a greater seasonal difference.

The final effect on Earth's orbit is the moon. The moon also pulls on the Earth, slightly changing the length and shape of its orbit.

There are enough factors for orbital change to have a significant effect on seasons, and many of these aren't very predictable. This could produce unpredictable seasons.

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This news article gives us another important option. NASA (which published the linked article) used the Kepler telescope to find a very unusual planet. This planet "wobbles" on its axis, much like a spinning top. NASA's first line in the article is

Imagine living on a planet with seasons so erratic you would hardly know whether to wear Bermuda shorts or a heavy overcoat.

Which sounds exactly like what you are looking for. The planet, called Kepler-413b, can tilt on its axis as much as thirty degrees in a year. This means that seasons will change drastically depending on the tilt. Kepler-413b's orbit also wobbles, moving up and down, as well as left to right. It moves enough that it cannot be seen from Earth for years at a time. This would make seasons fluctuate even more unpredictably.

Here is an image show Kepler-413b's interesting orbits:

Note that Kepler-413b's system is a binary system, so it is possible that could have an effect.

Answer 7

Our orbit and our sun's output are fairly constant. The sun does however, have a very large impact on our weather, both on a daily basis and over a much longer period of time. The sun has a cycle as well, where it has a 'hot' and 'cold' cycle. A sun with more varied and unstable cycles could very well lengthen or shorten a season, mostly noticeably winter or summer.

Answer 8

With the right planetary and orbital configurations season lengths could easily vary. Two possibilities come to mind:

• A much larger planet with a orbit slightly more eliptical than the target planet but fairly close; this would cause perturbations in the orbit of the smaller inner planet. However it would not be a fast thing. Also at least using earth as a guide for development timing, that situation would resolve most likely before much life could develop. Most likely either the smaller planet being captured as a moon by the larger, being torn up into an asteroid belt due to the opposing gravitic forces or being shot out of it's orbit.

• Second possibility though it is on a short time frame; an exoplanet drifts by close enough to affect the orbit - though that would likely affect it once and then slowly stabilize to a new normal.

Other gravity disturbances could conceivably do the same thing but a long-term (in stellar terms) situation like that I can't envision any reasonable reason.

Answer 9

Off course you can have some strange orbits, but something like this is possible in planets with ordinary orbits too. For Earth, there are Milankovich cycles and other influences that make the climate warmer/colder and dryer/wetter. However, another factor is how stable the climate is. Nowadays the climate is not very stable.

Let's compare last two winters where I live (Czech Republic). No need for exact numbers, just how they feel. For the last winter (2013/2014), there were just few days when temperature stayed below 0°C (1 to 3 weeks when added together from some two months, not at the same time), and I noticed only one day when the temperature was around -5°C and there was at least 10 centimeters of snow (i.e. what we call "winter" here). I don't remember the winter before (2012/2013) so exactly, so I'm not sure whether the "true winter" started in mid-January or early February. Anyway, it lasted for definitely more than a month, and temperatures around 0°C spanned lasted to April.

So no need for erratic orbit, just place your story in some period when the climate is even less stable than now in Central Europe.

Answer 10

Yes it's possible also with a binary star system. I tried different combinations with the software Universal sandbox2 and I can say that the only way (that I've found) to really affect the weather is by placing your planet at the same distance Earth-Sun to the closest star and have another star farther away. This second star can be the major or junior partner.

If you want something similar to Earth's climate, you could have a star similar to the Sun (G class star) or a little smaller (K star). The other star would be much smaller , like a red dwarf. A small star can be put closer, meaning that it's impact on the climates is more tangible. You can also use a larger star but even if it's several billions of km away, it's heating the planet and it could take centuries to have 1 cycle. Cycles with a smaller star would be shorter. I can't tell how much you can shorten it. At some point, the small star could make the orbit of the planet unstable. Red star could be relatively small, a couple of times the mass of Jupiter. So I guess it could be more or less at 10 AU.

