Our solar system is a dynamic neighborhood, with planets not always orbiting alone. Many worlds host their own celestial companions, natural satellites that dance around them.
What is a Moon?
A moon, in astronomical terms, is a natural satellite, meaning it is a celestial body that orbits a larger astronomical object like a planet or a dwarf planet. The gravity of the larger body keeps its moon in a consistent path. Natural satellites vary greatly in size, composition, and origin, from large, spherical worlds to small, irregularly shaped rocks.
Inner Planets and Their Moons
The inner, rocky planets of our solar system exhibit a wide range in their moon counts. Mercury, the planet closest to the Sun, possesses no moons. Venus, Earth’s neighboring planet, is also moonless.
Earth has one natural satellite, the Moon. Our Moon is notably large relative to its host planet, influencing Earth’s tides and stabilizing its axial tilt. Moving further out, Mars has two small moons, Phobos and Deimos. These Martian moons are irregularly shaped, resembling asteroids, and are thought to be either captured asteroids or remnants from a past impact. Phobos orbits extremely close to Mars and is gradually spiraling inward, eventually destined to either crash into the planet or break apart to form a ring.
Outer Planets and Their Moons
Beyond Mars lie the gas and ice giants, which boast impressive collections of moons. Jupiter, the largest planet, has 95 officially recognized moons, making its system a miniature solar system in itself. Among its most famous are the four Galilean moons—Io, Europa, Ganymede, and Callisto—first observed by Galileo Galilei in 1610. Ganymede stands out as the largest moon in the entire solar system, even surpassing the planet Mercury in size. Europa is particularly intriguing due to evidence suggesting a vast liquid-water ocean beneath its icy crust, a potential habitat for life.
Saturn, renowned for its prominent rings, currently holds the record for the most moons in our solar system, with 274 confirmed satellites as of March 2025. This remarkable number includes planet-sized Titan, the only moon in the solar system known to possess a substantial atmosphere and stable bodies of liquid on its surface, similar to Earth’s water cycle but with liquid hydrocarbons. Another notable Saturnian moon, Enceladus, harbors a subsurface ocean beneath its icy shell, from which plumes of water ice and vapor erupt into space.
Uranus, the ice giant, has 28 known moons, many of which are named after characters from the works of William Shakespeare and Alexander Pope. Its moon system is less massive than those of the other giant planets, with its five largest moons being Titania, Oberon, Ariel, Umbriel, and Miranda. Lastly, Neptune, the outermost planet, has 16 known moons. Its largest and most significant moon is Triton, notable for its retrograde orbit, meaning it orbits Neptune in the opposite direction of the planet’s rotation.