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I would like a planet that's a tropical paradise on one side but covered in ice and glaciers on the opposite side, whether it be the Southern-Northern or the Western-Eastern hemisphere.
I know it would be theoretically possible by making the planet tidally locked to its star but I'd like to avoid that because I want normal day and night cycles and a normal year length (26 Earth hours long days and an orbital period of 390 Earth days).
How could I make all of that happen?
Mount Kilimanjaro, despite being close to the Equator, has permanent glaciers on its top.
This hints toward the solution to your problem: orography!
While one hemisphere is mostly flat, with limited elevation and can enjoy the benefit of a warm climate, the other hemisphere is much more mountainous with most of its peaks above the snow line. The precipitation that one side would be tropical rain falls as snow on the mountains, and stay frozen because of the low temperatures.
Combine an significant axial tilt with a elliptical orbit.
Axial tilt allows one hemisphere to be in winter while the other is in summer. Earth has approximately 23.44 degrees of tilt. Even in mid-summer the temperature at the south pole is far from tropical because the sun never rises above 23 degrees and sunlight is correspondingly dim. 45 degrees is too much since the south pole would get twice as much sun as the equator (when south pole is pointed toward the sun). 30 degrees might be a good guess, the pole would get more sun than the equator at the peak of summer, but the equator would get more sun at other times.
Earth has a slightly eccentric orbit; it is nearly circular. Because of the duration of summer and winter is almost the same. Since Earth is closer to the sun (and moving faster) when it is winter in the northern hemisphere, the northern hemisphere has longer summers and the southern hemisphere has longer winters. In 2006, the northern hemisphere summer was 4.66 days longer than winter
Now, increase the eccentricity of Earth's orbit until the winter 6 times as long as the summer (in the colder southern hemisphere). Consider the northern hemisphere first, summer is now 5 times longer than the winter, but the solar radiance is not at high as on Earth because it is further away from the some (keeping the same orbital period), but first day of summer is not the hottest because of thermal inertia. The much longer summer would allow thermal inertia to create a much longer tropical conditions period than might otherwise be expected.
The southern hemisphere will be bitterly cold for months on end (during the dog days of the norther hemisphere).
Now, the northern hemisphere will endure a brief (about 1.5 month) winter, the would have have little sunlight and decidedly non-tropical. Thermal inertia will keep the temperatures from being too frigid.
I am assuming significant amount of ocean as a prerequisite for thermal inertia - because this gives a result more closely in line with the desired outcome. Without oceans, temperature change will be more intense (hotter summers, colder winters).
Due to precession, the winter / summer conditions will change over time, i.e., the northern hemisphere will experience the long winters, then thousands of years later it will experience short winters, and so as as the precession cycle repeats. 26,000 years in the case of Earth.
This won't precisely yield a planet with the conditions desired, but I think it is probably closer than you will get with other realistic conditions.
It occurred to me later that there will be large storm patterns associated with such a planet. The large temperature differences between the 2 hemispheres will drive large storm systems. While this would be true of any planet meeting the tropical on hemisphere, glaciers on the other, the alternating temperature patterns under this arrangement guarantees that there are not going to be any areas of mild weather on a year-round basis.
1. The planet is close to the galactic core, but far from galactic equator so light from the core is only hitting one hemisphere.
You don't actually need much light to get the difference you want, making one hemisphere around 2 degrees warmer will give you what you want. That is the difference between modern day chicago and a mile of ice over chicago.
Your systems plane happens to run parallel to the galactic plane while being fairly close to the galactic core/center, but it is far from the galactic equator. The galactic core produces a lot of light, and this light is coming in roughly perpendicular to the orbit of your planet, you can lessen the axial tilt of the planet to further exaggerate the effect but that will mess with your seasons.
The large number of star on one side and the comparatively small number on the other means one hemisphere is receiving considerably more light, to the point its night would not be as dark as night in the other hemisphere. Keep in mind the light side is also going to receive a lot more radiation.
This is what the night sky looks like on your warm hemisphere
And this is what it looks like from the cold hemisphere
The earth does not receive this effect because we are near the galactic equator so the light we receive from the core is roughly equal and we are far from the core so we don't receive that much to begin with.
