Celestia Forums

Can someone tell me how to figure out what the gravity, rotation and orbit of a planet would have to be in order for a planet 25 times larger than Earth to have a gravity of between .90 and 1.10 of Earth Normal Gravity?

I am writing a SciFi story and the planet is ideally very similar to Earth in most planetary chacteristics the size is the one thing I am trying to figure out. Any suggestions on this one?

You will have to define 'larger' a bit more clearly. Do you mean 25 times the diameter? 25 times the volume? 25 times the mass? 25 times the surface area?

Choose one of these, and we can try to figure something out.

But to get an Earth-like gravity on such a big world would require that the planet either has a very low density- practically no more than a gas giant-
or the planet must spin very fast, so that the centrifugal force at the equator counteracts the pull of gravity from the plant's mass.

Mesklin is a famous example in fiction of the second type of world; but I doubt that such a world would exist in real life.
http://en.wikipedia.org/wiki/Mesklin

Thank you for the Hal Clement reference. I have never heard of him before. I am amazed at the volume of 1950's SciFi references that people keep sending me in relation to parts of my story that I ask questions about.

The surface area was mainly what I was thinking about being 25x Earth normal. I have had several people now question the stability of a planet that size and a gravity that low. I have the means in the story to artificially accomplish this but I was hoping for a more natural solution. Though it is looking more and more like it will require those artificial means to accomplish what I am after.

With a surface area 25 times Earth, the radius of your planet would be 5 times as large as Earth.

For a gravity 1.10 of Earth's that makes the density 1228kg per cubic metre; lower than the density of Callisto, the least dense large object in the solar system that I can find by Googling. (1860kg/m3)

Unlikely, to say the least. Even if your planet were made of pure ice, I think it would compress under its own weight until it was denser than the figure you need. But a rapid spin might do the trick, although only in a thin band around the equator.

What artificial means are you thinking of, by the way? One way would be to make the planet artificial; a shell suspended above a smaller, denser planet at the right height to produce the surface area you require. But this requires megascale engineering and a constant supply of energy to hold the shell in place.

Thanks for your efforts. I think that about kills the "naturally occuring" idea. So plan B it is.

Ironically, that is very close to what a good friend suggested as a solution. The shells would be artificially suspended via a repulser tech. though the energy source question is a good point that I haven't had anytime to consider. Any ideas?

The macroengineering is for the most part child's play to the architects of this decaying civilization. The true keepers of the system are dead but the autosystems are more than sufficient for several more millenia provided nobody starts blowing things up... =D

In Orion's Arm we have a kind of artificial planet suspended above a small black hole; a hole the mass of the Earth would be about the size of a golfball, and you could use it to convert mass into energy to support the shell. Trouble is, a hole that size evaporates too slowly to compensate for the material which falls in during the energy generation process, so your hole slowly increases in mass.

This means your artificial planet needs to get bigger over time to maintain the same surface gravity.
(and no, there isn't a page about this concept yet...)