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I'm embarking on a fantasy book so I guess in essence you can write about anything but I do want an element of plausibility. Could you have hot desert caps on a planet and an icy/winter equator?

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  • $\begingroup$ Welcome to WorldBuilding! If you have a moment please take the tour and visit the help center to learn more about the site. A little tip for the future: it's normally a good idea to wait at least 24 hours before accepting an answer. Currently only 34 people have seen your question and you would be surprised how creative WorldBuilders can be. Some people may be discouraged from answering if they think you already found your solution. You can accept and unaccept as often as you like. Also please edit your question to provide requested information. How big is the planet? Have fun! $\endgroup$ – Secespitus Jun 30 '17 at 14:49
  • $\begingroup$ Check this out:worldbuilding.stackexchange.com/questions/73340/… $\endgroup$ – Willk Jun 30 '17 at 15:29
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    $\begingroup$ Possible duplicate of Planet with poles warmer than equator $\endgroup$ – L.Dutch Jun 30 '17 at 15:50
  • $\begingroup$ This is not a duplicate. The alleged duplicate is science-based, this one is fantasy. That opens completely different perspectives on the issue. $\endgroup$ – Philipp Jun 30 '17 at 18:41
  • $\begingroup$ coincidentally I was just reading this very similar if not duplicate question. please could you edit your question if you feel that none of the related questions answer your scenario. you can thing specify what is different in your question and we can focus on helping your fantasy story. worldbuilding.stackexchange.com/questions/21508/… $\endgroup$ – EveryBitHelps Jun 30 '17 at 18:50
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Certainly. I will assume by desert you mean "hot and arid" rather than just arid. A planet could obviously be entirely frigid with the poles significantly more arid than the rest of the planet.

One possibility could be a cold planet with limited volcanic activity which, for whatever reason, is only observable or easily measurable near the poles. The geothermal activity near the surface heats the polar regions to near boiling, effectively turning the water on the surface and in the lower atmosphere to vapor. Combined with an interesting weather stream which frequently generates high winds, the poles are a near absolute desert.

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  • $\begingroup$ Thanks for your help. Could the world be inhabited by a biped species similar to humans do you think? $\endgroup$ – antjar Jun 30 '17 at 14:25
  • $\begingroup$ Welcome to WorldBuilding! If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! $\endgroup$ – Secespitus Jun 30 '17 at 14:47
  • $\begingroup$ I tend to follow a logic for this sort of thing which is best described as an imminent and overwhelmingly likely result of evolution. The fact most species of humanoid species in my creations are 'human-like' in many ways is attributable to a commonality between the evolutionary paths of many intellligent, sentient beings which have reached a level of development similar to human beings (e.g. evolved from a water based, mild temperature planet with a magnetic cosmic ray shield, minimal tectonic activity, intelligence derived from a need to use tools, etc.) $\endgroup$ – Starrdaark Jun 30 '17 at 14:48
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    $\begingroup$ It is worth noting that Antartica is the driest desert in the world. $\endgroup$ – BobTheAverage Jun 30 '17 at 16:15
  • $\begingroup$ It is also worth noting it may be difficult to find large expanses of ground not covered in water. Frozen water it may be, but pretty sure ice and snow are just liquid water which have changed state phase. $\endgroup$ – Starrdaark Jul 1 '17 at 10:24
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The planet might have a 90° axial tilt.

That means each pole will be in direct sunlight for half a sidereal year and in shadow the other half year.

The equator will receive exactly 50% sunlight and 50% nighttime each day. But it will still experience drastic seasons, because over the course of a year the sun's height at noon will vary from perfect zenith to just above the horizon. Twice a year there will be a sunset which will take all night and directly transition into the sunrise of the next day.

The poles will receive twice as much sunlight as the equator, so they will be hotter on average. Just like deserts on earth, they will have very hot days and very cold nights. Just that both take far longer than on earth, so the temperature differences will be even more extreme. It's unlikely that much fauna and flora will survive under these extreme conditions, so they will be quite desert-like.

