There is a strong correlation observed across the Solar System between a planet’s size and the number of moons it possesses. This relationship is rooted in fundamental principles of physics and the specific history of planetary formation. The ability of a planet to acquire and keep these satellites depends on a combination of its own gravitational strength and the environment it formed in.
The Observable Relationship: Size and Moon Count in the Solar System
The evidence for a size-moon count relationship comes from dividing the Solar System’s planets into two distinct groups. The four inner, smaller, rocky worlds—Mercury, Venus, Earth, and Mars—are categorized as the Terrestrial planets. This group collectively possesses only three known moons: Earth’s single large satellite and the two tiny satellites orbiting Mars.
In stark contrast, the four outer, massive, gas and ice giants—Jupiter, Saturn, Uranus, and Neptune—host vast, complex satellite systems. As of early 2025, Jupiter has 97 confirmed satellites, while Saturn holds the record with 274 confirmed moons. Uranus and Neptune still command significant retinues with 28 and 16 moons, respectively.
This massive disparity demonstrates that planetary mass is the primary factor driving the observed pattern. The planet’s gravitational pull, which is directly proportional to its mass, dictates its capacity to maintain a large collection of orbiting bodies.
Gravitational Dominance: Why Mass Matters for Moon Retention
The core physical mechanism that allows massive planets to retain large numbers of moons is the size of their gravitational sphere of influence, known as the Hill sphere. The Hill sphere defines the region in space where a planet’s gravitational pull is stronger than the competing gravitational pull of the Sun.
The more massive a planet is, the larger its Hill sphere extends into space, creating an immense volume where its gravity is the dominant force. The gas giants, being hundreds of times more massive than Earth, possess Hill spheres that are millions of kilometers across. This expansive gravitational territory provides a much larger target area for moons to occupy stable orbits.
Conversely, the smaller Terrestrial planets have relatively tiny Hill spheres. This severely limits the amount of space available for long-term satellite retention.
Two Paths to Moons: Formation and Capture Mechanisms
The size of a planet’s Hill sphere not only allows for the retention of moons but also dictates the two primary acquisition mechanisms: formation and capture.
The most massive planets were able to form vast, rotating disks of gas and dust around them early in the Solar System’s history. Material within these circumplanetary disks accreted and condensed, forming the large, inner, or “regular” moons. These satellites, such as Jupiter’s Galilean moons or Saturn’s Titan, travel in stable, prograde, and nearly circular orbits aligned with the planet’s equator. The terrestrial planets generally missed out on this formation pathway, with Earth’s Moon being the result of a distinct, violent impact event.
The second path, capture, is responsible for the massive counts of small, outer, or “irregular” moons. The vast Hill spheres of the giant planets act as gravitational nets, highly effective at snagging passing asteroids, comets, and Kuiper Belt objects. These captured objects become irregular moons, characterized by their distant, highly inclined, and often retrograde orbits. The terrestrial planets rarely succeed at permanent capture because their small gravitational reach requires a passing object to lose a large amount of energy very quickly, a process that is highly improbable.