SitheseParadox wrote:Hello, lads! I am new around here...
Anyways, I am making a planet for a roleplay forum I'm involved with, and I've stumbled across a rattling concern: In order to make my planet have Earth gravity with 0.6 Earth radii, I've calculated that my planet's mean density must match 21.77 g(cm^3)^-1. I have allocated most of the planetary mass to osmium-iridium (It is an alien construct forged from nuclear-transmuted elements, perhaps) , but now I need to fill in the rest of the mass with other elements.
This may be more mathematical than conceptual, but might someone lend me a hand in making a well-rounded planetary composition capable of (barely) tolerating human life?
You should check out Stephen L. Gillette's World-Building
, part of Ben Bova's Science Fiction Writing Series
(There are at least four books in the series, but the only two I've ever had are the above and Stanley Shmidt's Aliens and Alien Societies
; which I also recommend. Probably the others would also be good and helpful, but I've never seen them.)
The main thing, as far as whether or not the planet would support life, is the makeup of its "volatile" or "atmophilic" substances.
A compound is called "volatile" or "atmophilic" if it remains on or above the crust because it is fluid (liquid or gas) in the conditions ambient on the planet's surface. An element is called "atmophilic" if it readily forms atmophilic compounds in the conditions ambient on the planet's surface.
Clearly, which substances are "atmophilic" or "volatile" depends on the planet -- how big it is, how hot it is, how close to its primary star it is, what the relative chemical abundances were in the dust-cloud out of which it was formed, etc. (Or what the aliens made it out of.)
For technology you also want to know about the make-up of its "lithophilic" ("rock-loving") substances, and its "chalcophilic" ("ore-loving") substances.
An element is called "lithophilic" if it remains on or in the crust because it readily combines with oxygen to form compounds that don't sink into the mantle.
An element is called "chalcophilic" if it remains in the crust or upper mantle because it readily combines with sulfur or some other chalcogen (e.g. selenium or tellurium -- but oxygen and sulfur are most of the chalcogens) to form compounds that don't sink into the lower mantle or the core.
The remaining major category of chemical constituents of a planet, are compounds are called "siderophilic" ("iron-loving"). These sink into the lower mantle or the core -- at least the outer core -- because they readily dissolve in iron. Elements that form siderophilic compounds are called siderophilic elements.
Again, which compounds and which elements are lithophilic, which are chalcophilic, and which are siderophilic, depends on various other information about the planet -- the answer may be different from one planet to another. For instance, for some planets H2
O is more lithophilic than atmophilic.
Anyway; Gillette's book gives "recipes" for making several types of worlds. If memory serves that includes what I think is the type you are interested in making.
Go ahead and look up the other stuff on World-Building in this search