I am writing a science fiction movie script and a planet idea came to me a few weeks ago: A planet with equatorial icecaps and polar jungles. As interesting and intriguing as this is to think about, it has been the bane of my sanity as I try to find a way to make this planet even PLAUSIBLE.

Welcome to Nabirmo:

Nabirmo as seen with its equatorial icecaps, alpine forests, mid latitude deserts, seas, and polar jungles.

My best attempt to reconcile this fantasy world to within sight of reality is as follows:

  • Nabirmo is a moon of a very low density gas planet which is in an elliptical orbit around a small star.
  • Nabirmo's tilt on its axis is nearly 45 degrees
  • Most of the planet's orbit lies within the habitable zone, except for closest approach and again at the farthest reach.
  • There is a resonance between the time it takes Nabirmo to orbit is parent planet, and the length of its year.
  • The star system may or may not be part of a binary system.

So how does this work out? And where are the questions? Be patient.

I am well aware that for just about any planet not completely tipped over on its axis, that the equator receives more energy from its star than the poles. So in order to combat this I had to find a way to "block" this energy from its star when it would usually shine on the equator. The moon's axis of rotation is lined so that at its closest and farthest away points the equator is lined up with the star (our equivalent of spring or fall) and on approach or the receding parts of the orbit one of the poles are titled toward the sun, where the jungles are.

"But how would this limit the amount of sunlight received by the equator?" This is where the resonance comes in.

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Thus, the orbit of the moon is timed with the eccentric orbit of its parent gas planet so that the moon is eclipsed to the maximum as it passes on closest approach. This has the added benefit of keeping the moon cooler than normal, allowing me to place the vast majority of its orbit in the habitable zone without fear of overcooking it. I have also added the bit of fun that the gas parent planet swells up on closest approach roughly doubling its volume and so increases the width of its shadow. Something we have witness of with a new Kepler discovery of a swelled up gas giant with the density of Styrofoam.

However despite all this arrangement I still have this nagging suspicion in my mind that this still wouldn't produce the effects I am looking for.

I am well aware of eyeball planets (tidally locked worlds) and planets like Uranus that have a heavily tipped axis.

Here come the questions:

Would this arrangement work with perhaps some minor tweaking?

Or is there another scenario that could produce the desired results of a planet with equatorial icecaps and polar jungles?

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    $\begingroup$ Have you considered how the orbital speed varies but the rotation does not? The periaster, where you've drawn the situation, is traversed rapidly. $\endgroup$
    – JDługosz
    Jul 29, 2015 at 22:14
  • $\begingroup$ Yes indeed it is. It is also very close to the star which is why I designed the resonance to shade it for at least a major percentage of periaster. $\endgroup$ Jul 29, 2015 at 23:19
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    $\begingroup$ Where do you get this planet pictures from? Do you make them yourself? $\endgroup$
    – Green
    Jul 30, 2015 at 11:17
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    $\begingroup$ @user6760 -- wouldn't that make one pole tropical and the other arctic? $\endgroup$ Jul 30, 2015 at 13:56
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    $\begingroup$ @Green, yes I make them myself. $\endgroup$ Aug 4, 2015 at 0:59

8 Answers 8


First, as JDługosz pointed out, the orbit you've drawn has a very high eccentricity - much higher than any of the planets in the Solar System, or many other planetary systems. To have something that is briefly out of the habitable zone, try something like Gliese 832c:

It's orbit has an eccentricity of 0.180, letting it pass out of the habitable zone. The only issue is that it is in the zone for half its orbit and out for half, not like your arrangement.

However, whether you choose your orbit or mine, you still won't get the effects you desire all the time. The solution is to change the albedo of the planet in different places. You can then use the effective temperature as an approximation: $$T=\left( \frac{L(1-a)}{16 \pi \sigma D^2} \right)^{\frac{1}{4}}$$ A greater albedo means a cooler temperature, and a lower albedo means a hotter temperature. Make the albedo greater at the equator and lower at the poles.

I think that your comment suggests the best way to change albedo:

We know from Iapetus that huge changes in albedo can occur with relatively little transition in between. With a very high albedo at the equator and dark poles we should be able to get the desired feedback. Snow falling at the equator would further lighten the albedo, and dark green plants growing at the poles would absorb more light.

They create a positive feedback loop.

