Worldbuilding Seasons on Planets with High Axial Tilts

When worldbuilding speculative fiction settings, all manner of planets are considered, from ocean planets to desert planets and everything in between, but one class of planet that you don’t hear much about is the high obliquity planet. This is somewhat surprising, considering that we have an excellent example of a large planet with a high axial tilt in our own solar system, namely Uranus. This is more than you could say for tidally locked planets, which by now are rather commonplace in science fiction, no doubt inspired by all the exoplanetary discoveries of recent decades as well as scientific studies that show that such worlds are much more habitable than previously thought.

We know little if anything about the axial tilts of planets outside our own solar system, and so this issue has gotten less attention than the likes of density, composition, eccentricity of orbits, and tidal locking, characteristics we can more easily discern from afar. For science fiction and fantasy worldbuilders, though, modifying the axial tilt has a wide array of possibilities that are seldom touched.

Fundamentals of High Axial Tilt

Earth’s tilt on its axis is 23.5 degrees or so, which is what gives us the seasons we know and love: bright warm summers and dark cool winters of equal length. Seasons can easily be unequal in length with different setups; eccentric orbits cause longer winters and shorter summers, for reasons we might explore in another post. Anyway, we already have a helpful prototype for how axial tilt impacts climate on an Earth-like planet, namely Earth. Increasing the axial tilt naturally leads to more extreme seasons; a tilt of, say, 30 degrees instead of 23 would lead to the temperate zones experiencing, relative to today, winters with longer nights and colder weather and summers with longer days and hotter weather. Such a change would certainly be noticeable, although not drastic, if it happened to our own planet.

The most obvious effect would be on the sun’s path through the sky. The tropics, the highest latitudes where the sun appears directly overhead at any point during the year, are 23 degrees away from the equator because Earth’s tilt is 23 degrees. Similarly, the polar circles, the lowest latitudes that experience polar night and midnight sun, are 23 degrees away from the poles (and 67 from the equator). A tilt of 30 degrees brings the tropics up to 30 degrees and the polar circles down to 60 degrees.

Once axial tilt exceeds 45 degrees there is no more temperate zone in terms of sunlight; under a 45 degree axial tilt both the polar circles and tropics are at 45 degrees latitude. At 45 degrees latitude in this world on the summer solstice the sun peaks directly overhead and doesn’t quite sink below the horizon at midnight; on the winter solstice the sun doesn’t quite rise above the horizon at noon. This constant day and night will produce more extreme seasons, but at these ranges of tilt the Earth will still be rather Earth-like; the tropics will be hot and the poles will be cold year-round. This doesn’t remain the case at higher levels of tilt.

A World Turned on its Side

At higher tilts, such as 85 degrees, which was modeled in this excellent 2003 study, the tropics rise to 85 degrees above the equator, while the polar circles drop to 5 degrees above the equator. The bulk of the planet now experiences half a year of constant daylight and half a year of constant night, with only brief periods near the equinoxes featuring a “normal” day-night cycle. At these levels of tilt the poles experience the most intense sunlight of any part of the planet during the summer, while the tropics experience something closer to perpetual twilight. This effect is so pronounced that at truly high tilts the poles actually receive more total solar energy than the tropics! Thus as measured by annual mean temperature the tropics are the coldest part of the planet.

If this scenario were to apply to Earth, according to the 2003 study the northern parts of the ocean, holding more heat in from the summer months and having a higher annual mean temperature anyway, never quite fall below freezing! This eliminates the arctic ice caps, and the toasty polar oceans keep the coastal regions warm enough in the winter to prevent persistent snowpack. In North America the Desert Southwest has the coldest winters, at under -10 degrees Celsius, with the American Rockies being the northernmost extent of the winter snowpack. Greenland and Alaska actually have the warmest winters on the continent. During the summer the arctic really heats up, with parts of northern Canada having the hottest summers, averaging over 80 degrees Celsius. That’s over 170 degrees Fahrenheit! All of Canada, Alaska, and Greenland average over 50 degrees Celsius, or around 120 degrees Fahrenheit. Central and southern Mexico have the coldest summers, averaging near freezing, with snow persisting over the summer and forming glaciers in the higher altitudes of southern Mexico.

