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I have wanted to write a story before about people on an arctic planet and other animals, particularly reptiles metamorphosing into polar bear people who hibernate through the winter.

But is it possible for a planet in the habitable zone to have a polar climate throughout the planet and all the time? If so, what would be required? Are there any gases that would cool the atmosphere, thus giving a reverse greenhouse effect? Would it have to rotate faster to make winds stronger and thus a colder wind chill?

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  • $\begingroup$ Yes it would be possible. If the planet was farther from the sun, but the equator region would be warmer $\endgroup$ – Bryan Feb 20 '18 at 23:49
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    $\begingroup$ You are using specific words, such as "arctic" (= northern polar) and "polar" (= north of the northen polar circle, or south of the southen polar circle), when you mean "cold". So no, "polar" bears at the equator are absurd, an entire "arctic" planet is absurd, a world-wide "polar" climate is absurd. On the contrary, is is perfectly reasonable and possible to have white bears at the equator (they would be just bears, not "polar"), a planet-wide cold climate (as Mars has), or an entire world covered in ice (as Earth was in the Cryogenian period). $\endgroup$ – AlexP Feb 20 '18 at 23:51
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    $\begingroup$ Given that you want some plants to produce oxygen and that your people need to eat something, would it be enough that all the continents are in polar areas? Everybody would then live on the coast of the polar continent eating sea food. There would be warm tropical oceans keeping the planet habitable but people would never go there. Or at least nobody would ever come back. $\endgroup$ – Ville Niemi Feb 21 '18 at 1:31
  • $\begingroup$ The base of the food chain (on land) is plants. You may notice that plants don't grow when it is below freezing. How do your polar bear people live and eat? $\endgroup$ – kingledion Feb 21 '18 at 2:17
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    $\begingroup$ @AlexP If I'm an astronaut and I see alien, white-furred, bear-like creatures fumbling around at the equator, imma call 'em "space polar bears", geographical correctness be damned. $\endgroup$ – AmiralPatate Feb 21 '18 at 8:19
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If a planet is far enough from its star, it will be cold, water will be frozen etc. To have plants, you'd need liquid water for at least part of the year (something like the tundra, at least.), or you'd need liquid seas where you'd have algae. If you don't have plants, you can't have all the higher order of organisms.

Winds are produced by differences in air pressure, which would be related to temperature, not to how fast the planet spins. I'm not sure how lower temperatures would affect that.

Basically, look at Earth, then stretch the polar areas towards the equator. In the depth of Antarctica, nothing lives - too cold. On the shore near the ocean, you have penguins. They live on earth, but they hunt in the water. In turn, they are also hunted in the water - there's a whole ecosystem there, with algae, krill, whales and more. All underwater, where it's warmer (marginally) and water remains liquid. In the arctic area, it's much the same - polar bears live on the ice, but hunt in the water, where you also find algae, fish, whales etc. The tundra is much more lively - plants, birds, insects, mammals - but only in summer. Once it gets too cold, everything either goes into hibernation or migrates south.

So, you can have cold habitable areas near your equator and uninhabitable large poles. You can have an ice desert with hot-spring oases. You can have the whole planet covered in ice, no liquid water, but then it wouldn't be habitable. Thing is, our cells are filled with liquid water. So if you don't have liquid water somewhere, you can't have a living cell. If you can't have a living cell, you can't have a multi-cellular organism that would produce and preserve body heat, which would allow it to live in more extreme environments.

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The actual Earth suffered from one or more global ice ages during the period known as "Snowball Earth". Early in the Earth's history, the Sun was much less energetic and delivered less warmth. With a luminosity as low as 30% of todays, sudden changes in climate, such as the "great oxygenation" would result in massive positive feedback loops, including runaway glaciation as the ice reflected ever more radiation into space. The Earth's equator may have resembled modern Antarctica at the height of the glaciation.

enter image description here

Possible reconstruction of Snowball Earth

So in general you would need to have less solar energy reaching the planet to begin with (smaller, dimmer star or being farther from the sun), and a sudden drop in either the ability of the atmosphere to hold radiation (such as the great oxygenation) or a runaway feedback loop caused when there is the natural onset of an ice age due to a Milankovitch cycle. Either way could lead to a period of tens of thousands to millions of years of intense cold.

