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I'm trying to create a world where there's an equator-like divide running along the axis(?) of the planet and the two hemispheres are perpetually different in terms of temperature. Basically, I want one side of the world to always be warmer than the other side and for the temperature change to be noticeable when crossing over an invisible "divide".

I'm not a physicist and I know very little about astronomy so my thoughts on this might be completely off track, but I was thinking that maybe a circumbinary planet with two stars of differing heat intensities might work? But I think that would affect the changing seasons more so than the relative temperatures of the hemispheres, is that right?

From what I've read, the angle of the planet in relation to the sun is what causes the difference in global temperature, but I don't think that wouldn't create the drastic change that I'm looking for. Any thoughts on how I could make this work?

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  • $\begingroup$ Welcome to Worldbuilding, Morgan! If you have a moment please take the tour and visit the help center to learn more about the site. You may also find Worldbuilding Meta and The Sandbox (both of which require 5 rep to post on) useful. Have fun! $\endgroup$ – FoxElemental Jun 26 '18 at 16:19
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    $\begingroup$ This sounds a lot like a world that is tidally-locked to its sun, although the temperature divide on such a planet would be pretty extreme. Have you considered that option? $\endgroup$ – Gryphon Jun 26 '18 at 16:22
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    $\begingroup$ @Gryphon I hadn't considered a tidally-locked planet so thank you for that suggestion! I was hoping for something less extreme though. I want both halves of the world to be habitable, if possible. $\endgroup$ – Morgan Jun 26 '18 at 16:55
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    $\begingroup$ While, just say that aliens did it. Maybe the aliens later became primitive, a la fall of a great empire type story. $\endgroup$ – Gryphon Jun 26 '18 at 17:33
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    $\begingroup$ Mars is like this right now. The summer temperatures in the north can be 30 °C higher than in the south. This is mostly due to the elevations involved. $\endgroup$ – Brian Jun 26 '18 at 21:37
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Tidal locking is a fine method. Here is a different method. Asymmetric magmatic heating.

mars being hit https://www.reuters.com/article/us-mars-crater/giant-crater-explains-strange-shape-of-mars-idUSN2530000020080626

Your world was hit by a large impactor. On resolidifying, the iron core of the planet was no longer in the center of mass. A large lobe of silicaeous material overlays one hemisphere and what was once the metal core is displaced, to the opposite hemisphere.

The metal core conducts heat, and also provides substrate for eddy currents and ohmic heating. The hot metal core is close to the surface in that hemisphere. The entire hemisphere has a situation much like Iceland with ample geothermal heat. This is the hot side.

The far site is insulated from the core by the overlying light materials and has essentially no geothermal activity. It is warmed minimally from the core and only by the sun, which is distant and dim. This is the cold side.

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    $\begingroup$ I was gonna say you beat me to it but your answer is way better and more detailed yhan what I was going to post :) $\endgroup$ – Renan Jun 26 '18 at 17:38
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    $\begingroup$ I've never tried to calculate the viscous creep flow around a planetary core through the outer core and mantle of a planet. (I know, makes me a noob around here.) But surely this is only a temporary condition, with re-centering completing in (perhaps much) less than 1000 years? $\endgroup$ – Eric Towers Jun 26 '18 at 20:22
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    $\begingroup$ @Tim : Using Stokes' Law, the mass of the core of the Earth (1/3 mass of Earth) and gravitational force in the neighborhood of ~1/2 G, the Earth's core radius of 1220 km, and $10^{19.5) Pa \cdot s$ as the viscosity of the mantle (if you believe earthscience.SE), gives velocities nearer 2 cm/s, so about 10 million times faster than your estimates (and 200-ish times faster than my guess). $\endgroup$ – Eric Towers Jun 26 '18 at 21:23
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    $\begingroup$ Why does the metal core not sink to the geometric center of the planet while it is still molten? This should be a very quick process, at least on geologic timescales. Much faster than the core will cool and solidify. $\endgroup$ – Ryan_L Jun 27 '18 at 0:23
  • $\begingroup$ So, this wouldn't work at all? Why is it so heavily upvoted and ticked, then? $\endgroup$ – lly Jun 27 '18 at 13:33
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Take a planet like Mercury, with no atmosphere, and have it tidally locked with the central star. When you cross the terminator (the line dividing the day region from the night region) you quickly go from "scorching hot" to "hell has frozen cold".

