# 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? – bowlturner Aug 25 '15 at 20:19
• By 46.7 times the size, you mean physical size, and not mass, right? – HDE 226868 Aug 25 '15 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? – Frostfyre Aug 25 '15 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. – Micheal Eldridge Aug 25 '15 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. – HDE 226868 Aug 25 '15 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. – Micheal Eldridge Aug 25 '15 at 21:10
• @MichealEldridge So, the mass could change, then? – HDE 226868 Aug 25 '15 at 21:10
• Yes if that would make the size work. – Micheal Eldridge Aug 25 '15 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. – Micheal Eldridge Aug 25 '15 at 21:14
• @MichealEldridge Well if it had a Lithium core you could get a bit bigger with less mass. – bowlturner Aug 25 '15 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. – Micheal Eldridge Aug 25 '15 at 21:23
• @MichealEldridge Yes Lithium is the lightest metal in existence. Third lightest element after Hydrogen and Helium – bowlturner Aug 25 '15 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. – Micheal Eldridge Aug 25 '15 at 21:40