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    $\begingroup$ If it had a 90 degree axial tilt and rotated about that axis wouldn't it have one pole facing the star all year long? $\endgroup$ – Tracy Cramer Jun 30 '17 at 18:54
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    $\begingroup$ @TracyCramer No, it would not, for the same reason axial tilt on Earth causes summer and winter on Earth and not just one hemisphere being hot and the other being cold. $\endgroup$ – Philipp Jun 30 '17 at 19:08
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    $\begingroup$ How does it transit from one pole to the other? Isn't a 90 degree tilt essentially tidal locking that pole to the sun as it rotates around its axis? $\endgroup$ – Tracy Cramer Jun 30 '17 at 19:18
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    $\begingroup$ @TracyCramer No, it's not. The rotation of a planet is in a reference frame which is independent from its orbit around its sun. Looking from outside of the solar system, the rotation axis of a planet would always point in the same direction, no matter where it is on its orbit. So if a planet would have a 90° axial tilt, the poles would experience one solar-day each sidereal year. $\endgroup$ – Philipp Jun 30 '17 at 19:22
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    $\begingroup$ Tracy, I think I can help. You are picturing the planet rotating about the sun as if an axle were attached at the centers of the sun and planet, with the planet rolling around with one pole always pointed at the sun. Let's pretend that this example system was on a sheet of paper on your desk and you were seated at the desk. The sun is in the middle of the paper and the planet is at the top with the south pole pointed toward you (and the sun). Now move the planet to the 9 o'clock position with the pole still pointed toward you. It is no longer pointed at the sun, the equator is. Savvy? $\endgroup$ – ozone Jun 30 '17 at 19:45
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Yes, it's entirely possible.

Your planet would have a pretty extreme axial tilt, something like 50- or 60-plus degrees. You would see the following effects:

The poles would have extremely long 'days' equal to years, as we currently see on earth. The difference is that during the summer, the 24-hour sunlight would be much, much warmer, potentially warm enough to dry out the whole region - hence hot desert. During the winter, the area would freeze solid for six months.

The equator would be much cooler, since it never gets close enough to the sun to warm up. At extreme tilts, this would be similar to the conditions in the arctic circle, where even in summer it's generally not warm enough to melt permafrost. The equivalent of summer for the equatorial band would be the equinox, when the sun would be directly overhead. Whether the days at this time are long enough for ice to melt is up to you.

Between the two would be temperate bands, just as we have today. This is likely where most life would be found.

You'd also likely see some strange weather. Strong, hot winds would blow from the summer pole towards the winter pole. This could bring energy to the equatorial regions, again, depending on your design.

You could make more of a regular day/night cycle at the poles by having the star a relatively cool red dwarf and having the planet orbit much closer, making years much shorter.

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  • $\begingroup$ "Whether the days at this time are long enough for ice to melt is up to you." - Not really. The days would be 12 hours long. (Unless your planet spins slower... but in any case, it will be day half the time at the equator no matter the season.) And the so-called "temperate" bands will still be MORE frozen for half the year than the equator. $\endgroup$ – D M Jun 30 '17 at 18:44
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    $\begingroup$ "The equator would be much cooler, since it never gets close enough to the sun to warm up." This is incorrect: for about half the year (split into two parts), the equator would be closer than either pole to the sun, unless the axis of the planet was also rotating around the star. $\endgroup$ – Nathaniel Ford Jun 30 '17 at 21:38
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Two stars orbiting each other with a planet in between. The planet is tidally locked to both stars so each pole always faces 'its' star.

As some have pointed out this system is not 'naturally' stable. But, in the comments, Jose Antonio Dura Olmos suggested an intriguing notion that could introduce a dizzying array of potential plot points - the system is stable due to an ancient civilization's experiment with their new technology. After they had successfully created and stabilized this system they applied the knowledge to their home world and in the successive millennia life arose independently on this world. Or perhaps they seeded this world with microbes and intelligent life came about through evolution.