To find the impact on temperature on the planet, check this page out.

Lastly, you don't have to worry about the risk of having two stars in the sky. At best, it's a close small star or a bright but faraway star. I doubt it could be larger than the full moon. To be sure, you might need to calculate the apparent magnitude of the second star...

Note: I said this was the only possibility but it's also possible to have a binary star system with both partner close to the center of the system. In that case you might have the n body problem, making the planet orbit possibly unstable. It's possible but the problem is that the movement of the star only provide a small variation in temperature and the cycle is shorter than a season.

Answer 11

Another possibility that produces varying but predictable seasons:

The planet is in a close orbit about a neutron star, it gets nowhere near the jets from the poles. This is the most massive body in the system but it's dark, it doesn't fry the planet. It's close enough that it's sure to be tidally locked so it's "year" is it's day--but this "year" is meaningless as the star is dark. Thus the inhabitants see it as a day.

There is a second star in the system in a fairly circular orbit. This defines the year but it is eccentric enough to make seasons.

There is a third star in the system, somewhat more distant but close enough to shed substantial heat. How close or far that star is has a big effect on the time of the transition between seasons--when it's close summer is long and winter is short, when it's distant you get the reverse. The year does not change, though.

Adding more stars to the mix will make it much harder to predict the seasons (but it also makes the system more unstable.) Having a planet with a non-transparent atmosphere so they inhabitants can't plot the path of the stars will also make it much harder to predict because they can't associate bright or dim stars with the seasons.

Answer 12

If you combine a highly eccentric orbit, an axis tilt like ours, a variable star like a cepheid, a short axis precession cycle and a short orbit precession one, you'll be able to make almost anything you want.

Answer 13

The astronomy of seasons can be regular, while the effective seasons (i.e. the weather, despite a constant sun position per date) varies quite a bit from year to year. This happens in many places on Earth, particularly recently, but even before industrial climate abuse. It happens because a planet's weather is complex and not only based on its orbital position, and goes through various phases and anomalies. Some planets' weather seems to be more variable than others, and/or to have shorter- or longer-term storm systems (see Venus vs. Jupiter). In the case you mentioned of Game of Thrones, there is also magic.

Answer 14

One significant thing that people haven't noted is that there is a difference between culturally established seasons and astronomical seasons. The 'growing season' isn't the astronomical 'summer', but the time between frosts, which is going to be widely different depending on where on the globe you are. Some societies, especially pre-industrial, aren't going to be that anal about marking seasons by the stars, but instead concentrating on the environmental effects that define the season.

Answer 15

I would like to suggest solar activity as one option (sun spots, etc). An unstable star, maybe an old star or something. I think a star near the end of it's life goes through a lot of oscillations beyond just turning into a world destroying red giant.

If the planet was on a very nearly circular orbit with a very straight rotational axis, the only noticeable change in seasons would come from solar activity. Basically what they would call winters and summers would be like Earth's ice ages or something. Anything that changes the climate over long periods could be the thing they call seasons. Maybe regular eruptions of some large volcano system somewhere on the planet.

An ice age could be too long. But maybe just a period of high solar activity that lasts a few years and caused the planet to be warmer and summery for a long period of time.

On the other hand, this would have no effect on the length of a day, so it would be different from winters on Earth. I don't know if they mentioned nights being longer in the winters in the books. But if not, this seems plausible.

Answer 16

A planet could have 3 axis of different length and rotate on the unstable axis. This makes it tumble, or podehole. If the tumbling is slow, perhaps on the scale of a year, you not only have different seasons depending on the current tilt and which hemisphere you end up in, but the sunrise can be in any direction too! Isn't that fun?

For a real example, look at a moon called Hyperion.

Answer 17

Under Ocean Volcanic Activity

Most answers so far have focused on a planet similar to ours, having some odd change to the orbital path. I will take a very different approach.

