(A table of contents is available. This series will remain open for additional posts and the table of contents up-to-date as new posts are added.)
Part Nine: Moons, Moons, and More Moons
Just like how planets orbit a star and the stars orbit the center of a galaxy, and two stars might orbit each other, and two pairs of stars might orbit each other, and asteroids sometimes orbit each other, so too do planets get their own tag-alongs orbiting them. These celestial bodies–natural ones, mind, not constructed ones–are called moons or satellites.
Moons are defined by both size and composition. Major moons have enough mass that they retain a mostly spherical shape while minor moons are smaller, with less mass, and therefore are frequently oddly shaped. The key to size and shape is 200-300km in size, with minor moons being less than that size and major moons being larger. Moons tend to be comprised of either rock or ice, depending on where in the system it’s located. Obviously, like the formation of icy planets, icy moons must form outside the frost line.
Moons collect around gas giants in kind of three groups:
- Small, irregular rocky minor moons gathered in an orbit on the outer edge of where the planet could potentially have rings. They are generally only 10-100km in size and make sure to leave 0.25-1.5 Earth-radii of space between each other.
- A handful of large major moons gathered in orbit outside where the planet’s rings are. They leave more than half the planet’s size between each other. They may even have minor moons of their own.
- Tiny, irregular minor moons–often captured asteroids–orbiting distantly, at the very edge of where they’re able to orbit. Generally in the 10s of kilometers in size, these minor moons orbit on very eccentric and inclined paths, and sometimes even backwards (retrograde).
Don’t forget! Moons clear out the debris from their orbits paths! So if you position a moon in the midst of a gas giant’s rings, they’ll create a gap in the rings where they orbit. Some major moons of gas giants cause major gaps in the rings, even though they’re not actively orbiting in the rings to clear it out. The reasons for this are a bit complex, but if you wanted to build a ringed gas giant with a major moon close-ish to the planet and a major gap in its rings, you totally could!
Terrestrial planets don’t tend to accumulate many moons. They may have enough gravity to pull in some asteroids to become moons, like Mars’ two moons. (Remember that asteroids are mostly chunks of rock that are small enough that they would be considered minor moons.) To go along with this, distant planets are likely to have more moons, and close-in planets will have less.