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?

  • $\begingroup$ Volume or area? $\endgroup$
    – bowlturner
    Aug 25, 2015 at 20:19
  • $\begingroup$ By 46.7 times the size, you mean physical size, and not mass, right? $\endgroup$
    – HDE 226868
    Aug 25, 2015 at 20:20
  • 1
    $\begingroup$ Random Inspired Question: How big could a mass of cotton candy get before it collapsed into a solid and/or became a planet? $\endgroup$
    – Frostfyre
    Aug 25, 2015 at 20:22
  • $\begingroup$ 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. $\endgroup$ Aug 25, 2015 at 21:05
  • $\begingroup$ @MichealEldridge Is that a comment on my answer? If so, comment beneath it, and point out the specific parts you want clarification on. $\endgroup$
    – HDE 226868
    Aug 25, 2015 at 21:06

2 Answers 2


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.

  • $\begingroup$ 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. $\endgroup$ Aug 25, 2015 at 21:10
  • $\begingroup$ @MichealEldridge So, the mass could change, then? $\endgroup$
    – HDE 226868
    Aug 25, 2015 at 21:10
  • $\begingroup$ Yes if that would make the size work. $\endgroup$ 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.

  • $\begingroup$ 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. $\endgroup$ Aug 25, 2015 at 21:14
  • $\begingroup$ @MichealEldridge Well if it had a Lithium core you could get a bit bigger with less mass. $\endgroup$
    – bowlturner
    Aug 25, 2015 at 21:18
  • $\begingroup$ 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. $\endgroup$ Aug 25, 2015 at 21:23
  • $\begingroup$ @MichealEldridge Yes Lithium is the lightest metal in existence. Third lightest element after Hydrogen and Helium $\endgroup$
    – bowlturner
    Aug 25, 2015 at 21:25
  • $\begingroup$ 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. $\endgroup$ Aug 25, 2015 at 21:40

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