Eurasia is much the same as North America, but more extreme. Almost all of Africa and Eurasia is cold enough for snow in the winter, with Tibet having the coldest winters (at -30 Celsius) and coldest summers (-10 Celsius), spending the entire year well below freezing and covered in an ice cap, by far the Earth’s largest ice cap in this scenario. Siberia, as is the case today, has the most extreme seasons, ranging from around freezing in the winter to, in parts of the north, well above 90 degrees Celsius in the summer. That’s around 200 degrees Fahrenheit, almost hot enough to boil water!

The Antarctic has less extreme seasonal variation than the Arctic does, ranging from somewhat below freezing in the winter to around 20 degrees Celsius in the summer. The southern parts of the ocean range from 10 degrees Celsius in the winter to perhaps up to 30 degrees Celsius in the summer. The Southern Hemisphere on such an Earth would be the largest zone that still has a climate most people would recognize as Earth-like; the maritime influence that keeps its temperature almost the same year-round today (as opposed to the conventionally Earth-like north) is only strong enough to keep it Earth-like (as opposed to the alien extremes of the north) in this scenario.

Even in the Southern Hemisphere six months of darkness and six months of daylight would still seem strange to most Earthlings of today. Even so, it doesn’t compare to the Arctic coast, which features the utterly alien combination of warm snowless winters and months of constant pitch-black night.

Science fiction, alternate history, and fantasy worldbuilders can certainly see the possibilities here. What sort of vegetation could survive the extremes in the north (or an equivalent region of an alien planet) is left as an exercise to the artist that wishes to accept the challenge, though one opinion I will offer is that among forests the trees of the taiga have the best chance to survive extreme seasons.

Summers hot enough to boil water are certainly an interesting setting to explore, as perhaps most prominently done by Frank Herbert with the Dune saga’s planet Arrakis. Unlike Arrakis, where it is that hot or even hotter most or all of the time, having only a part of the year where it is so hot and another part of the year where it is more temperate makes it much more plausible that Earth-like life can survive or even thrive. Most forms of life could simply go into hibernation (or estivation in this case, being over summer instead of winter) or some form of stasis during the summers, emerging once it becomes cooler.

Tilted Earth in Mars’s Orbit: A Super-Siberia?

An even more interesting setting might be on an alien planet, similar to Earth in this scenario, located in a colder part of its solar system than Earth is. After all, the same extremes that produce a temperate winter and an extremely hot summer could produce a temperate summer and an extremely cold winter. All we need is to lower the average annual temperature, easily accomplished by moving the planet’s orbit further out.

Simply moving Earth’s orbit out to Mars’s orbit (without accounting for albedo (reflectivity of the surface) effects) lowers the average temperature from 15 degrees Celsius to -40 degrees Celsius, which would cause Earth to become an ice planet like Hoth from Star Wars; even the equator would be glaciated, though relatively clement. However, high axial tilt changes this; a general 55 degree reduction in temperature cools the 90 degree Celsius summers in Siberia down to 35 degrees Celsius. This is around 90 degrees Fahrenheit, which while still very hot (even at Mars’s distance!) is well within the Earth-like range. Winters in northern Siberia cool from near freezing to -50 degrees Celsius, yielding a climate similar to parts of modern Siberia but with hotter summers. Northern Canada, interestingly, gets a climate remarkably similar to parts of modern Siberia; even the summers are about the same.

The Arctic Ocean, meanwhile, stays frozen year-round assuming a uniform temperature reduction, with perhaps some of the coast melting in the summer, much like today. Unlike today, however, the rest of the world’s oceans will be cold enough to develop ice cover year-round, with the possible exception of a warm belt in the southern oceans during the summer there. Indeed, all land south of Canada and Russia would never warm up enough to melt snow during the summer, and thus would likely be glaciated. Tibet, the coldest part of the world, plummets down to -90 degrees Celsius (-130 Fahrenheit) during the winter, colder than modern Antarctica, albeit not by much.

