LET THERE BE DRY LAND...
Planets are the main background in our stories, like dimensions in a magical universe, each planet is a different world, containing different many environments in itself...
The planets in your stories could be of all types, from rocky hot Mercuries, cold deserts like Mars, or hot Jupiters, or similar to Earth in some extent...
Whatever planet or moon you may choose, keep in mind that everything can be written down to math.
All values, for Density, Mass, Radii and Volume will be given in Earth relatives, ie, Earth values are equal to 1.
For reference, Earth density is around 5,51 g/cm³. Is very recomended your density wander in the realm of solid stuff like silicates and metals, unless you are building a Waterworld or a Gas giant.
Theoretically, habitable worlds "comfy" stats goes as:
g = 0,68 ~ 1,5
M = 0,4 ~ 2,35
R = 0,78 ~ 1,25
But could go as far as these, if you extrapolate habitability for settling:
g = 0,4 ~ 1,6
M = 0,1 ~ 3,5
R = 0,5 ~ 1,5
These last Extensive values includes worlds as small as Mars in mass, gravity and radii, just for comparison.
As for density, you can easily solve through directly crunching the numbers into average values, for rocky planets, use densities similar to Earth, or go through material densities, if they are abundant.
Here is a list of metal densities for use:
| Element | Symbol | Density g/cm3 |
|---|---|---|
| Actinium | Ac | 10 |
| Aluminum | Al | 2.70 |
| Antimony | Sb | 6.68 |
| Barium | Ba | 3.62 |
| Beryllium | Be | 1.85 |
| Bismuth | Bi | 9.79 |
| Cadmium | Cd | 8.69 |
| Calcium | Ca | 1.54 |
| Cerium | Ce | 6.77 |
| Cesium | Cs | 1.93 |
| Chromium | Cr | 7.15 |
| Cobalt | Co | 8.86 |
| Copper | Cu | 8.96 |
| Dysprosium | Dy | 8.55 |
| Erbium | Er | 9.07 |
| Europium | Eu | 5.24 |
| Gadolinium | Gd | 7.90 |
| Gallium | Ga | 5.91 |
| Gold | Au | 19.3 |
| Hafnium | Hf | 13.3 |
| Holmium | Ho | 8.80 |
| Indium | In | 7.31 |
| Iridium | Ir | 22.5 |
| Iron | Fe | 7.87 |
| Lanthanum | La | 6.15 |
| Lead | Pb | 11.3 |
| Lithium | Li | 0.53 |
| Lutetium | Lu | 9.84 |
| Magnesium | Mg | 1.74 |
| Manganese | Mn | 7.3 |
| Mercury | Hg | 13.53 |
| Molybdenum | Mo | 10.2 |
| Neodymium | Nd | 7.01 |
| Neptunium | Np | 20.2 |
| Nickel | Ni | 8.90 |
| Niobium | Nb | 8.57 |
| Osmium | Os | 22.59 |
| Palladium | Pd | 12.0 |
| Platinum | Pt | 21.5 |
| Plutonium | Pu | 19.7 |
| Polonium | Po | 9.20 |
| Potassium | K | 0.89 |
| Praseodymium | Pr | 6.77 |
| Promethium | Pm | 7.26 |
| Protactinium | Pa | 15.4 |
| Radium | Ra | 5 |
| Rhenium | Re | 20.8 |
| Rhodium | Rh | 12.4 |
| Rubidium | Rb | 1.53 |
| Ruthenium | Ru | 12.1 |
| Samarium | Sm | 7.52 |
| Scandium | Sc | 2.99 |
| Silver | Ag | 10.5 |
| Sodium | Na | 0.97 |
| Strontium | Sr | 2.64 |
| Tantalum | Ta | 16.4 |
| Technetium | Tc | 11 |
| Terbium | Tb | 8.23 |
| Thallium | Tl | 11.8 |
| Thorium | Th | 11.7 |
| Thulium | Tm | 9.32 |
| Tin | Sn | 7.26 |
| Titanium | Ti | 4.51 |
| Tungsten | W | 19.3 |
| Uranium | U | 19.1 |
| Vanadium | V | 6.0 |
| Ytterbium | Yb | 6.90 |
| Yttrium | Y | 4.47 |
| Zinc | Zn | 7.14 |
| Zirconium | Zr | 6.52 |
Lets say the planet I want to build is 70% Iron, 20% Titanium, 5% Tungsten, 3% Bismuth, 1% Nickel and 1% Water Ice, because I want a very cold metal world, like a mining colony for instance.
ρ = %(ρ A)+%(ρ B)+%(ρ C)+%(ρ D)
ρ = 0.7*(7.87)+0.2*(4.51)+0.05*(19.3)+0.03*(9.79)+0.01*(8.90)+0.01*(0.92)
ρ = 7.7679g/cm³ or 1,4x Earth's density
Even tho you might already know what to make, I wanna show you new possibilities:
Super-Mercury
You know the planet Mercury is very dense, it could be summarized to being a big iron ball with a thin layer of rock over it. Know what's better than making a Mercury-like world in your system? Make it real BIG, a Super-Mercury.
Super-Earth
The planet Earth is cool, may universe hide smaller versions of Earth, like Venus would have been in the past, but why not make it big? Most of the terrestrial exoplanets found are Super-Earths, larger, heavier, but still habitable having not so strong gravity and lots of land and sea to conquer.
Puffy Worlds
The opposite of a Super-Mercury, a puffy world is a planet made of light material, like silicates and aluminum, rather than Iron and Nickel. Imagine a world the size of Earth, but with 2/3rds as Earth's gravity.
Helium Titans
Gas giants are huge, take Jupiter for instance, the entire Earth could fit inside its Great Red Spot, but what if some gas giants were made from heavier materials? The second most abundant element is Helium, with the mass of 2 Hydrogens in itself, so it's not rather possible, but totally likely that any gas giant might form not from only Hydrogen but maybe 20% to 90% Helium, imagine, a Jupiter sized giant, but weighting twice as Jupiter, this could lead to some crazy heavily packed systems. Imagine explorers around a new discovered planet, smaller than Saturn with apparently way more moons and heavier ones than it could theoretically hold, but ends up that by being made of Helium, and having half radii as Jupiter, having the same mass, its gravitational pull would be 4x as powerful as our king of the planets.
Now, lets go crazy an create the planets of our system :D
- M. O. Valent, 19/02/2019
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