Climate on a slow spinning planet

Question

I'm currently working on a story set in a planet whose rotation period lasts around 24 years (Earth years, or 8766 days). This planet orbits at more or less 0,75 AU from a K-type star, and the translation period lasts about 365 days. It is similar in size and mass to the Earth except it has no tectonic activity. Due to slow rotation, water tends to flow to the poles, leaving a ring-like supercontinent between the 40th parallel north and the 40th parallel south. For sake of simplicity lets say its axial tilt is 0º. The temperature in the day zone is 150ºC (302ºF) and -150ºC (-238ºF) in the night zone (although these numbers can change as long as there is impossible for humans to live in those zones).

In tidally locked planets we can expect huge hurricanes and storms in the day side as water evaporates, strong winds from the night zone to the day zone and frozen water storing on the night side, and I think these conditions may be similar to my planet, but not the same.

Since this planet spins (very slowly, but it spins), I see the sunlight sort of like a "heat ray" (to put it in plain words), from the surface perspective. This heat ray, with the diameter of a whole hemisphere, burns the land and evaporates the water, creating storms, as it slowly moves on eastward to the temperate zone. But in the other edge of the ray, the land reclaimed by the temperate zone is already burnt and dry, so hot air ascends, but it has no moisture to form rain clouds. Likewise, as the night side moves on, it freezes the dry land, but as it retreats, the frozen water stored in it melts by action of the day side.

I assume there would be a "drier" convection cell in the temperate stripe moving sunward, and a "wetter" convection cell in the temperate stripe moving nightward. This would be the diagram:

As the sun heats and boils the water, this hot humid air would flow in every direction to fill the void, so I would expect humid winds traveling through the poles, losing water and then regaining it from the ocean and creating new rain clouds. This could moderate the weather in the temperate zones and supply water tho the drier stripe moving sunward. I'm thinking that maybe, as the continent get cooler faster than the ocean, we could expect a phenomenon like the monsoon in India or El Niño in South America.

In regard to the stripe moving nightward, I would expect heavy rainstorms, violent winds, and in general a more inclement weather than in the sunward stripe. Also, colder water released by the night zone would cool down the continent, but maybe the sun moving on and boiling the oceans could regulate the temperatures.

In the poles the climate would be calmer, but since they're covered by water, we couldn't expect but sealife.

I think although it might seem complex, it's actually a very simple climate pattern. The planet doesn't have tectonic plates, so it lacks high mountains that would deflect winds. The planet has an orogeny produced mostly (if not completely) by meteorite collisions in a distant past. Also, the continent shape would be pretty regular, with few lakes, inner seas or pronounce gulfs.

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I would like some feedback from you, guys. I don't know if I'm missing some crucial points, maybe something I'm not taking into consideration about oceanic currents, albedo, magnetism, satellites, atmospheric composition... you name it. The main goal of all of this is creating a planet though hostile, could harbor human life (and as scientifically accurate as possible!).

Human life in this planet would have developed naturally, evolving from a life form to another until reaching human species. All of the exposed here may be changed, but my only requirements are:

• Human life. Normal, Earth-like humans. They wouldn't be strictly humans since they are not exactly Earth-born and raise Homo sapiens, but they should be similar to humans in all aspects.

• A day of 24 years (or more). It would be similar to be tidally locked but it’s NOT tidally locked. One hemisphere of the planet would be exposed to the sunlight for a long period of time, while the other is dark, but it would rotate slowly so life could exist, but animals and people would have to be always migrating. I want them to have to constantly travel and face the risks of this hostile environment.

• Uninhabitable day and night. Temperatures can be different from 150ºC and -150ºC. They may be more extreme if needed.

About axial tilt, distance to the star, heat/size of the star, satellites orbiting the planet or other celestial bodies, we can touch up all these things. But please bear in mind those three requirements.

Thank you very much!!

PD: If something isn't clear enough please tell me and I would be glad to clarify it!

Answer 1

There's a potentially very important phenomenon that you haven't considered, and that is atmospheric super-rotation, whereby strong winds can effectively blow around the world faster than the world's own rotation rate.