As I said, the plot points for where this could go are amazing. The intelligent life gains enough understanding that their home world is unstable and yet it exits. Who were the ancients? The system is degrading and they need to fix it. They can't and they need to come to terms with their demise. Or they go off in search of the ancients who would share their DNA.

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    $\begingroup$ That's unstable. The planet will fall into one of the stars. $\endgroup$ – Emilio M Bumachar Jun 30 '17 at 19:23
  • $\begingroup$ Really? Bummer. Sounds cool. $\endgroup$ – ozone Jun 30 '17 at 19:47
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    $\begingroup$ Yes, it is unstable and would likely never have formed in the first place. But this is world building so I thought I'd throw it out there. $\endgroup$ – Tracy Cramer Jun 30 '17 at 20:13
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    $\begingroup$ The system is the remains of an experiment by a civilization at 2.2 in the Kardashev scale. The mechanism which compensates deviations of the planet still works. In the long absence of its creators new life has arisen. $\endgroup$ – Jose Antonio Dura Olmos Jun 30 '17 at 21:30
  • $\begingroup$ See astronomy.stackexchange.com/q/11389/1559 for why it's unstable. $\endgroup$ – Mark Jun 30 '17 at 22:56
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Your planet is actually a moon with one side always facing a hot gas giant. One side of the planet would always get warmed up by the gas giant and would have two day-like periods where the sun rises on the horizon, then sets behind the gas giant, then rises from behind the gas giant and sets on the other side of the horizon, giving an effect similar to a daily eclipse. The other pole would be more temperate and have only one day-like period, where the pole faces out from the planet towards the star and gets warmed up, and where the pole faces out from the planet away from the star and gets cooled at night.

The tricky part is getting the distance from the star and gas giant just right so that, given a livable atmosphere, liquid water and solid ice can both exist on the surface of the moon. You would also need to get the orbital period to be such that you don't get nightly frost on the side that faces away from the gas giant. To make the poles arid, spin the planet; this can keep most weather from approaching the poles. Some severe storms could still form in polar regions, but these would happen much more often on the outward-facing pole than the inward, and they would last longer on the outward-facing pole. The inner pole might still get storms from time to time, but they would likely be much milder because these are more influenced by the gas giant than the star.

The result is either a moon with an icy band around the middle and deserts on both poles, or you get an outer pole that is wet, stormy, and full of jungle life and an inner pole that is a dry desert, with a massive wasteland of ice in between.

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  • $\begingroup$ It looks as if you're suggesting that the moon should spin around the axis pointing towards the gas giant. That's not really possible, since the moon is already spinning around its orbital axis (otherwise it wouldn't stay tidelocked!) and it's not possible for rigid bodies in three dimensions to spin around two axes at the same time. At best you could have the spin axis precess one revolution per orbit, but I don't see any practical mechanism that could plausibly drive such rapid precession. $\endgroup$ – Ilmari Karonen Jun 30 '17 at 21:01
  • $\begingroup$ Good point; one would need something extra, like crazy magnetic fields, to keep the moon aligned with the pole towards the gas giant, which would cause the moon's orbit to deteriorate relatively quickly. Tidally locked would probably be the best way to do this, though technically the planet-facing side of the moon and the start-facing side are not poles, which doesn't match the question, but it could still result in an icy band around the planet. $\endgroup$ – Adam Jul 3 '17 at 18:36
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This is very difficult to pull off with normal physics, because of the reasons why poles are typically cooler:

  • There is little direct sunlight constantly heating the poles.
  • The magnetosphere holds less atmosphere near the poles, meaning there is less insulation in those places. Heat radiating outward has less atmosphere to get through.
  • Counterintuitively, light coming from the sun has to pass through more atmosphere in order to reach the surface, because it's coming in at a low angle, and thus is more likely to be deflected into space.
  • Antartica is colder than the Arctic, largely because it's mostly land and at a higher elevation, whereas the heat capacity of the Arctic stabilizes the temperature at a higher level.