"Near the center of our galaxy, the average distance between neighboring stars is only 1000 A.U. If the star Sirius were only 1000 A.U. from the Sun, it would be twelve times brighter than the full moon. If the Sun were located within a parsec of the galactic center, there would be a million stars in our sky with apparent brightness greater than Sirius. The total starlight in the night sky would be about 200 times greater than the light of the full moon; you could easily read the newspaper at midnight, relying on starlight alone. astronomy.ohio-state.edu/~ryden/ast162_7/notes31.html –
You can drastically boost the effect by correctly positioning your continents. First on your cold side you want a large (asia sized) continent centered over the pole with an equatorial ocean. this will encourage the growth of an icecap, and minimize thermal transfer between the hemispheres.
On the warm side you want a polar ocean to minimize ice formation. You also want several continents closer to the equator, preferably wide near the equator and thin near the pole, with about 3 atlantic sized oceans that rum from pole to equator. That will encourage heat transfer from equator to pole on that side keeping the higher latitudes warm. This global map alone will get you most of the way there. Now you only need about 0.5-1 degree difference between your hemispheres to get what you want.
1/(1E10)^2 as much radiation from any star in the core. Which again means you'd need some 1E100 stars in the center of the galaxy to get the same radiation we get from the sun. That's more stars than there are atoms in the universe...
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1E36 stars in/near the core, which is 25 orders of magnitude more than exist in the entire milky way... I'm simply not convinced the numbers pan out, because as you move your hypothetical system closer and closer to the galactic core the required stars would need to occupy a smaller and smaller space. I'm happy to be proven wrong by actual sources though.
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Have the planet be half of a binary pair with the two tidally locked to each other but very close together (close to the Roche limit but not within it).
The two would orbit around the star and their mutual spin would provide a normal day-night cycle but the side towards the partner would receive less sunlight due to the shade from the other half of the binary and as a result would be colder.
Have one hemisphere to have much higher terrain features than the other. The higher you go, the colder it gets due to atmospheric pressure.
That happens because for most gases, if you take a constant volume of them, temperature is proportional to pressure and mass.
Mars has the features you need. This is a rendition of what it would look like if you filled the lower plains with water:
If instead we kept the planet dry and pressurized it to have Esrth-like pressure at any point, the plateaus would be naturally colder than the lower lands.
This works with any rocky planet capavle of holding an atmosphere.
Messing with the axial tilt would be the closest you may get to your scenario in a clean way, but even the pole-equator climate switch will only get you a ring of moderate "tropics" around the equator and a seasonal switch from Hell to Helheim on the poles. I would recommend this as it is the least odd and most probable scenario.
However, there might be another way. If the sides have a significantly different albedo one could be cooler than the other. The following scenario is just one way to do this, artificial structures and terraforming gone wrong would also be good explanations. Imagine a planet, half solar-collector farm, and half nature preserve.
The Martian Way
Mars used to have a big ocean covering almost the entire northern hemisphere. Imagine this ocean, but covering half of both hemispheres. Now, this world is really cold. So cold that it mimics Earth during the Cryogenian Period and the ocean side nearly freezes over completely. The continent side, however, has a much higher albedo (the world will need some very dark rocks) and absorbs much more heat than the reflective frozen ocean side. This may allow for continent based zones with a microclimate fit for rainforests near the equator.
This won´t be a clean solution, the ice will probably have an equatorial "channel" and the "warm side will have a lot of tundra and glaciers. So not a tropical paradise on one side but covered in ice and glaciers on the opposite side but one frozen ocean side and one warmer tundra side with the planet's rainforests in equatorial retreats. If you make the tropical retreats geothermally active this could also aid the survival of the forest and create valleys with actually pleasant microclimate. I would furthermore suggest keeping the planets axial tilt low, so seasons don´t mess up the planets delicate climate setup.
Massive Geo-Engineering
Tilt the planet by 90°
Place a self-stabilizing sunshade at the L1 point
Place massive, controllable mirror-arrays at L3 and L4 and selectively heat the desired hemisphere and somehow manage the heat-transfer of the atmosphere
You don't need anything fancy or clever here,
JUST USE THE EARTH AS A MODEL
Like your fictional planet the Earth is not tidally-locked. Yet the average temperature of earth's continents vary wildly. They can be simply grouped as follows:
The big obvious difference between them is their latitude.
So all you have to do is to position their latitudes to give the average temperature that you want.
