Just imagine a world in which seasons could last for years and in which long summers are followed by still longer winters. Seasons which no one could say how long will they last and when they will be over. Does this sound familiar?
For sure, most of you have already understood that I am speaking of the world created by G.R.R Martin for his Chronicles of Ice and Fire or the TV show Game of Thrones.
At this point, I can imagine that many of you will be thinking: weren't you an astronomer? Actually, I am. And I still haven't change my job, but often many stereotypes have some truth. I am also a great fan of fantasy in general and of these books and the TV show in particular. Therefore, I found it quite natural to ask myself the following question, which recently became the subject of discussion with my colleagues and supervisors between a coffee break and another: could a situation like the one present in Westeros be physically possible?
Let's see. First of all, let me briefly explain the nature of the problem for those who are not aware about the wars between Starks, Lannisters and Targaryens. Basically, as I said before, we are speaking of a fantastic world in which the story is mainly placed on two continents, Westeros and Essos. The main characteristic of this world concerns the duration of seasons. What emerges from the books is that these can last several years and their length is not fixed, varying from season to season.
Before trying to understand which physical configuration could allow a planet such a behaviour in the climate cycle, let me specify something. Everything that will follow from now on is just based on several acceptable hypothesis, but do not believe this is an absolute truth. In particular, none of us made real calculations which, speaking of physical theories, is the only way to ensure definitive answers to our questions. That is to say, this is just a nice game between fans and it would be interesting to know your opinion as well.
Let's then start from what we know, based on what it is written in the books and in the TV show. Westeros (and similarly Essos, placed to the east) is a continent in which the climate is in some sense similar to our northern hemisphere. In fact, we can find an extremely cold north, temperate areas in the central part and a hot south. In the far north there are the so called "lands of always winter", where ice is eternal and that we could assume is near the north pole of the planet. Also in the south there are the "summer islands", where people with dark skin live, so we could assume that they are very hot and placed probably near the equator. Moreover, we know that there is a sun and a moon, like for the Earth.
At this point it could be useful to do a quick summary on some concepts. On the Earth, seasons are caused not, as many people think, by the distance of our planet from the Sun (that varies not so much, since the orbit is practically circular). Instead they are caused by the inclination of the rotation axis. Solar rays beat on the surface of the planet at a different angle, with respect to the position of the Earth, during the orbit around the Sun. For the same reason, seasons in the two hemispheres are flipped. To be precise, the duration of terrestrial seasons is not always exactly the same, due to several reasons that are beyond our purposes, and anyway it tends to vary in time. But these changes are too slow to be appreciable during a lifetime. There is no way, in such a configuration, to obtain a situation similar to Westeros.
So, I started to think of an orbital configuration that could be responsible for an irregular variation of the seasons cycle. My first hypothesis has been to fix the length of the years, on Martin's planet so that it is coincident with terrestrial years (honestly this assumption is not necessary, but it is consistent with what we can extrapolate from the books). Then, my idea was the following: let's imagine that the rotation axis of the planet is not inclined, but practically perpendicular to the orbit. In this case, we will have no seasons. But on the Earth there are phenomena, as I said before, that modify the duration of seasons and cause climate changes over millenniums. Such phenomena, like equinoxes precession and axis nutations, could be present anyway, at the cost of a very tiny axis inclination. On Earth they are considered jointly responsible for glacial eras. Therefore, if we assume them to be more extreme on the temporal scale and we combine their effects with the normal stellar activity and the variation of the magnetic field, they could cause very irregular and hard to predict mini-glacial eras.
But this hypothesis has a huge bug, as my supervisor Kai Polsterer told me, that I had not noticed. A planet with a rotation axis perpendicular to the orbit will be subject to tidal effects due to the gravitational force of the star, which will probably cause the planet to become tidally locked. Basically, it should be the same as what happened to our moon: the rotational speed around the axis becomes equal to the revolution speed, and therefore the planet always shows the same face to the star. The consequence is quite simple, no day and night cycle. A similar case, for example, is found in the recently discovered TRAPPIST-1 system, and scientists are still discussing the possibility that such a configuration could be able to support life. Anyway, we know from the books and from the show that in Westeros the day and night cycle is absolutely normal, so this hypothesis has to be discarded.
Do we have to conclude that there is no possibility to one day find a planet adapt to wait for the long winter? Not necessarily. From every side we look at the problem, in an orbital configuration similar to Earth, there is no way to obtain such irregular seasons. But the orbit is not the only thing responsible for climate changes. The cause could be found instead in the star around which the planet is orbiting. We know that the solar activity, characterised by sunspots and flares, influences Earth's climate, but in a limited way because these variations are quite slow.
If we now imagine a star in which this activity is more prominent, we could obtain the desired effect on the seasons. Let's think about the following situation: a planet with an inclined rotation axis, so that the day and night cycle is preserved, but with a smaller angle, in order to allow minimum seasonal variations of the climate. If the star is characterised by the presence of sunspots that in certain moments cover a great part of its surface, then irregular and unpredictable long winters and summers could be possible. But now the question is: do these stars really exist? For what we know, there are stars with sunspots, or stellar spots to be more precise, which cover up to 30% of the surface during the periods of strong activity. Moreover, it seems that in some stars the coverage could reach more than the 50% of the surface. If we consider that the difference in temperature between spots and surface is about 2-3000 kelvin, it is clear that a planet in the habitable zone will necessarily suffer the influence of these variations.
During our chats another hypothesis came out, which maybe even more fascinating. It is a configuration in which a planet is placed in a triple system, so the orbit, assuming it stable (this is absolutely not easy), could be so irregular that the seasons cycle will be completely disrupted. Obviously many of you will now be thinking that in Westeros there is just one star and not three. The solution to this problem could be, for example, a system in which two components are black holes and only one a main sequence star. It is a rare event, but not impossible (we have to remember that most of the stars are in multiple systems).
Clearly, all these hypothesis, after analysing them carefully, will be non robust and one could always find contradictions, because the only clear explanation is the magic that a clever author has transferred on the paper, creating a fantastic world.
But I believe that these games are useful, because they force us to think about all the strange things in the Universe, and about the incredible stuff we could still find out there. Human beings have always tried to adapt what we see on our small grain of dust to the vastness of the Universe, assuming that what we observe is normal. But things can be quite different and every time we look a bit farther, we find something able to astonish us.
Finally, it is interesting to think of our luck. Because life on this planet is based on a very tiny and delicate equilibrium, not just limited to the distance of the Earth from the Sun, which places us in the habitable zone. A certain axis inclination, "non extreme" solar activity and an almost circular orbit, allowed the possibility to generate and develop life as we know it today. And perhaps we should consider ourself lucky that we have not need to pronounce the well known sentence "winter is coming!"