Venus notably exhibits this, with very fast winds compared to its rotation rate which is quite slow, a little like your world. Venus has a denser atmosphere by quite a large margin, and rotates somewhat faster so it isn't entirely comparable.

There's been some research done on this, partially driven by the number of exoplanets found that are likely to be tidally locked. A potentially interesting one take a peek at is Atmospheric Circulation and Thermal Phase-curve Offset of Tidallyand Nontidally Locked Terrestrial Exoplanets which looks at the conditions that can cause the sorts of prevailing winds likely to help even out temperatures on a slow-rotating world.

Given the difficulty in getting a good look at alien atmospheres, there's clearly a great deal of wiggle room here... you could reasonably handwave in as much or as little of the effect as you wished. With too much, your planet would likely be too hot around the dawn terminator for you needs, but you clearly need a little in order to drive the air currents you're after on the habitable zone may end up being a bit too small.

The planet doesn't have tectonic plates, so it lacks high mountains

Venus doesn't appear to have plates either, but the Maxwell Montes are 11km above datum height. Probably doesn't affect your setting much, but something to bear in mind.

magnetism

Probably none. Between a minimal magnetosphere and a very hot atmosphere at the substellar point, you do risk accelerated atmosphere loss, though at least under a K-type star the amount of ionizing radiation will be lower.

could harbor human life

I wonder where all your oxygen is coming from. A relatively small portion of the planetary surface is capable of producing it from photosynthesis, after all, and anything on the surface that can oxidize will oxidize under years of hot sunlight. Oxygen is the gas most at risk of being split by UV and lost via atmospheric escape, too, and there's a whole hemisphere where that's going on. Your world also lacks convenient phytoplankton too, which are a major source of oxygen on Earth.

That bit probably bears more thought... I don't see it mentioned in your question.

Answer 2

Sorry about this not being in comment form. But just as a small statement, it looks like all the conditions for human or human style life developing are present, as if your climate description is elaborated one and the fauna is at least vaguely earth like it encourages a migratory style for animals, and so would probably encourage any migratory tree dwelling mammals to develop a bipedal form, and feet or a foot analogue, as such a creature would do a large degree of migration, and so strike the balance between walking and tree climbing( or a tree analogue) and resources would be scarce enough to justify living in groups, and so any sapient life not adjusted to eat something like a leave analogue or grass analogue or the like would probably be encouraged to adapt a hunter gather lifestyle, though that logic on my part is possibly a bit shakey. The uninhabitable day and night encouraging migration. It might help a bit if there where plentiful places with heights to justify development of a bipedal frame and so not make almost all life four legged, and enough change present in the climate in lieu of the rotation to justify continuous migration throughout the habitable area and not a just a slow migration northward(?) as to make sure that the tree dwellers end up having to move a lot (or cave etc)but otherwise I can’t really find any problems with selective pressure to justify the creation of a humanoid frame in the first place. That was messy. Bleegh.

Answer 3

The question seems to be a bit vague to me.

I don't know what you mean by a translation period of 365 days.

You say the the rotation period should be 24 years, without specifying whether it is planet years or Earth years.

You say that the planet orbits around a K type star at about 0.75 AU. I assume that the planet has an average temperature similar to Earth's, but has great extremes of heat and cold in the long, long days and nights.

Here is a link to a question:

https://astronomy.stackexchange.com/questions/40746/how-would-the-characteristics-of-a-habitable-planet-change-with-stars-of-differe/40758#40758

The answer by user177107 has a table of the characteristics of various types of stars. It includes the radius of the orbit where a planet would received exactly as much radiation from its star as Earth gets from the Sun. I call that distance the Earth Equivalent Distance or EED. And it also has the orbital speed and the orbital period or year of the planet.

Stars are divided into various spectral class which have capital Latin letters, in the order of O, B, F, G, K, & M (and some other more unusual ones) going from hotter photospheres to cooler ones. And each spectral class is subdivided into 10 subclasses with Arab numberals from 0 to 9 in order of decreasing surface temperatures. Stars are also classed by luminosity using Roman numerals. All main sequence stars are in luminosity class V.

According to the table, a K8V star would have an EED of 0.281 AU and an orbital period, or year, of 70.95 Earth days.