Therefore, to establish a warm polar situation we might do a few things. First, we note that our own poles are relatively arid, receiving in places the same precipitation as the Sahara desert. If we put land in both places which is just above sea level, we are off to a good start. We can posit we get it warm enough that snow (a primary reflector of sunlight) is not there, and we say that most of the area is covered with black sand or the like. Increased volcanic activity in this area would obviously help with this, meaning we are getting heat from both the planet core and from the sun.

This suggests a strange magnetosphere as well - if the magnetic 'pole' were not only close to the equator but rotating (rather than being mostly fixed), it would mean that heavier atmosphere would concentrate in the polar regions and also that volcanic activity was more turbid. (I have no idea if a magnetic pole would reasonably do this, but we can stretch a bit here I think.)

A significant tilt to the planet would help, but too much won't help because for a quarter of the year the area would be entirely in the dark and therefore cold, and for half the year the equator would actually be nearer the sun than either pole (because the axial tilt of a planet stays pretty constant in relation to the fixed solar plane). Some tilt, though, means more direct sunlight for part of the year.

Meanwhile, the equator will need to largely be comprised of very high mountain ranges. On an Earth-sized planet, you lose about 6.5 degrees Celsius per kilometer of height, so we'd want as much of the equator to be as high as possible. Maccu Piccu is about 8.5km up, Tibet is on average 4km, to give you an idea of what 'high' is. The surface should be largely reflective: sand or white stone, and some hostility to plant life.

To really make this work out, I'd posit some high-altitude high-albedo clouds in the equatorial regions - far more than you'd see on Earth, and periods of high-intensity short-burst rainfall in the north, where water that evaporates captures a lot of heat and forms flash storms, flushing that heat back to the surface while still trapping it in the atmosphere. Having a secondary heat source (large moon, second star, or gas giant) also helps, to get direct light on at least one polar region.

All that said, this strains credulity to the maximum, because most physics simply don't favor these conditions, especially not for very long.

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You said fantasy, so rules of physic don't apply.

The planet could be orbited by a moon on an equatorial orbit which actively cools the planet through a magic cold radiation. This radiation gets partially absorbed by the atmosphere, just like the heat radiation by the sun. The moon is closer to the planet than the sun, so latitude will have an even larger effect on the intensity of its radiation than it has on sunlight.

The orbit might in fact be so close that even the difference in distance between surface and moon becomes non-negligible (that would likely put the moon inside the planet's Roche-limit, but we already established that it's magic, so magic powers might keep it from breaking apart).

Without this moon's cooling, the planet would likely be far too hot to be habitable. The people living in polar regions might understand that and worship it as a power of good which relieves them of the scorching heat whenever it appears over the horizon. Those living near the equator, however, will curse it as an evil entity which brings them unbearable frost.

Btw: Now that you have placed a huge ice-magic related BDO in a low orbit around your planet it would be a shame if it wouldn't be somehow involved in your main plot.

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We can use a see through material at the poles as a green house. Have a see through material like quartz cover the top of the water ice and over time the heat from the sun will gather under it, melting the ice down to the surface. This will make the environment warm.

The surface should be an easily drain able material like sand. This way as the ice melts to reveal the ground the water drains down into the earth. This will provide the environment to be dry.

The planet will need a 0 degree axis to sustain this and simulate a night/day at the poles.

The rest of the planet, and even above the pole it can be a ball of ice making the planet much like you wish. As per one of your comments, this sort of environment will doubtfully create human like species.

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I have two ways this can possibly happen: The planet is tidally locked to a red dwarf and is in a polar orbit around it, or the planet is orbiting around a regular star, but it is tilted like Uranus

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