This is Part I of hopefully a lengthy series, posted as I graduate to new levels or sections, of a top-down method of creating a game world, starting at the "top" - the celestial body itself. I'll be using Earth and our solar system as my base examples, so let me put its stats up first.
| PLANET | DIA (MI) | SqMi | Land% | Land SqMi | NumCont |
| Earth | 7,930 | 197,000,000 | 30% | 57,000,000 | 7 |
Now, looking at the other planets in just our solar system, I find that they range in diameter from 3,000 to 90,000 miles in diameter. I'll also include the relatively new classification of "dwarf planets", of which Pluto is now a member; these range from 600 to 1,600 miles in diameter. So overall, the planets and "planetoids" in our solar system range from 600 to 90,000 miles in diameter.
After extrapolating both directions from the above averages, I arrived at a fairly consistent chart for random dice determination for new planets. You could either use the 3d6 roll to see which type of planet you're making, or just choose whatever strikes your fancy.
1. Planet Size and Type
| 3d6 | PLANET TYPE | DIAMETER (MI) | RESULT RANGE | EXAMPLES |
| 3-4 | Dwarf | 200 + 3d6 x100 | 500-2000 | Pluto, Ceres, Eris |
| 5-9 | Small | 1000 + 3d6 x500 | 2500 to 10,000 | Earth |
| 10-13 | Medium | 8000 + 3d6 x1000 | 11,000 to 26,000 | |
| 14-16 | Large | 24,000 + 3d6 x1000 | 27,000 to 42,000 | Neptune, Uranus |
| 17 | Giant | 35,000 + 3d6 x3000 | 44,000 to 89,000 | Saturn, Jupiter |
| 18 | Super Giant | 2,000 + 3d6 x6000 | 90,000 to 180,000 | TrES-4b |
2. Total Planetary Area (Square Miles)
Multiply the planet's diameter (in square miles) by itself (in other words, square it), then multiply that result by Pi (3.1416). So the formula is:
Total Planetary Area Formula: Planet Diameter^2 x 3.1416 (Diameter x Diameter x 3.1416)
The Earth example is:
7,930 x 7,930 = 62,885,000 x 3.1416 = 197,560,000 square miles
3. Land/Water Ratio
Use the below chart to either choose or randomly determine what percent of your planet is land. The remaining percent is of course, water.
Total Planetary Land
Multiply your Total Planetary Area by your Land% (40 = 0.40) to get the actual square miles of land of Total Planetary Land.
| 2d6 x5 | Land% | World Type |
| 2 | 40 | Aquatic |
| 3 | 45 | Near-Aquatic |
| 4 | 50 | Very Wet |
| 5 | 55 | Wet |
| 6 | 60 | Tropical |
| 7 | 65 | Near-Tropical |
| 8 | 70 | Earthlike/Temperate |
| 9 | 75 | Dry |
| 10 | 80 | Arid |
| 11 | 85 | Near-Desert |
| 12 | 90 | Desert |
4. Continental Area (Square Miles)
Now we'll determine both the number of continents on your planet, and their area in square miles.
Smallest Continent Area
We will start by determining the size of your smallest continent, by multiplying your Total Planetary Land by 5% (0.05). This constitutes the minimum square mile criteria required for a landmass to be considered a continent. If you generate any area of land roughly this size, it likely signifies you have determined your last continent.
Largest Continent Area
Make the roll indicated below and multiply it by your Total Planetary Land to determine your largest continent's area in square miles.
Continental Area Percent Formula: 15 + 2d6 x5 (yields 25-75% or 0.25-0.75)Second and Additional Continent Areas
For your additional continents, subtract the largest continent's area from the Total Planetary Land to get your Remaining Land Area. Make a second Continental Area Percent Formula roll, multiplying the result by your Remaining Land Area to get your new Remaining Land Area.
Do this, multiplying your new Continental Area Percent by the most recent square miles of your Remaining Land Area, until your Remaining Land Area is approximately equal to your Smallest Continent Area, at which point you assign this last area of land as your last continent. You should wind up with somewhere between 4 and 16 continents, depending on planet diameter and total land area values.
Next: Now What? Maps and Continents
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