# How big can a (human) habitable planet be while having around x0.7 the surface gravity of earth?

I've been working on the world-building for my setting and I realized I haven't really thought about the implications the lowered gravity I'm working with would have on the size of the planet.

Initially, I was thinking of a much bigger planet than earth (several times the radius), a quick google search (How to make a bigger planet be as similar to Earth as possible, Stumped: How can I get a huge Earth-like planet? and Gravity of a Super-Earth) and a bit of common sense ruled that out.

So, the question is: how big (or smaller most likely) would my planet need to be for it to have $$\times0.7 \sim\times0.8$$ the surface gravity of earth and the ability to sustain life forms similar to the ones we have around us, in particular, humans.

I've found other similar questions, but they tend to want either $$9.8 ms^{-1}$$ or don't require the ability to harbour life at all (like here).

Additionally, I've found that Chthonian planets could maybe allow for bigger than usual planets with similar gravities, but I couldn't find anything in particular regarding that, much less the lower value I'm working with.

• Don't forget that the surface gravty is important for comfort and for ease of flying, but the escape velocity is important for retention o rot of the atmosphere. And the escape velocity is not directly proportional to the surface gravity. Commented Aug 29, 2021 at 16:59

Here you go.

Our moon is about 60% of the density of Earth, being as the moon is mostly made of crust rock splashed off of the Earth, and Earth kept most of the metal. Earths density is 5.5 g/cm^3. The moon is about 3.3 g/cm^3.

I tried some numbers. A planet with radius 8000 km (Earth is 6378) and density 3.3 g/cm^3 has surface gravity of 7.38; 0.75g. SO you can have it be a little bigger.

But who cares about "little bigger". I know you want GIANT PLANET!! So let us make it out of water. We will use 1 g/cm^3. Yes I know there are funky ices with different densities but the number 1 is a good number.

The calculator gave me a planet with radius 25000 km and surface gravity 0.71g. Earth has a surface area of 5111859 square km. The water planet has a surface area of 78539816 square km! The water planet has 15 times the area of Earth!

• While I love the answer, the science-based question appears to expect human-habitable solutions and the calculator, as fun as it is, is ignoring the collapse of mass as you get closer to the center.
– JBH
Commented Aug 29, 2021 at 3:38
• Thank you! I've seen the idea of a water world, even that specific calculator I think, but like JHB said it isn't scientifically plausible, which is what I was going for. I don't mind stretching things a bit, but for this setting/story a pure water world would just raise eyebrows from my players/readers. Commented Aug 29, 2021 at 8:42
• Don't forget that the surface gravty is important for comfort and for ease of flying, but the escape velocity is important for retention o rot of the atmosphere. And the escape velocity is not directly proportional to the surface gravity. Commented Aug 29, 2021 at 16:59

If it spins fast enough, then the centrifugal force can decrease gravity more than mass increases it. That's why gravity is stronger at the poles and weaker at the equator, because they move at different speeds.

Then the only limit in size would be the point where the planet get's big enough to become a brown dwarf.

• There is also the problem of the planet's stability if it has to spin fast enough for the surface gravity to be bearable. Pages 59-61 at Habitable Planets for Man discuss limits of planetary rotation rates. rand.org/content/dam/rand/pubs/commercial_books/2007/… Commented Aug 29, 2021 at 17:06