According to the table, a K5V star would have an EED of 0.406 AU and an orbital period, or year, of 114.84 Earth days.

According to the table, a K2V star would have an EED of 0.58 AU and an orbital period, or year, of 182.93 Earth days.

A rotation period of 24 years that were 70.95 Earth Days long would be 1,702.8 Earth days, or 4.66 Earth years long.

A rotation period of 24 years that were 182.93 Earth Days long would be 4,390.32 Earth days, or 12.0200 Earth years long.

So 24 years of a planet in the EED of a k2V star would be 2.579 times as long as 24 years of a planet in the EED of a k8V star

And of course there were would even greater difference between the orbital periods around a K0V star and a K9V star.

Your planet doesn't have to orbit exactly at the EED distance from its star, and could be closer to or farther from its star. I am not certain that a planet orbiting 0.75 AU from even a K0V star would receive enough radiation, but I don't know much about climate.

Thus there is considerable variation in the possible year length of your planet, and thus in 24 times that year.

If you mean that the planet rotates once in 24 Earth years, or about 8,766 Earth days, that would be about 47.9199 to 123.55 of the years of your planet.

In any case, your planet has a rotation period, persumable its sidreal rotation relative to the distant stars, that is many times as long as its orbital period around its star.

I note that tidal interactions with a star can only slow the rotation of a planet to the point where it is tidally locked and has a rotation period equal to its year. Tidal interactions cannot slow the planet's rotation beyond that point.

I note that the very odd rotations of Venus and Uranus are believed to have been caused by giant impacts very early in the history of hte solar system when such events were more common. And I recently read a statement that the rotation rates of planets are mainly determined by the giant impacts they encounter during their formation and early history.

If that is the case, it is possible that the rotation rate of your planet was slowed drastically by a giant impact it received.

Of course, I don't know for certain whether the question said that 24 years was the length of the sidereal day of the planet, the time it takes to rotate 360 degrees with respect to the stars.

Possibly you ment that the stellar day of the planet would be 24 years long. The stellar day woudl be the equivalent of Earth's solar day, the time it would take for the planet to rotate 360 degrees with respect to its star.

Since the planet is orbiting around the star and changing the direction to the star while it is also rotationg on its axis, those two movements combine make the stellar day. If a planet's orbit and rotation is similar to that of Earth's its stellar day will be very similar in length to its sidereal day.

If the stellar day of the planet is supposed to be 24 Earth days or about 8766 ath days, the planet will rotate 15 degrees with respect to the star in every Earth year, and 0.4106 degrees every Earth day, and 0.0017111 degrees every Earth hour.

The equatorial circumference of Earth is about 40,075 kilometers or 24,901 miles. So 1 degree along the equator is about 111.31944 kilometers or 69.169 miles. So animals and people trying to stay in the temperate zone would have to travel 1,669.79 kilometers or 1,037.54 miles per Earth year, 4.57 kilometers or 2.84 miles per Earth day, and 0.19 kilometers or 0.118 miles per Earth hour.

What do you mean by:

translation period lasts about 365 days

Is the translation period the length of the stellar day of the planet. In that case the animals and people trying to stay in the temperate zone would have to travel 24 times faster than in the previous calculation.

Or is the translation period the amount of time that any particular longitude of the planet will remain in the temperate zone, and the maximum amount of time that people and animals can stop in one spot during their endless migrations aroudn the planet.

If the temperate zone is 10 degrees wide, and a spot will remain in the temperate zone for about 365 Earth days, the stellar day of the planet would be about 36 Earth years or about 13,140 Earth days long.

PS.

If the planet takes 24 Earth years for each rotation with respect to the stars, it will be very close to a perfect sphere in shape, and there should be no tendency for the oceans to flow toward the poles.

I suspect that the shape and location of continents and oceans will be due to the plate tectonics of the planet, and they will move over periods of tens and hundreds of millions of years.

On Earth at the moment the north polar ocean is almost totally surrounded by two land masses. The south polar region is a continent, but once it moves far away from the pole in the vast southern oceans, there will be a polar ocean. Then there would just have to be enough continental movment to make Eurasia and the Americas join together into a continent stetching around the world.

So a planet with polar oceans and one giant equatorial continent is possible.