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I want to create a planet similar to Earth but with narrow temperate bands between much wider tropical and polar bands. How can this be achieved?

The planet must be similar to Earth and capable of sustaining human life in the (diminished) temperate bands.

The polar regions must have ice and the tropical region deserts.

The planet rotates roughly once every 24 hours and is not tidally locked.

related questions:

Habitable planet with extreme hot and cold regions that is not tidally locked

How Could A Planet With Extremely Enlarged Subtropical Zones Exist?

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  • $\begingroup$ My assumption would be an overall warmer planet (as in the second related question) with more severe axial tilt and perhaps a longer year. I'm not confident enough to post this as an answer. $\endgroup$
    – rek
    Oct 4 '20 at 15:32
  • $\begingroup$ "Desert" does not impose any temperature restriction, only rainfall. There are plenty of deserts in artic and near-artic conditions. $\endgroup$
    – Mary
    Oct 4 '20 at 16:27
  • $\begingroup$ @Mary Very true $\endgroup$
    – Slarty
    Oct 4 '20 at 22:06
  • $\begingroup$ There is a rather simple solution, don't put continents in the temperate zone. $\endgroup$
    – John
    Oct 10 '20 at 4:08
  • $\begingroup$ @John, yes that would work, however for my purposes they need to be connected on land. $\endgroup$
    – Slarty
    Oct 10 '20 at 15:21
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You are looking for a planet with lots of tropical zones, lots of polar zones, and little to nothing in the temperate range. I'm not sure how much you can manage without artificial intervention, but I can put you on a good path at least.

Before I start, I'll make sure those zones are properly defined; temperatures here are monthly averages, not minimums or maximums. Tropical regions want an average temperature above 18C year-round (that is part of the definition of a tropical climate). Arctic regions want to average less than 10C year-round; tundra gets above 0C in some months, while polar zones are below 0C all year. You've lumped temperate and continental climates into the same basket, so I'll separate those for clarity: temperate zones remain above 0C year-round, but in at least some months fall below 18C, while continental zones fall below 0C in some months but exceed 10C for at least their warmest months.

Your goal is to maximize arctic and tropical climates. Those are the two types with simple year-round measures: they don't look for yearly variation. Your best solution is also quite simple: have a planet without any real axial tilt. This does mean you lose any concept of the conventional four seasons most of us know: summer and winter won't exist as separate things. If you instead wanted to maximize temperate and continental zones, it would be the opposite: you'd want a large axial tilt (although pushing too far beyond what Earth has isn't advisable, because you get some bizarre climates if tilt is greater than 45 degrees).

Your other parameter of choice is the planet's distance from its sun: if you want to exaggerate this dynamic farther, move the planet closer to its sun and make the polar regions mountainous (higher altitudes generally mean a plunge in temperatures). Doing this will likely render the equator too hot to be feasibly survivable, though if you want that to be impassable without technology it's a bonus.

Deserts are a function of precipitation, not temperature; there are cold deserts as well as warm ones. Having deserts in your tropical zone is easily explained; look no farther than the Sahara desert right on Earth, a large portion of which is in the tropical band. If it's too hot, you can also stop precipitation (you aren't likely to get water vapour condensing into rainfall at 40C). If you want a desert in a specific place, you'll want an understanding of wind patterns and probably a carefully positioned mountain range, but without seeing your world map I can't give you any details on that. Incidentally, arctic regions are technically also deserts in most cases: their snowfall per year is a rather tiny measure.

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Dense rings and a tight orbit around a cold star

The planet is in a tight orbit around its tiny, coldish, host star - "years" are a few hours, (the "24 hour day" is actually a beat in the day / year rhythms.)

The planet also has an extremely thick ring system, and as the planet goes through a "year" in a few hours, with axial tilt, it goes through summer and winter every few hours. The rings shadow keep the higher northern latitudes cold for half the "year", and the lower southern latitudes cold for half the "year".

The end result is 2 sharp lines of latitude where the total amount of daylight is halved (the sun rises and sets behind the rings every few hours).

The close orbit makes things blindingly hot along the equator. And there's a tiny temperate zone straddling the two.

Partial terraforming

The planet is a little outside of the suns habitable zone, just a little too cold. A resourceful race comes along and notices a valuable ore in deep deposits below the frozen equator. Rather than tunnel hundreds of shafts through 10km of frozen solid planet, they decide to make the ice go away. So they build orbiting mirrors in geostationary orbit to reflect extra sun onto the planet, focusing on the equator, as that's where they want to mine first.

And then, nothing. The project was cancelled. The mirrors are still there, blasting heat around the equator onto the frozen planet.

The end result is big polar frozen regions stretching a long way, a big desert region around the equator where the mirrors have over melted, and a tiny habitable zone straddling the two

Shiny, elliptical moons

If aliens are a hard no, a bit more of a stretch but don't involve aliens, we can replace the mirrors with shiny, metallic moons.

A number of highly reflective moons In equatorial orbit could reflect light onto the equator at night time. The moons are slightly elliptical so the light is distributed more over the equator than the rest of the planet, making it very hot. Otherwise the setup is same as terraforming aliens.

Polar volcanoes

Active volcanoes at the poles keep the sky outside of the tropics covered in an ash cloud, resulting in year round freezing temperatures.

The hot sun keeps the equator very hot. There's a tiny zone just under the edge of the ash clouds where the sun doesn't boil you, and you don't freeze. Except for the epic sunsets, the ash is too thin to see with the naked eye.

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One way to accomplish this is to change the planet's axial tilt. On Earth, the tropics of Capricorn and Cancer lie 23.5 degrees north and south from the Equator. 23.5 degrees is the axial tilt of planet Earth, so we can develop the formula t = a, where t is the deviation of the tropical circles from the Equator and a is the axial tilt of the planet. Now onto the Polar Circles, which on Earth lie 66.5 degrees North and South. In other words, we can make the formula p = 90 - a, where a is the axial tilt and p is the deviation of the Polar circles from the Equator. Now for your case, you'd want the axial tilts close to 45 degrees for a small temperate zone. Too low or too high and the temperate zone will be larger. Just note that the more extreme the axial tilt is, the more extreme storms will get in the spring and autumn on the planet. For evidence of this look at Uranus. Scientists detected extreme storms during spring on the ice giant, so a similar phenomenon will happen for your planet if it has too high an axial tilt.

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