So we have constructed a world that is an ice planet but with a summer strong enough to thaw out the northern lands, permitting the habitation of Earth-like life in these regions. The southern oceans may also thaw out, with life adapting to these conditions; migratory birds (or similar creatures) could decamp from the frigid north in autumn and subsist off the thawing south’s rich waters when it’s summer there, while less-mobile creatures in the north hibernate or face more difficult hunting conditions. Vegetation similar to the taiga may prevail in such a world, since the climate in the temperate lands will be remarkably similar to present-day Siberia.

The rest of the planet will resemble the Antarctic, and be frozen over year-round, excepting the warm belt in the southern oceans, which will thaw out to just above freezing during the summer. This is all accomplished merely by turning Earth on its side and moving it to Mars’s orbit! Keep in mind that these calculations assume a uniform temperature swing, which won’t occur in real-life due to feedback effects from ice cap buildup and the like, but the general concept should be viable both in real-life and in fiction. In particular this strikes me as a great place for a fantasy setting: a vast taiga surrounded by an ice sheet in every direction with six months of day and six months of night sounds really cool.

This is probably about as extreme a setting as you could construct with axial tilt alone; thinning the atmosphere or adding more land at the expense of sea would make it more extreme, but not by enough to change the setting all that much without taking it outside the Earth-like range. There is another possibility, though: orbital eccentricity.

Maximizing the Seasons with Elliptical Orbits

As mentioned earlier, making the orbit more elliptical produces homogeneous changes over the whole planet; the entire planet gets warmer when closer to the sun (perihelion), and the entire planet gets cooler when further from the sun (aphelion). Due to the laws of orbital motion, planets move faster when closer to their sun and slower when further from their sun, so eccentric Earths will have long winters and short summers. Where this helps us is that an eccentric orbit can be combined with a high axial tilt.

This can work any way you want; for example, perihelion could occur during northern summer, making the northern summer much more extreme, or it could occur during southern summer, making the southern summer much more extreme and the northern winter much more temperate. Perihelion and aphelion could also occur near equinox, muting their effects on the seasons. On Earth, aphelion occurs in July, meaning southern seasons are slightly more extreme than northern seasons than they would be otherwise, but this effect is too small to be significant. Mars, on the other hand, has a more eccentric orbit, so southern seasons on Mars are significantly more extreme than in the north.

How this helps our scenario is that if you want a setup like the planet above, an ice planet with a summer thaw in the north permitting a vast boreal forest there, but with more or less extreme seasons, you can do that by modifying the orbital eccentricity. For more extreme northern seasons, hotter summers and colder winters, have aphelion be in the winter and perihelion be in the summer. Keep in mind that this will cause most of the year to be colder, especially in the winter, with a sub-seasonal “dog days” period in the summer when temperatures will spike to their maximum. Also this setup will make southern hemisphere seasons less extreme; the summer that was strong enough to thaw out the southern seas previously now occurs at aphelion, and so the south remains completely frozen year-round.

Conversely, putting aphelion in northern summer and perihelion in northern winter makes the south’s seasons more extreme, yielding a situation where both temperate bands warm up enough to thaw the ice during the summer, but without ever getting truly warm like in most of today’s taiga and in the “circular orbit” scenario.

The Ultimate Limit of Winter Habitability

Another way to use this technique would be to keep the summer temperate but make the winters even colder; a combination of higher eccentricity and moving the semi-major axis (the overall average distance) even further away from the sun would accomplish this. At the most extreme end, the planet could have an orbital eccentricity more like a comet, with a period of a few days of Siberian-like summer weather at perihelion combined with a years-long outer-solar-system-like winter.

At very high (but not necessarily comet-like) eccentricities, combined with the planet swinging further out in northern winter, an Earth-like or even temperate summer could be coupled with truly exotic winters well outside the Earth-like range. How exotic? Colder than -78 Celsius carbon dioxide freezes and can fall as snow, much like it has done on Mars to produce the dry ice caps found there; since that’s -108 degrees Fahrenheit and parts of Antarctica regularly get that cold in the winter, dry-ice precipitation would have to occur on Earth but apparently isn’t that noticeable. This may be because carbon dioxide has a very small presence in the atmosphere compared to water, so there’s less of it to precipitate in the first place.

