Natural satellites are celestial bodies that orbit a planet, dwarf planet, or minor planet, commonly referred to as moons. The number of these companions varies dramatically across the Solar System, ranging from none to hundreds. This variation is largely determined by a planet’s mass and its location relative to the Sun and the asteroid belts. The official counts highlight the distinct environments of the inner, rocky planets versus the massive, outer gas and ice giants.
The Terrestrial Planets
The four innermost planets, known as the terrestrial or rocky planets, possess a relatively small number of moons. Mercury and Venus, the two closest to the Sun, have no natural satellites, as their proximity to the Sun’s immense gravity makes it difficult to retain an orbiting body.
Earth is unique among the inner planets for having one exceptionally large moon, which stabilizes the planet’s axial tilt. Mars, the fourth planet, possesses two small, irregularly shaped moons named Phobos and Deimos. Scientists believe these small, dark bodies are likely captured asteroids from the nearby main belt.
Jupiter and Saturn
The gas giants, Jupiter and Saturn, dominate the Solar System’s moon count, hosting vast and complex satellite systems. These numbers are frequently revised as new telescopic surveys identify smaller, fainter objects. Saturn currently holds the record, having amassed the highest total of confirmed moons, many of which are small, irregular bodies orbiting at great distances.
Saturn’s system includes Titan, the second-largest moon in the Solar System, notable for being the only moon with a dense atmosphere and stable surface liquids. Jupiter closely follows Saturn, with an official count of 95 natural satellites.
The four largest of Jupiter’s moons are the famous Galilean moons: Io, Europa, Ganymede, and Callisto, first observed by Galileo Galilei in 1610. Ganymede, the largest moon in the Solar System, is even bigger than the planet Mercury. Most of Jupiter’s remaining moons are small, distant objects likely captured from the asteroid belt.
Uranus and Neptune
The ice giants, Uranus and Neptune, host significant satellite populations, though their totals are smaller than those of Jupiter and Saturn. Uranus has 28 known moons, including five major satellites:
- Miranda
- Ariel
- Umbriel
- Titania
- Oberon
Uranus’s naming convention is unique, with moons named after characters from the works of William Shakespeare and Alexander Pope.
Neptune currently has 16 confirmed moons in its orbit. Its largest satellite, Triton, is notable for having a retrograde orbit, revolving opposite to Neptune’s rotation. This suggests Triton was not formed around Neptune but was captured from the distant Kuiper Belt. Triton also exhibits cryovolcanism, or “ice volcanoes,” making it one of the few geologically active moons in the outer Solar System.
Criteria for Counting Natural Satellites
The fluctuating moon counts for the outer planets stem from the lack of a formal definition for a “moon” by the International Astronomical Union (IAU). Unlike the precise criteria for a planet, a moon is generally defined only as a natural object in a stable orbit. This broad definition includes everything from immense, round bodies like Ganymede to tiny, kilometer-sized captured asteroids.
Scientists often distinguish between a “moon” and a “moonlet,” with the latter term applied to objects under a certain size. To be included in the official count, a newly detected object must have its orbit confirmed through multiple observations over time. This rigorous, multi-year process of orbital confirmation is why the official number of satellites continues to grow as telescope technology improves.