There are other factors also. For instance, Europe is on average warmer than North America even at the same latitudes because the Gulf Stream warms the ocean air to the west of Europe, which then gets blown onto it by the prevailing winds. And North America is somewhat colder that we might think from latitude alone because arctic air frequently gets blown southernly over it.
Likewise, the winds and current around Australia make it significantly hotter (and dryer) than it's latitude would suggest.
With some mechanism to prevent the atmospheres mixing, simple climate change on one hemisphere would not affect the other. For example, an equatorial bulge or chain of mountains high enough to prevent most of the weather from crossing. If you didn't need stability in geological scale, even weather structures could form a suitable barrier for hundreds or even thousands of years.
With a barrier between them, one hemisphere could be runaway greenhouse CO2-rich jungle (whether from volcanic action, cultural effects, differences in plant respiration, shortage of water, higher albedo, etc etc...) while the other is an icy wasteland.
The separation of the mountains would form a wall against the ice all too reminiscent of the wall in the Game of Thrones, though.
In the comments, I think you said that two continents would be acceptable. In that case, the only thing you need to change is the planet's axial tilt. Make it nearly zero. Right now, Sweden has pleasant summers because its days are so long. With uniform day lengths, it would be pretty cold all year long. Then you could have one cold continent near one of the poles, and one warm one in the tropics.
You have to realize, the Earth we live on isn't so far from what you describe. Parts are a tropical paradise, and parts are covered in ice.
On Earth, much of Antarctica's cold climate is due to Antarctic Circumpolar Current, and much of Europe's warm climate is thanks to the Gulf Stream. You can conveniently combine the two - open ocean along the equator with a strategically positioned archipelago diverting warm waters northwards (and inland through conveniently placed straits) while keeping the southern continent surrounded by circumpolar cold water trap.
The planet is a part of a binary star system. It orbits one star in a plane which is perpendicular to the line connecting the stars. This way the other star will always be over one pole of the planet.
This answer assumes an earthlike planet orbiting further out from its primary star. Effectively the planet will be locked in a permanent glacial epoch. Except it isn't.
Overhead its northern hemisphere in the position corresponding to Earth's Pole Star is a pulsar. The solar system containing is bathed in the beam of the pulsar and the pulsar is closer enough to significantly warm the planetary environments of all the planets and moons.
A pulsar (from pulse and -ar as in quasar)[1] is a highly magnetized rotating neutron star or white dwarf that emits a beam of electromagnetic radiation. This radiation can be observed only when the beam of emission is pointing toward Earth (much like the way a lighthouse can be seen only when the light is pointed in the direction of an observer), and is responsible for the pulsed appearance of emission.
If the pulsar is rapidly rotated the flicker effect of its lighthouse-like beam will be hardly noticeable.
Significantly pulsars can emit radiation right across the electromagnetic spectrum.
In addition to radio emissions, neutron stars have also been identified in other parts of the electromagnetic spectrum. This includes visible light, near infrared, ultraviolet, X-rays and gamma rays.
The pulsar doesn't need to be as bright as the planet's primary star. It only needs to deliver sufficient radiant energy to raise the temperature enough to keep the hemisphere under the pulsar compared to its opposite hemisphere. If the radiation spectrum of the pulsar favours electromagnetic radiation that will pass the atmosphere this will warm the planet's surface.
If radiation is absorbed buy the planet's atmosphere, this will be contributing mechanism to raise the northern hemisphere's temperature, but not all the energy will be absorbed by the atmosphere. The ground can store large amounts of heat from the Sun, as is the case here on Earth, so this can play an important role of the planet we are considering. If the northern hemisphere has significantly large land masses, say, like a super-continent, then this land will be warmed sustainably by the radiant flux from the pulsar. The land-mass will act as a thermal mass sufficient to maintain a higher temperature in the northern hemisphere while the southern hemisphere languishes in the deep freezer.
A close pulsar acting as a pole star can significantly warm one hemisphere over another. This will be especially the situation if the pulsar can deliver enough energy to the hemisphere that lies below its position in the planet's sky, like a Pole Star.
One way you could have an effect somewhat similar to what you describe is to have a planet with a variable axial tilt. Having for example the North hemisphere permanently tilted away from the sun would keep it in permanent winter, while the South would be in permanent summer. Where the variable part comes in is that the tilt itself would need to be "locked" in such a way that it shifts exactly with the rotation of the planet around its star so as to always keep the same pole tilted towards the sun. I will admit that this does seem highly improbable, however you could probably fit it into the story.
