First, this is my current understanding of things, using the Universe Sandbox.

To have the same surface gravity as Earth's on another planet that's smaller than Earth, you would have to increase the density of the smaller planet.

For example, if you would keep the density at Earth's current 5.51 g/cm³, Planet B with 0.5 of Earth radius would have 0.125 of Earth mass and only 0.5 of Earth surface gravity. To get to the surface gravity up to normal Earth, you would have to double the mass to 0.25 of Earth, which would mean a density of about 11 g/cm³ for Planet B. Now, as far as I understand, that density would not really be possible, even with a very heavy core, like pure iron. Fictional materials aside, a Planet B with 0.5 of Earth radius and same surface gravity cannot exist. I think I'm pretty safe so far.

Now let's assume a Planet C with 0.8 of Earth's radius. If my calculations in Universe Sandbox are correct, then that planet would have about the same surface gravity as Earth when its mass is about 0.644 of Earth which would mean a density of about 6.93 g/cm³. I think that would be plausible.

Now, to my actual questions:

Q1: Assume Planet C with 0.8 of Earth's radius and 1 Earth surface gravity which would be achieved by a higher percentage of Iron (I really have no idea about planet compositions, I just read somewhere that more iron would be one way to increase density). What would the weather be like on that planet? Especially regarding storms, would they be more/less frequent or more/less intense?

Q2: Assume the same Planet C as in Q1, except the land masses are more like in this rough image:enter image description here Map as a rotating Globe: https://drive.google.com/file/d/1yCxESjBvE53VRc_qQPJHISGH2ZS1d3bP/view?usp=sharing

What would the weather be like on that planet? Especially regarding storms, would they be more/less frequent or more/less intense?

Q3: Assume the same Planet C as in Q2. Which variables would have to change for more frequent and more intense storms. For reasons I won't get into here, the 0.8 Earth radius is fixed, so I guess the only actually variable variables here could be the Planet's density and the core materials. If someone of you really knows their stuff, perhaps other variables could be the distance to the Sun or intensity of the Sun, but that's where I have virtually no knowledge in and I would be hesitant to change those things in my setting.

One more thing I should probably clarify: Apart from Storms, the average temperature and atmosphere should stay similar to Earth, or at least habitable.

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    $\begingroup$ The only major two factors that a planet size would change are the strength of the Coriolis Effect, though rotation rate is by far the more important factor here, and surface area, thus by extention the size of climate zones. The latter would only really become interesting if the world rotates quickly and climate zones are narrow and fragmented anyway, because the planet has 5 or 7 atmospheric circulation cells insted of earths 3. Check out this blog for a great bereakdon of more relevant details. $\endgroup$ Jun 28, 2021 at 15:51
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    $\begingroup$ What software did you use to create your map & globe, if I may ask? (Also, I like your geography, especially that long northern strait. Lots of story potential.) $\endgroup$
    – BMF
    Jun 28, 2021 at 20:55
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    $\begingroup$ @BMF I'm creating my map with inkarnate.com. I've tried the free version and it was pretty good, but there was one thing (I think it was max resolution) that made me try the subscription, I don't regret it. To make a globe, I use maptoglobe.com, you just gotta remember to create the map in a 2:1 resolution to be able to turn it into a globe that way. The distortion on the poles in globe form is a bit annoying but I haven't found any better way. And thanks on the geography, I really do have a story for it in mind. The map is not nearly finished though, just finished it roughly to show here ^^ $\endgroup$
    – Arimeris
    Jun 28, 2021 at 23:05
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    $\begingroup$ @Arimeris thank you, I'll sure be giving that a look. Also, I believe it's possible to upload the gif in your link directly to your Q as an image. It's a cool graphic that probably wouldn't hurt the readability of your Q. $\endgroup$
    – BMF
    Jun 28, 2021 at 23:24
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    $\begingroup$ Something to consider, regarding a planet that dense. Disregarding pure random chance. The origins of the nebula this planet formed from is likely to be a 4th even 5th generation star. Meaning the material making up our solar system went through forming a star > supernova > reforming >another supernova for many cycles, thus concentrating the heavier elements. There are certain other possibilities but this is the most likely. And may well open up fascinating possibilities of building your world if you explore its genesis. $\endgroup$
    – Gillgamesh
    Jun 2, 2022 at 16:12

1 Answer 1


The source of weather for a planet is it's star. The weather then results from it's interaction with the albido of it's atmosphere, landmasses and seas. Example, the reason why ice ages on earth last so long is because the albedo of the mostly frozen planet reflects light and thus heat away from the planet. The reason why global warming happens (doesn't matter if it's human caused or just a natural cycle), is the increase of greenhouse effect...moisture, carbon dioxide, methane, etc causing reflected light to be reflected back down to the lower atmosphere.

Anyway, the star warms landmasses which is the source of wind, and the star warms seas which is the source of humidity. The coriolis effect influences the primary direction of winds, like the trade winds, but local weather will likely be most influenced by the rise and fall of land temps, that's why on the coast the air is usually pretty still in the early morning, and the breeze picks up in the after noon after the warmed landmass causes an updraft.

Major storms are caused by steady warm spots causing long term updrafts that gain energy over time. That's why hurricanes start in the african tropical atlantic, and gain energy as it crosses the equatorial atlantic ocean where the air is warmer, the air is drawn in from the north and south, warmed as it gets closer to the equator and is drafted up when it reaches the storm. The storms hit critical strength as they reach the carribean because the shallower summer waters are even warmer than the equatorial ocean water, and that's when they change from just a good tropical storm to 1,2 and even 3 4 and 5 rated hurricanes. Then once they hit land, the land gets wet, it cools the land down...the updraft is halted and the land (and dense vegetation like trees) creates resistance.

What does this mean for your smaller planet? I don't know....if it's got the same gravity, it's probably denser and spinning faster. Would the faster spinning create a more homogenous and even heating of the planet surface by its star? Probably some, but to what degree? I don't think we know for sure since we don't have a close truly earth like neighbor with seas to watch. Not to mention, our moon helps a great deal in ensure the seas move and mix well, and that probably has some impact as well. Does your planet have a significant moon in close orbit like we have?


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