The lack of atmosphere will take care that no diffusion of heat takes place.

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    $\begingroup$ Or an atmosphere could make the difference less pronounced $\endgroup$ – Rafael Jun 26 '18 at 16:25
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    $\begingroup$ @Rafael, I am not even sure that on Mercury an atmosphere could last. But, yes, if there was, it would not mix that much. $\endgroup$ – L.Dutch Jun 26 '18 at 16:27
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    $\begingroup$ A tidally locked planet was also the first thing that came to my mind - I guess the OP wants something less extreme than Mercury though, so can you still have a tidally locked planet in a binary system? $\endgroup$ – Nicolai Jun 26 '18 at 16:45
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    $\begingroup$ Mercury does have an atmosphere, it is just so thin that it takes a spectograph to see it. But for all practical purposes it does not have one. $\endgroup$ – Renan Jun 26 '18 at 17:36
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TL;DR It can be done, but there would be huge storms as a result.

Long explanation The only natural phenomena that I can think about that would be able to produce this sort of result would be tidal locking. Basically what happens is that a planet is so close to its star that the gravity makes it spin at the same rate it orbits the star, making one face of the planet permanently fixed to the star. From the planet's perspective, it would be like a sun that never moves. Now, this has its complications, mainly the following:

1.- Having a tidal lock on a star like our sun would melt half of the planet so in order to be habitable the star would have to be a red dwarf: in other words, a very small and cold star.

2.- Even with a small star the temperature would be really high on the bright side. Just to put in perspective, the sun for such a planet would occupy probably at least half of the sky; on the equator, temperatures would be enough to melt rock. Probably the habitable part would be on the twilight between the dark side and the bright side.

3.- But talking of the dark side, it would be incredibly cold, freezing solid oxygen kind of cold due to its permanent shadow if it weren't due to the next point.

4.- You have atmosphere, and storms within atmospheres are driven mainly due to heat differences. When air is hot, it expands; when it's cold, it contracts; and when a cold mass of air crashes with a hotter mass of air, they start to combine, expanding and contracting respectively creating something called eddie currents, which are common to what we call a tornado. On Earth, this happens when the sun heats the sea and its hot air crashes with the cold air coming from the poles, and the greater the difference the greater the force of the storm. Now on your world we are not talking just about a small gradient like here on Earth. We are talking about air as hot as melting rock crashing with air cold enough to freeze oxygen; there would be global storms stupendously huge engulfing the planet all the time, making the dark side somewhat warmer and the bright side somewhat colder than they would be otherwise, but the air speeds would be just amazing, probably in the hundreds of mph. Actually there are some scientific studies on this.

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  • $\begingroup$ What's good about this answer is the emphasis on atmospheric effects and how that can balance the extremes of more mundane tidal locking, but at a cost. This would require a very particular kind of atmosphere. It can't be a greenhouse trap like Venus (that is hotter than Mercury!), but it still has to be thick enough to transport and distribute vast amounts of energy. A binary star system with tidal locking on one star, and the second star being much more faint, might help reduce the extremity. If Im not mistaken the energy of the storms will be about proportional to the temperature difference $\endgroup$ – World Outsider Jun 27 '18 at 1:52
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The Earth's seasons are caused by the axial tilt of the planet. In June, the Northern hemisphere is tilted 21 degrees toward the sun, the Southern hemisphere is tilted 21 degrees away from the sun. This means the Northern hemisphere will get more and more direct sunlight, which makes it heat up.

Over the course of ~25,000 years, Earth's axis precesses; the North pole points at different places in the sky. This is caused by tidal forces between the Earth, Moon, and Sun. It should be possible in a more complex solar system for this precession to happen much faster, such that the axis precesses at the same rate that the planet orbits the sun. This would lead to one hemisphere always being tilted toward the sun, and one always tilted away; a summer hemisphere and a winter hemisphere. Unlike a truly tidally locked planet, both hemispheres will have a day-night cycle. The summer hemisphere will have longer days, the winter hemisphere will have longer nights. Depending on the elevation, there may be peaks of eternal daylight at the summer pole, or valleys of eternal night at the winter pole. The tidal forces necessary to make the precession happen this quickly would lead to some extreme tides; perhaps hundreds of meters or more. The unbalanced heating would also lead to some pretty extreme weather, especially near the equator.

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