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    $\begingroup$ I never thought about the albedo angle of things. You're right. We know from Iapetus that huge changes in albedo can occur with relatively little transition in between. With a very high albedo at the equator and dark poles we should be able to get the desired feedback. Snow falling at the equator would further lighten the albedo, and dark green plants growing at the poles would absorb more light. $\endgroup$ Jul 29, 2015 at 23:24
  • $\begingroup$ But can albedo alone account for such a difference in temperature, with such different angles of incidence of sunlight? Perhaps with a very thick atmosphere? $\endgroup$
    – Rafael
    Aug 15, 2018 at 13:40

Give the planet rings which constantly shade the equator.

Depending on the thickness of the ring and how far its outer edge is from the planet, it could shade the equator enough to cause it to remain significantly cooler than the rest of the planet. Obviously, the planet needs to be close enough to its star so that its poles would normally be tropical, but the ring shade would keep things from getting out of control.

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    $\begingroup$ I think we're on to something here. This combined with what HDE 226868 said would provide the desired effect if I kept the moon's axis tilt to something like 5 degrees. Plus the ring material might be responsible for the changes in albedo mentioned above. An elegant solution with a probable cause I like it. $\endgroup$ Jul 29, 2015 at 23:30
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    $\begingroup$ @JoshBelmont I sure hope it helps. Seeing the "science" in science fiction on television and movies makes me think you're the only one who cares about plausibility though. Please work more hours. $\endgroup$
    – Samuel
    Jul 29, 2015 at 23:39
  • $\begingroup$ L.O.L. Thanks. I always find truth to be stranger than fiction. $\endgroup$ Jul 29, 2015 at 23:42
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    $\begingroup$ Rings don't shade the equator as they are too thin. They shade the temperate zone if the planet's axis is tilted. $\endgroup$
    – Oldcat
    Jul 30, 2015 at 0:25
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    $\begingroup$ @Samuel I'll find out. $\endgroup$
    – HDE 226868
    Jul 30, 2015 at 14:02

Assume a planet with a core which is proportionately larger than Earth's. This will be less rigid and hence be more oblate than Earth, causing the equator to be higher. This increases the chance of equatorial glaciers, which could join to produce an ice ring (rather than ice cap).

Meanwhile the poles will be closer to the core and more tectonically active, giving direct geothermal heat and increased carbon dioxide levels for a greenhouse effect.

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    $\begingroup$ I like this answer, though I suspect that if parts of the planet are higher due to oblateness, then that won't make them colder. $\endgroup$ Jul 30, 2015 at 17:09
  • $\begingroup$ Wouldn't the atmosphere also be thicker at the equator then? Kinda like Earth? $\endgroup$ Jul 30, 2015 at 21:01

A bit of a cheat, but if in this era the land masses of the planet have gathered together to form a largely ring shaped supercontinent around the equatorial zone, then the upthrust would produce ranges of mountains which would become covered with snow and glaciers, giving the effect of a "polar band" around the planet when seen from orbit. Even on the ground, the generally high elevations would serve to keep the landmass cooler, and the circulation patterns of the atmosphere should provide a steady stream of precipitation to maintain the snowcap.

With your proposed axial tilt, the "jungles" on the polar islands would be very seasonal, with the jungle exploding into life during the period of "midnight sun" and the opposite polar jungle quickly becoming dormant (plants seeding and dropping spores, animals going into hibernation or whatever equivalent exists, extreme migrations to the opposite poles), which would make for an interesting environment for the heroes as the ecosphere would rapidly change, and most life would evolve to be growing and adapting to the changing light conditions at breakneck speed compared to Earthly life.

Details about weather patterns would depend on environmental variables (how high are the mountain chains, is the supercontinent a complete ring around the planet or are there gaps, how close are we to the star etc.), so this is a possible starting point, but could change drastically depending on what variables are in play.


Another way to make this world plausible is to locate it in a binary star system.

Put Nabirmo into orbit around a very dim red dwarf star near or even a little beyond the outer edge of its habitability zone. It would become tidally locked to the red dwarf, with its axis of rotation pointing toward the red dwarf, but if the red dwarf/Nabirmo system was itself in orbit around a larger, more luminous star, also near the outer edge of its habitable zone, we would get a situation where Nabirmo's non-constantly-illuminated hemisphere would receive just enough heat to prevent it from freezing entirely. If the orbital planes of the three bodies were closely aligned, the brighter star would be eclipsed by the red dwarf when Nabirmo passed behind its red dwarf star.

