A moon, also known as a natural satellite, is a celestial body that orbits a larger astronomical object, such as a planet. This orbital motion is maintained by the gravitational pull exerted by the larger body. While some planets in our solar system host numerous moons, others have none at all, and the number varies significantly across the planets. These natural companions play various roles, from influencing planetary stability to potentially harboring conditions for life.
Moons of the Inner Solar System
The inner solar system exhibits a notable scarcity of natural satellites. Mercury and Venus, the two innermost planets, do not possess any moons. Their proximity to the Sun and its powerful gravitational pull makes it challenging to retain orbiting bodies.
Earth, the third planet from the Sun, is orbited by a single, relatively large natural satellite, the Moon. Earth’s Moon has a diameter of about 3,474 kilometers, making it roughly one-quarter the size of Earth. Its substantial size for a planetary moon contributes to Earth’s axial tilt stability, influencing our planet’s climate over long periods.
Mars, the fourth planet, possesses two small moons named Phobos and Deimos. These moons are irregular in shape; Phobos has a diameter of about 22.2 kilometers, while Deimos measures approximately 12.6 kilometers across. Both are heavily cratered and resemble asteroids. Scientists believe they are likely captured asteroids rather than objects formed alongside Mars. Phobos orbits extremely close to Mars, completing an orbit in just 7.66 hours.
Moons of the Outer Solar System
The gas and ice giants of the outer solar system host numerous systems of natural satellites. Jupiter, the largest planet, has 97 confirmed moons as of April 2025, the second-highest count in the solar system. Among these, the four largest, known as the Galilean moons—Io, Europa, Ganymede, and Callisto—are notable. Ganymede is the largest moon, even surpassing the planet Mercury in size, and is the only moon with its own magnetic field. Io is the most volcanically active body in the solar system, while Europa shows strong evidence of a subsurface ocean beneath its icy crust, making it a target for astrobiological research.
Saturn holds the record for the most moons, with 274 confirmed as of March 2025. Its largest moon, Titan, is unique with a dense, nitrogen-rich atmosphere and stable bodies of liquid methane and ethane on its surface, resembling Earth’s water cycle. Another significant moon, Enceladus, exhibits geysers erupting from its south polar region, indicating a subsurface liquid water ocean that could potentially support life.
Uranus, an ice giant, has 29 known moons, with recent discoveries. Its five largest moons are Miranda, Ariel, Umbriel, Titania, and Oberon, all named after characters from the works of William Shakespeare and Alexander Pope. These moons are composed of roughly equal parts rock and ice, with some showing signs of past geological activity like canyons and volcanic features.
Neptune, the outermost planet, has 16 known moons. Its largest moon, Triton, is intriguing due to its retrograde orbit, meaning it revolves in the opposite direction to Neptune’s rotation. This unusual orbit suggests Triton was likely a dwarf planet from the Kuiper Belt, gravitationally captured by Neptune. Triton is also one of the coldest objects in the solar system, yet it displays cryovolcanism, where icy material erupts from its surface.
Why Moon Counts Vary
The differing number of moons orbiting planets throughout the solar system stems from factors like a planet’s gravitational influence, its formation history, and interactions with other celestial bodies. A planet’s mass plays a substantial role; more massive planets exert a stronger gravitational pull, attracting and retaining more natural satellites. This explains why the gas and ice giants possess significantly more moons than the smaller inner planets.
Many moons formed from the same disks of gas and dust that surrounded their parent planets during the early solar system, particularly for the large, regular moons of the giant planets. This process is similar to how planets themselves formed around the Sun. Other moons, especially smaller, irregularly shaped ones, are thought to be asteroids or comets that were gravitationally captured by a planet. This capture mechanism likely explains the origin of Mars’s moons, Phobos and Deimos, and many outer, irregular moons of the gas giants.
The formation of Earth’s Moon is attributed to a colossal impact event, where a Mars-sized object collided with early Earth, ejecting debris that eventually coalesced into our Moon. Conversely, Mercury and Venus lack moons partly because their proximity to the Sun means the Sun’s strong gravity makes it difficult to retain orbiting bodies. The dynamic and chaotic environment of the early solar system, with frequent collisions and gravitational interactions, shaped the diverse moon systems we observe today.