# How to make a bigger planet be as similar to Earth as possible

My fantasy world is slightly larger than earth so that discovery of the continents becomes harder (about 20% surface should do it). But at the same time I want to preserve most aspects to make it like the earth, it has a moon, similar animal species, similar climate, four seasons, temperatures ranging from 50C to -60 or so, magnetic field etc. Also similar proportion of minerals in the crust. To achieve this I thought of making the planet spin faster and it's tilt angle to be slightly lesser. Would that work?

How would the increased weight come into play? Would density of the inner mantles be important to preserve or could we make them more porous so to keep 1G without much changes in the surface? Would we still have the same tectonic activity or should we change the nature or thickness of the crust? Would the climate change a lot or the scale of the cyclon/anticyclone system would remain the same? Should I make the atmosfhere lighter?

Any other solutions will be welcomed. I found this post very useful.

• How big do you want it? – Vincent Dec 9 '18 at 22:05
• Edited (about 20%), thank you for your question Vincent. – Tomás Dec 9 '18 at 22:09
• +20% area or radius of planet? – Artemijs Danilovs Dec 9 '18 at 22:27

Gravity: Without some sort of unobtanium it will be difficult to substantially increase the diameter of an earth-like wold without also substantially increasing its surface gravity.

If you replace most/all of the iron in the core with lighter materials like silicates and magnesium, you might be able to reduce the density of the core by ~50%. But as the core only makes up about 10% by volume, this would only reduce the average density by around 5% - assuming this could still produce a planet with a crust still containing substantial amounts of iron and other heavy elements as found on the earth. You could possibly do a little bit better by changing the ratio of silicon to other elements, but probably only a couple more percent difference in density. Voids and porosity won't work - at the sort of pressures you find in the core there are no voids. You could go a bit more extreme by replacing substantial amounts of silicon and similar elements with water, carbon and other lighter materials. This could reduce density substantially, but now the crust (and the tectonics) is definitely not going to resemble that of earth (maybe substantial engineering by a super-civilization could get you a reasonable approximation - but it would not form 'naturally').

So assuming a density 7-10% less than that of the earth, surface gravity scales as density $$\times$$ radius2. So if you allowed for surface gravity to be maybe 10% higher than earth's (is that similar enough?) - you could get away with approximately 10% greater radius. This would provide you with ~30% more surface area. If you want to retain ~1g then you are probably limited to radius increase of no more than 4-5% without weird global engineering.

Spinning faster: Spinning faster won't help reduce surface gravity unless you spin ridiculously fast - 20 minute days anyone?

Tilt: Increasing the tilt angle would make the climate more extreme (possibly much more extreme). And as a larger world would likely have more extreme weather anyway (more space for thermal gradients etc to develop), you would likely get a pretty stormy climate with much higher average wind speed. Perhaps that will help you get what you want - a slightly bigger planet with a slightly higher frequency of horrible weather would indeed be harder to explore.

As an added note - re-positioning the continents can also make things more difficult for exploration. If the Pacific was narrowed by 30% then the Atlantic could be widened by ~50% (without significantly altering the shape of the continents). This would have made it harder/impossible for Columbus etc. to have 'discovered' the new world (which could have led to at least a century delay till it was found by Europeans).

• Thanks for the answer, it's practically perfect, I didn't intend more than 30% more of surface. Spinning faster is about the day/night cycle, not gravity. Tilt, I guess I got it backwards, shouldn't it be useful to compensate the extreme wheather? The occasional horrible weather could fit perfectly. – Tomás Dec 9 '18 at 21:07
• It turns out that planetary size doesn't affect surface gravity all that much once you get larger than Venus. The planet will likely have ~1g even if it is significantly larger than Earth. – eyeballfrog Dec 10 '18 at 1:19
• @eyeballfrog A careful reading of that paper shows that this is only true for 'transitional' super-earth type planets that fit between rocky earth-like planets and gas giants. The reason is that most 'super-earths' have a very significant atmosphere so the effective planetary radius increases more quickly with mass. But these super-earths bear little resemblance to our planet - the 'surface' is measured at the gas surface. If you stick with rocky planets with an atmosphere approximately comparable to ours (say no more than 10-20 km deep), surface gravity will scale as I suggested. – Penguino Dec 10 '18 at 1:58
• Great answer and very thorough! +1 Just to comment on the spin, faster spinning to compensate gravity would make traveling north or south impossible, as the centrifugal forces get reduced the further from the equator you are. Also I think Coriolis forces would be quite significant. – spcan Dec 10 '18 at 11:21
• And if it really was spinning sufficiently fast to make a significant diffidence to surface gravity it would be very noticeably oblate... – Penguino Dec 10 '18 at 20:14