The downside of Snowball Earth is the frigid conditions are very hostile to life, and during the historical period, life seems to have retreated to extremophile bacteria, algae and other very primitive life which can survive with minimal energy inputs. Polar bears or penguins are not going to survive for long in this environment.

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Below is a terrestrial planet with an axial tilt of 58°. As a consequence of this the polar circles (32° N&S) are closer to the equator than the poles, and the tropic circles (58° N&S) are closer to the poles than the equator:

Diagram of planet with 58° axial tilt

Each pole takes turns pointed toward and away from the sun for about half the year, as on Earth, but because the tilt here is more than twice that of Earth the winter freeze actually extends to and beyond the equator, to the summer half of the planet (depending on geography – on Earth Arctic sea ice can reach as far south as 45°N and Antarctic sea ice as far north as 20°S).

This planet is also farther from its sun than Earth so it receives less energy, has a thinner atmosphere, which retains less heat and produces less of a greenhouse effect, and may also have a shorter year and day than Earth. By combining some of these variations it's likely snow and sea ice dominate the landscape for most of the year for most of the equatorial region, and it's entirely possible winter snow and ice could be found year-round.

Note: This assumes a planet with significant land in the area of the poles. Climate at high obliquity (PDF) concludes (in section 6. Conclusion and Appendix A) that an ocean world with axial tilt of 54°-90° would largely remain above freezing thanks to the thermal transport properties of the oceans:

Our simulations employ a configuration without any land. Although it reproduces (at 23.5° obliquity) the main features of present day climate, it is plausible that particular continental configurations would have a first order impact on the climate, for example in cases where large areas of land are removed from all oceanic influences (e.g. a large polar continent). In such cases, only the atmospheric heat transport could modulate the extreme seasonal fluctuations, possibly resulting in temperature excursions beyond the habitability range.

For a cold-adapted species the most habitable area of this would would be roughly 32°N–32°S, extending poleward in the winter and as local geography (such as elevation) allows.

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is it possible for a planet in the habitable zone to have a polar climate throughout the planet and all the time? If so, what would be required?

A serious reduction in the amount of greenhouse gases.

If the temperature goes up from an increase in CO2, then it will go down from a decrease in CO2. Decrease it enough and the winters will get longer and more severe. The extra snow will reflect more energy back into space, and so the planet won't warm up as much. This will lead to a vicious cycle leading to very cold temperatures.

It wouldn't be very habitable, though...

enter image description here

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  • $\begingroup$ This answer could be expanded a bit; the greenhouse contribution to climate and temperature is significant, but you've only presented one aspect of it. $\endgroup$ – rek Feb 21 '18 at 19:43
  • $\begingroup$ @rek unfortunately, I don't know much/any more than this. What more needs to be said? $\endgroup$ – RonJohn Feb 21 '18 at 19:47
  • $\begingroup$ There are other greenhouse gases, such as methane and water vapour, and the greenhouse effect is also a product of atmospheric density, so there are more variables than just CO$_2$ to consider. $\endgroup$ – rek Feb 23 '18 at 21:10
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    $\begingroup$ @rek very true. My first sentence mentioned "gases". Methane is (relatively; half-life of 7 years) short lived in the upper atmosphere (so once "life" slows down and organic matter is trapped in the permafrost). And the absolute humidity will drop as the temperature drops. Regarding atmospheric density: not every factor controlling temperature needs to drop in order for the temperature to drop significantly and the climate to get caught in a vicious cycle, just as the Earth is now, in the opposite direction. $\endgroup$ – RonJohn Feb 23 '18 at 23:45
  • $\begingroup$ It's a good answer. Not sure why the downvote, perhaps just changing $CO_2$ to the more general "greenhouse gases" would satisfy. $\endgroup$ – Samuel Mar 7 '18 at 0:16

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