Assume a planet similar to ours, with the following exceptions:

1. Oceans are just as large in surface area, but perhaps only a hundred feet deep
2. Crust of planet has large cracks that lead to magma
3. Atmospheric pressure is higher
4. Elevations on the planet vary greatly

A general pattern of seasons would emerge, but with unknown length.

Water in the oceans covering the cracks dissipates heat slowly, causing higher elevations to cool. The water that does evaporate accumulates slowly in the higher elevations as snow until the oceans are too thin to keep the cracks from directly releasing heat to the atmosphere. The atmosphere is heated directly by magma (unimpeded due to increase in atmospheric pressure), increasing the temperature in the higher elevations, causing the snow to melt and run off back into the oceans. This new runoff creates new river patterns. Depending on the speed of that runoff, it will carry varying amount of sediment with it. The sediment amount and type will cause more variation in the amount of time for starting the cycle over.

Assuming the planet would have also had seasons similar to ours, the length of the two patterns intermingling will cause an unknown number and length of "winters", especially with the new pattern having varying length.

Answer 18

What about the L4 and L5 Lagrange points on the orbit of a giant planet that is in its Sun's goldilocks zone?

An object near one of these stable Lagrange points orbits around it, but the orbits are irregular and easily perturbed. They'll take the planet nearer and further from the sun as well as changing it's position with respect to the giant on its orbit.

Things get even less predictable if it's a dwarf star rather than a giant planet. They tend to be flare stars but it's the other star that's the main energy source for the planet so maybe the Lagrange planet is far enough away for life to survive the flares?

One thing I'm not sure of is whether a planet massive enough to hold its water is "small" enough to have a stable Lagrangian orbit.

Answer 19

A real-world example: in Cologne, we have four seasons (five, actually, the fifth being Carnival, but that's not a meteorological phenomena). A very short or non-existent Winter (typically a few days in the year where it's reasonably cold and maybe even snow), some Summer (which is either uncomfortably hot or non-existent; the hot state can last for days, weeks or months) and the ordinary crappy weather seasons in-between. An extreme case can be the years where we have no real Summer and no real Winter; in such case, the crappy weather season can span more than a year.

I am aware that the original question asks for >planet< having seasons of different length, not a location. However, seasons are always linked to locations on a planet. The equator regions on Earth have little to no seasons, the polar regions have very distinguished seasons. "Geographic anomalies" (oceans, mountains) can influence the local climate and the seasons quite a bit.

Answer 20

Unless I am very much mistaken, the difference between the moderate climate we had some 200 years ago and the last ice age was that the global mean temperature was 3°C lower during the Ice Age.

During an Ice Age, obviously there is much ice and snow, both of which tend to be bright and tend to reflect a lot of energy, preventing the ground below and the air above from warming.

So, consider a setting where, due to something like a fluctuation in the gulf stream, the polar regions are 1° cooler than average. This would likely result in a somewhat larger area receiving snow, which would create a little bit more cooling, and so forth. I think it is conceivable that this could lead to a mini-Ice-Age, at least regionally. If the spread reaches far enough to the south, the "corn belts" could be affected, and in turn affect the food supply of a much larger region.

Weather is quite chaotic and hard to predict, but I think that with a somewhat more fragile setup, it should be possible to have much more erratic seasons, simply based on small coincidences adding up.

Answer 21

I'm surprised no one has pointed out that the opening animation in the show displays a solar system of ring worlds around a central star and different rings at different orbital inclinations that appear to be changing over time, resulting in variations of shadow seasons for the outer rings. This is consistent with the zoomed in maps that are in the opening scenes as well where the world's curvature can clearly be seen to be consistent with a ring. I noticed it in the first episode when I watched and was a little surprised they never addressed it directly, but I watched the whole series and didn't notice any scenes that would be inconsistent with this world structure.

This discussion with the title sequence creator even mentions the concept of the world being inside a dyson sphere: Link