One important factor is that while Nabirmo is tidally locked to a relatively distant dim red star, it is still rotating with its axis of rotation locked toward the red dwarf, so that when it is illuminated by the brighter star, it receives a day-night cycle over the surface facing that star while Nabirmo is to either side of the red dwarf in its orbit. When Nabirmo is between the red dwarf and the brighter star, both inwards and outwards faces would receive illumination over longer periods. When the brighter star is at the outer pole's zenith (the outer pole being the one facing away from the red dwarf), both hemispheres would be receiving constant illumination, which would be much dimmer at the equator, hence the equatorial ice ring.


An alternative answer to the first I wrote is that Nabirmo has a heavily tilted axis of rotation about a fairly dim G-M-class star such that the axis of rotation lies close to the orbital plane rather than approaching being perpendicular to it.

In addition, Nabirmo has a highly eccentric orbit, such that its distance from its primary star varies considerably while remaining within the habitable zone, such that the primary star is close to the geometric centre of the elliptical orbit rather than being closer to one end of it.

If we arrange Nabirmo's eccentric orbit so that when the equator is facing the sun, it is more distant, and when a pole is facing the sun, it is closer. If the orbit is sufficiently rapid - possible only with a fairly dim star - then the long nights at the poles will not result in a particularly heavy covering of ice, which would rapidly melt as the day length began to increase again. However, when day and night length is equal, Nabirmo would be further from the sun, and the rate at which ice melted would slow.

The result of this, if the parameters of orbital eccentricity and solar luminosity and mass were carefully balanced, would be the desired equatorial ice band, where a thick accumulation of equatorial ice increases the local albedo and keeps the equator cool even when Nabirmo approaches its sun more closely at spring/autumn.


Two theories:


I envision a planet that spins significantly more rapidly than Earth and for this reasons and possibly other reasons has a more oblate shape. It has a ring of connected continents around the equator and has northern and southern oceans with other continents and islands in them.

The equatorial continents have very tall mountain ranges and plateaus and have an average altitude a mile or so higher than the land around the poles and the surface of the polar seas. Thus the equatorial continents have fewer green house gases above them than the areas around the poles.

The higher parts of the equatorial continents are covered with glaciers that reflect sunlight back into space. The poles are above thinner sections of crust and more internal heat seeps through in the polar regions than in the equatorial regions with much thicker crust due to the oblate shape of the planet, and the continents on top of that.

The spin axis of the planet is almost in the plane that it revolves around its sun in. Thus during northern hemisphere summer the northern polar regions are in direct almost vertical sunlight all during the summer and heat up greatly, while the southern polar regions are in shade and nighttime all through their winter and are cooling off.

In the northern winter it is the opposite, the northern polar regions are in darkness all winter and cool off while the southern polar regions are in summer and constant nearly vertical sunlight and heat up.

During those seasons the equatorial regions receive sunlight at very low angles and do not heat up much, and every hill and mountain casts a very long and cold shadow.

In the spring and fall seasons the equator receives direct vertical sunlight, but it is almost all reflected back into space and doesn't heat up the surface much. And since the planet rotates there will be fast days and nights all over the planet during those seasons so heat will not build up in any region.

The polar regions will receive sunlight during the day during the spring and fall seasons but it will be at very low angles and not heat up the surface much.

Thus the equatorial regions, because they are cold and icy all year, will not be able to heat up and so will remain cold and icy all year. The polar regions will have normal seasons being hotter in the summer and colder in the winter. But they may be warmer during their winter than the equatorial regions are all year.


Another theory is an Earth-sized moon A orbits a gas giant planet B that orbits a star C. Moon A's rotation has been slowed down until it always keeps the same face toward planet B, it's rotational period and orbital period have the same length.

The orbit of moon A around planet B could take about a single Earth day. The Galilean moons of Jupiter orbit at distances and periods of 421,700 kilometers and 1.769 days (Io), 676,938 kilometers and 3.551 days (Europa), 1,070,400 kilometers and 7.154 days (Ganymede), and 1,882,700 kilometers and 16.689 days (Callisto).

You would want the moon to orbit faster to have a strong enough magnetic field to protect it from solar wind.

And as the moon A obits planet B it will gradually recede farther and farther away from planet B, as Earth's moon gradually recedes from the Earth. Until eventually Moon A's orbital period around Planet B will equal in length planet B's orbital period around star C.

