Rocky Planets May Only Get Moons From One Source: Giant Impacts
“If your gravity rises up to a point where you can pull yourself into hydrostatic equilibrium — a sphere if you’re static, an ellipsoid if you’re rotating — you cannot be pulled apart by tidal forces so easily. But you could, in principle, develop moons through three methods: initial formation from a protoplanetary disk, capturing another passing body through gravitational forces, or from the debris of a large collision.
While the gas giants display moons that appear to have arisen from all three, the rocky planets, including both major and minor planets, appear to obtain moons from collisions alone. It may be the case that the other options are viable but rare, and we simply have yet to discover them. But following the evidence we have today, perhaps Earth’s moon isn’t atypical after all. Until further notice, giant impacts are the only known way for rocky planets to gain moons.”
When we first visited the Moon and returned samples back to Earth for analysis, scientists were surprised to learn that the lunar surface was practically identical to Earth’s surface. The elements were the same; the isotope ratios were the same; the ages were the same. Unlike the other moons around other planets, the Earth’s moon appeared to be made out of the exact same material as our own world. This helped lead to the giant impact hypothesis as the origin of the Moon. Surprisingly, Mars’ moons and Pluto’s moons appear to have a similar origin: from a giant impact. Of all the planets and dwarf planets with moons, in fact, it looks like giant impacts explain them all.