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This question is in reference to a previous question by TrEs-2b and SRM: I want to create a planet with a significantly longer day than Earth's. However, I was only able to find a question about a planet with 9-year long days. This is way too long of an rotational period. As evidenced by the linked question, longer days equal more extreme weather (high winds, rainfall, different hemispheres). I am looking for weather like this, but do not want the distinct hemispheres (one super hot and one super cold).

So, what would be the range of day lengths needed to achieve extreme weather without distinct hemispheres?

Clarifications:

  • The planet in question is just slightly larger than Earth
  • There is one super-continent, similar to the size and shape of Pangaea, in the middle of the planet surrounded by oceans
  • Everything else is identical to Earth (atmosphere, pressure, etc.)
  • I am looking for the minimum day length to achieve extreme weather and the maximum before the distinct hemispheres form
  • What I mean by distinct hemispheres is that the only habitable space is in the middle. Polar regions and deserts are ok.

Bonus points if the orbital period around the pseudo-Sun is one "day," like if the day length is equivalent to 400 Earth days, then it takes 400 Earth days to complete a full orbit.

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    $\begingroup$ To have a day length equal to the orbital period all you have to do is have your planet have a retrograde rotation. $\endgroup$
    – AlexP
    Dec 2, 2020 at 1:21
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    $\begingroup$ Difficult: the " distinct hemispheres (one super hot and one super cold)." is what causes the extreme weather. Even on Earth, most of the weather derives its energy from the day/night cycle of heating and cooling, plus the polar areas receiving no day worth mentioning. $\endgroup$
    – PcMan
    Dec 2, 2020 at 6:46

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We know of quite a few planets with long days (Mercury has a longer day than its orbital period), but I think that Venus is most similar to what you're looking for.

As you probably know, Venus is known for extremely harsh conditions on the surface with respect to pressure and temperature. But it also has wind speeds that can be up to 85 m/s and circle the planet once every four to five Earth days.

Venus days are 116 Earth days while a Venus year is 225 earth days. That puts the day at 62% of the year's length.

Venus is about the same size as Earth, but it definitely doesn't have the same atmospheric conditions. But if you go up in the atmosphere about 50km, you'll have the same pressure as earth, a similar temperature, and the winds are still 85 m/s.

And here's something that addresses your concern about one hemisphere being super hot and the other super cold: with such high winds, the whole planet is nearly equally hot because the heat is distributed by the strong winds. You could have a tidally locked planet with fast winds to distribute the heat around the planet. Basically, any period of rotation long enough to create extremely fast winds can do away with its distinct hemispheres.

In respect to having the orbital period be the same as the day, you're either going to need them to be tidally locked, meaning one side is always facing the sun (which you said you don't want, but you could do it with the winds as I said above) or have it spin in the opposite direction of the orbit.

I hope that helps. I tried to answer the question by giving a real-world example of a planet similar to what you're describing, which I personally find helpful when I'm designing planets, but it might not be helpful in your case. If not, let me know and I'd be happy to do more theorizing.

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  • $\begingroup$ I'm rather skeptical that even very high winds could "even out" the temperatures between hemispheres of a tidally locked or slowly-rotating world. An atmosphere will generally have far less thermal mass than the rock it sits above - it's a poor way of ferrying heat from one side of a planet to another. I expect it will be like filling a pool with a hose, and trying to simultaneously empty it with a thimble. $\endgroup$
    – Nuclear Hoagie
    Jul 7 at 19:05

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