So moon A will rotate at such a speed that it will always keep the same side facing toward planet B and away from star C and the other side facing away from planet B and toward star C.

The sub stellar point on moon A will always get direct vertical light from Star C and will get hotter and hotter. Hot water in the oceans and hot air in the atmosphere will flow away from the sub stellar point to the opposite side of the planet that gets no light from star C. Since they get no starlight there, they would normally freeze.

But the point opposite to the sub stellar point on moon A will be pointing toward planet B, a huge gas giant planet that might have a high albedo and might reflect a lot of light from star C back to the side of Moon C and heat it up. Thus moon A might have a hot area that gets constant direct light from star C, an opposite warm area that gets light from star C reflected off of planet B, and a cold area on the edge between the two hemispheres.

Could a gas giant planet and its hypothetical Earth sized moon orbit a star close enough to get as much light and heat from the star as Earth Gets from the Sun?

Yes. Such a planet is called a hot Jupiter and it is one of the most commonly detected types of extra solar planets. The hot Jupiter with the shortest year, WASP-19B, has a mass of 1.15 Jupiter masses and orbits WASP-19 at a distance of about 0.1655 astronomical units and a year of about 0.788 Earth days.

Could a planet orbit within the habitable zone of a star and have such a short year?

TRAPPIST-1g has an orbital radius of 0.0451 astronomical units and a year of 12.352 Earth days, and orbits within the habitable zone of TRAPPIST-1. TRAPPIST-1f has an orbital radius of 0.037 astronomical units and a year of 9.2066 Earth days, and orbits within the habitable zone of TRAPPIST-1. TRAPPIST-1e has an orbital radius of 0.028 astronomical units and a year of 6.099 Earth days, and orbits within the habitable zone of TRAPPIST-1.

Thus it is certainly possible to calculate the parameters of a star system where a tidally locked habitable Earth-sized moon A orbits a gas giant planet B that orbits a star C, and where the orbital period of moon A around planet B and the orbital period of planet B around star C are identical for a short era by astronomical standards.

Thus one side of moon A could always face star C and the other side could always face planet B.

On Earth and planets that have Earthlike orbital characteristics the ring around the equator is the hot tropical zone and the temperate zones are rings north and south of the tropics, and the cold polar zones are circles surrounded by the rings of the temperate zone.

On moon A, the circle around the sub stellar point would be the hot tropical zone, surrounded by a ring shaped temperate zone, and the cold zone would be a ring around the twilight zone on the edge between eternal day and eternal night. Except that light from star C reflected from planet B might make the opposite side have a similar climate pattern, though probably not as warm.

This orbital arrangement gives habitable moon A the desired arrangement of tropical, temperate, and polar zones, except that they do not center around moon A's poles of rotation. Of course one could always claim that the sub stellar point and the sub planetary point are the "temperature poles" of moon A, or maybe call them the east and west poles.

  • $\begingroup$ Oops! according to thsi study of Exomoon Habitablilty - ncbi.nlm.nih.gov/pmc/articles/PMC3549631 - says thatt he legenthof the moon's day must be a fraction of the length of hte planet's year. The longest possible length of a satellite's day compatible with Hill stability has been shown to be about Pp/9, Pp being the planet's orbital period about the star (Kipping, 2009a). Kipping D.M. Transit timing effects due to an exomoon. Mon Not R Astron Soc. 2009a;392:181–189. $\endgroup$ Apr 5, 2017 at 14:45

My answer is mildly more abstract, but is based around the rings suggestion.

Essentially, to get the colder ice equator effect, you would need something that 'inverts' the effect of the sun.

The easiest way to do this is to have it so there is a moon that is just the right size that always faces the sun (sorta like a permanent lunar eclipse). It'd have to be smaller than the earth's moon, as you only want to block out the equator, or perhaps further away from the planet.

Physically speaking, I'm not sure how sound this would be, but basically the moon would have a rotation of the planet, but would be tidal locked to the star you want to block the heat from.

Then you can point out the poles receive heat because they're outside the moon's shadow. Adjust the size of the moon to increase/reduce the size of the ice equator.

If sci-fi isn't outside the doorstep, then you could include a giant 'inhibitor ring' constructed to surround the planet by a previous set of aliens designed to produce a cool area for an otherwise 'almost too hot for life' planet.


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