Natural solution: Huge area where geothermal vents come to a surface. As in Yellowstone park, but on a much bigger scale.
Have you considered a technological solution?
As in solar satellites in orbit beaming energy to a specific population segment one one side of the planet. Maybe the builders of these satellites are trying to compensate for a dying sun?
Large domes keep the greenhouse effects?
The cold side would need to be a near desert, or in a few hundreds of years all the water would be trapped in the ice on the cold side. You would need some way to keep the moisture on the warm side.
Several existing answers talk about raising the elevation on the cold side, to make it cold. If you go that route you need to make the elevation high enough so that most of the rain falls on the coast of the cold side.
The weather on the coast of the cold side would be much like Western Washington and Oregon How Geography Shapes Regional Weather of the United States which is relatively wet and warm, most of your population would choose to live on the coast unless the slope was VERY sharp.
So have some two galaxies cross its ways, that just some solar systems on borders get somehow "near" each other to the effect, that they heavily interact with each other. And one planet was as an effect slingshoted by gravitations forces somewhere else (while other planets was just messed up or something).
Let this planet just dull burn out piece or rock, flying interstelar space for milenia and totally cold, until it hit other younger solar system (from that coliding galaxies) in very lucky way - it was traveling really near the Sun, got slingshot from it to lose a lot of inertia and got catched in that solar system as a strange comete.
Then it hit one of local planets, roughtly its own size, not fully colded, which made them merge and make new planet, where the cold one is one half, the hot one is other half (more a huge layer over half of the cold one) while at the connection was formed big ring many times higher, then Kilimanjaro around the whole planet. Also the impact change its orbital trace to more better one. (and destroyed everything on both parts, but we are forming planet now, life will came much later anyway)
Now we have planet in the right distance of (relatively weak) Sun, which is rotating and everything, but one its half is old dead rock, other is hot radioactive young planet, both parts are separated by mountain ring about atmosfere hight or at least much higher, then freezing water line, so the air does not mix much.
Over time it stabilize somehow, get continents, oceans and later even life, at least on the radioactive side, which is warmer, as it actively produce heat (and lose it to space), while the other side just get heated from the other part via masive, more then half planet huge mass of badly conducting rock) and lose that heat to space too. There is some snow, so it also reflects heat from sun much more, that the happy half.
The life here is accustomed to higher radioactivity and many vulcanos around, as it developed in this conditions. (Yes, we have a lot of natural radioactivity in background too, and there are organismus on Earth, who are even much more resistent to extensive radioactivity, so why not).
The local Sun provides a lot of energy, but not all needed. The younger part of planet provides also lot of energy, not as much as Sun, but enought to sum to pleasant climate. The older part does not, so it is arctic regardles its position.
We do not lose watter, as it rains and snows on the ring-hills on winter and on summer it flows back to oceans on warm side. There may be some caves thru the hills, maybe there are some Himalayan ways too, but there is not much of such ways, so not substantial watter and heat is lost or transferred to the cold side. But it may be possible to climb there and use some of the few ways to get onto the "other side" for motivated people - like it was to get to Kilimanjaro or to the poles on Earth. Hard to do, but possible, but only on few places, which are not so big, to change the climate substantially.
Depends on your plan, how bad is the cold side, but there may be also some life (vegetable or even animal), or at least there could be human stations, as the cold side may contain some valuable resources, like minerals.
Practically it is two different planets, with the same gravity and ecliptic, with some connections usable for humans, but otherwise with totally different clima and resources and lifeforms (more different, than rabits here, platypus in Australia and penguines in Antarctica)
If the sun is the main source of contributions to global temperature then this is gonna be rather unfeasible because the planet spins.
So, instead you need to have a smaller star which produces much less light, and an internal source of heat that is more dominant on one side.
You can have a planet which has an extremely hot core which warms up the planet, but due to chance and a large asteroid impact in the distant past, has very different atmospheres on different sides of the planet. One side might have extremely active volcanoes which spew out a lot of chemicals that break down global warming gases, while the other might have volcanoes which spew out global warming gases.
As such, one side is hot and one side is cold. The hot side will remain hot, day and night, and the cold side will remain cold day and night.
