Moons

A moon is an object that orbits a planet, like Planet Earth, not a star, like Sol, our sun.

Although, in recent decades, astronomers have detected thousands of exo-planets -- planets circling other stars, no exo-moons have been discovered. However, astronomers believe they are out there, to be discovered.

Luna, the object that orbits Planet Earth, is a moon. Five of the Solar system's eight planets. The oldest moons have more or less circular orbits, and either formed at the same time the planets accreted, or shortly thereafter. They orbit above the planet's equators. Younger moons, with more eccentric orbits, are believed to be captured asteroids, or possibly comets.

Galileo discovered four moons orbiting Jupiter not long after he was the first person to use a telescope to study the sky. Those first moons were Ganymeed, Callisto, Io and Europa.

Since then hundreds of moons have been discovered.