One might think all life would freeze in such weather, but the ecosystem in Yakutia (the coldest part of Siberia in winter) apparently is unfazed by -67 degree Celsius temperatures (the record low in that part of the world). It seems that with enough insulation creatures can withstand almost any cold, though survival in such conditions may require adaptations similar to Earth’s arctic and subarctic animals but more extreme. Worldbuilders that like to design creatures take note!

The real lower limit to Earth-like animals prowling around in the open is probably when oxygen starts to rain out of the atmosphere. This is really far down the temperature scale, as oxygen remains gaseous down to its boiling point of -183 degrees Celsius, or -297 degrees Fahrenheit. When the gas you breathe starts to condense and turn into rain, the point at which it’s rained out enough so that there isn’t enough gas to breathe is the absolute limit of survivability. Oxygen freezes at -218 degrees Celsius, or -361 degrees Fahrenheit; at that point it’s very unlikely there would be enough oxygen left to breathe, so somewhere in between these two numbers is likely the absolute lower limit for winter lows. Any form of life that breathes oxygen and isn’t in a high-grade form of stasis (like a tardigrade) will not survive such a winter.

The most extreme scenarios with such a planet, that border on the survivable for any animals still out in the open, might have northern hemisphere winters feature oxygen raining onto a hard-as-rock water-ice snowpack during the heart of winter, as temperatures drop below -300 Fahrenheit or so. Naturally it would be completely dark throughout the day, excepting any moons shining, which would be high in the sky at full phase; full moons follow the opposite track in the sky as the sun, so a sun that is almost exactly underneath you at the winter solstice would produce a full moon almost exactly overhead. At such high temperatures most of the oxygen would normally evaporate before it hit the ground, producing some beautiful virga if a moon were illuminating it over the frozen landscape.

As you can imagine this would be especially fruitful ground to explore for a fantasy world, as has been done to some extent in A Song of Ice and Fire, which features ultra-long seasons. Oxygen rains are at the most extreme end, requiring a very long trip far into the reaches of the outer solar system to get sufficiently cold, but even this extreme is perfectly plausible even using real-world science. As in there might actually be (and probably are) planets like this out there for us to find.

There is of course also the other extreme, a long temperate winter combined with a short summer well above water’s boiling point, which could certainly be explored but isn’t as cool as the various frozen landscapes and glaciers possible with the cold-winter version. In this scenario Tibet in the winter becomes the temperate habitable region, with everywhere else remaining boiling year-round and Tibet itself boiling off in the summer. The cooled wet mountainous roof of the world looking down a desertified hellscape beneath them is a cool vision. Not as cool as the icy glaciers in my view, but that too might be a fruitful avenue to explore if you’re more interested in that sort of thing.

Conclusion

So we see here that axial tilt alone is a greatly underappreciated part of worldbuilding a fascinating and alien, yet still Earth-like setting, much like tidal locking was a few decades ago. Interestingly from the equator’s point of view the effect isn’t even that different; in both cases it is in a sort of twilight zone, albeit in this case with the sun swinging to directly overhead near equinoxes, something which never happens on tidally locked planets.

High axial tilt alone turns Earth from mundane to quite alien, with the Amazon and Congolese rainforests becoming snowy arctic climes and the Arctic region itself becoming a boiling desert in summer far hotter than anything seen on Earth since at least the Messinian Salinity Crisis. Moving Earth further out in conjunction with tilting it on its side turns it into a planet characterized by a continent-sized island of Siberian taiga in a sea of ice and snow, which would be a fascinating setting for a speculative fiction work, especially fantasy.

It may offer something for everyone. The closest this concept has come to even approaching a mainstream breakthrough is A Song of Ice and Fire, but as we see science fiction, fantasy, and speculative fiction authors, worldbuilders, and artists have only tapped a small fraction of the creative power of these ideas. I strongly encourage artists, worldbuilders, and other creators looking for a creative and unusual idea for their work to fill this relative void in the annals of speculative fiction.