# How much play is there in building bigger-than-Earth planets that support humans?

Are there models, problems, and or engines for building life-sustaining worlds?

I believe that my approach is backwards. I have written a lot on my world and have been slowly working at making the humanoids, animals, plants and scale of the world all work.

This is the main question presently:

What would the makeup of a world need to be, to be 46.7 times the size of Earth, with 14% less gravity?

• Volume or area? Aug 25, 2015 at 20:19
• By 46.7 times the size, you mean physical size, and not mass, right? Aug 25, 2015 at 20:20
• Random Inspired Question: How big could a mass of cotton candy get before it collapsed into a solid and/or became a planet? Aug 25, 2015 at 20:22
• Yes the reference is to physical size, not mass. Which, although i see your math, i believe we can play with the amount of mass that is iron to affect the gravity. If not then help me understand the fixed points of the algebra. Aug 25, 2015 at 21:05
• @MichealEldridge Is that a comment on my answer? If so, comment beneath it, and point out the specific parts you want clarification on. Aug 25, 2015 at 21:06

# Not much.

I assume that by "14% less gravity" you mean that at the surface, the force of gravity is only 0.86 times its strength on Earth's surface, i.e $$g=0.86g_{\oplus}$$. You also say that the size is 46.7 times the size of Earth, or $$V=46.7V_{\oplus}$$. Putting these requirements together tells us that the planet's mass and radius must be $$M=11.2M_{\oplus}$$ and $$R=3.6R_{\oplus}$$. The density is therefore $$0.239$$ times that of Earth - that is, $$1.32\text{ g cm}^{-3}$$.

Looking at various mass-radius relation models, the planet should be composed largely of water, with a significant hydrogen/helium envelope. It's not significantly different from Uranus of Neptune in size, mass and density.

Such a planet will certainly be gaseous - it is far too big to be a super-Earth, although it could be a massive solid planet - because of how massive it is. It will likely have scooped up a fair amount of gas and dust from the original protoplanetary disk.

• Ok, well then, the size of me planet can not change. For it is far to large a part of much of the settings in the book. The gravity is not fixed. That was just a random thing i had come up with. Aug 25, 2015 at 21:10
• @MichealEldridge So, the mass could change, then? Aug 25, 2015 at 21:10
• Yes if that would make the size work. Aug 25, 2015 at 21:19

I'm going with the assumption that you mean volume 46.7 times the size of Earth.

I would say look at Neptune. It's radius is 15299 miles vs Earth's 3959 miles

this gives Neptune a volume of 8432960097242.145 cubic miles, vs. Earth at 146132702751.045

This makes Neptune 57.7 times the volume of Earth, a little bigger than your request.

Neptune's gravity is 11.15 m/s and Earth's is 9.8 m/s, So Neptune is %13 greater than Earth.

So as a rough estimate making a planet just like Neptune but at 46.7 times the size of earth should be pretty close. However, you really need to look at Neptune, since it is a gas giant and as such you will fall a long ways before you hit anything 'solid'...

To get a planet that large and 'solid' like earth, the gravity is going to be MUCH, MUCH more.

Kepler-10c! The largest rocky planet discovered to date. it is estimated that it is 15-19 times the mass of Earth, it has about 2.5 times the radius which is 12-15 times the volume of Earth.

• I think i understand, as far as "Much, Much more" goes. The composition of the planets core I believe could be adjusted. IE the amount of iron present in the core. Not the mass of iron itself, i realize that its mass is constant, no just the amount present combined with the rest of its said volume. Aug 25, 2015 at 21:14
• @MichealEldridge Well if it had a Lithium core you could get a bit bigger with less mass. Aug 25, 2015 at 21:18
• So lithium is less dense than iron i take it? (I will trust the facts presented, and not go look up all the elemental weights my self.) Not a scientist, just love the stuff and wish to learn so as to improve the real and wow factors of my world. Aug 25, 2015 at 21:23
• @MichealEldridge Yes Lithium is the lightest metal in existence. Third lightest element after Hydrogen and Helium Aug 25, 2015 at 21:25
• Not sure but I may have to invent a new element that has less weight but at least includes the properties of iron. So to have an atmosphere that can handle solar winds and such. Aug 25, 2015 at 21:40