What Makes a Moon a Moon?

A moon is a planetary body that goes around another planetary body. Usually, this is one or more moons going around a planet, but it doesn’t have to be a planet: the dwarf planet Pluto has five moons, and the asteroid Didymos has a small moon, Dimorphos. In Star Wars, the Death Star is not a moon because it isn’t a naturally occurring satellite: the International Space Station, the Juno spacecraft orbiting Jupiter, or any of the telecommunications satellites we rely on in low Earth orbit are satellites, but are not moons.

A lot of moons form out of the same clump of material as a planet. Jupiter, its rings, and many of its ~95 moons all formed together at about the same time. Moons can also be captured: Mars’s two moons, Phobos and Deimos were probably captured from the asteroid belt and Neptune’s large moon Triton was captured from the Kuiper Belt (where Pluto lives). Moons can form from a collision after a planet has formed: our Moon formed as the result of Earth’s collision with a Mars-sized object early on in Solar System history.

There are other special kinds of moons called Trojan moons. A ‘trojan’ is a smaller planetary body that shares the same orbit as a larger planetary body (this is sometimes also called a co-orbital). But trojans aren’t necessarily moons. The most well-known are Jupiter’s trojans, a group of asteroids that share Jupiter’s orbit around the Sun, but lots of planets have trojans. Saturn has trojans that are also moons! Saturn’s moons, Telesto and Calypso and Helene and Polydeuces, have trojans that follow in their same orbits around Saturn, so they are both trojans and moons, thus, trojan moons.

Then we have Venus. Earth’s twin in size, Venus is a special planet with a fascinating climate and geologic history. However, Venus doesn’t have a moon, and neither does the planet Mercury. Many scientists think that Mercury and Venus are too close to the Sun to keep a moon in orbit, but it’s possible that Venus (at least) had a moon but lost it. If a planetary body can capture a moon, why can’t it lose one?

Venus doesn’t have a moon like we do, but it does have a quasi-moon (as does Earth) called Zoozve. If you could stand on the surface of Venus and look up at the sky (10/10 do not recommend this) a quasi-moon sort of looks like a moon. But if you actually track its orbit, you find that quasi-moons are asteroids that orbit the Sun. They aren’t quite trojans, in that a quasi-moon doesn’t follow exactly the same orbit as a planet around the Sun, and for hundreds or thousands of years they kind of mimic a moon, so they have been given the name quasi-moon.

That leaves us with mini moons. A mini moon is a small body, like an asteroid that temporarily gets captured around a planet, and Earth has acquired a mini-moon recently! It’s only 33 feet across, has the romantic name 2024 PT5, and will only hang out with us for 57 days. This mini moon is an asteroid that got a little caught up Earth’s gravitational field, but only for a few months: 2024 PT5 will move on its cosmic travels and continue to follow its own pat.