The question of whether a moon can be larger than a planet highlights a common misunderstanding in astronomy. Many assume that the sheer physical size of a celestial body is the only factor determining its classification as a planet. The answer, however, is not a simple yes or no based on size alone, but rather depends entirely on the specific scientific definitions that govern the solar system. The true distinction lies not in a body’s diameter but in its specific orbital behavior.
Physical Size Comparison: The Solar System’s Largest Moons
When comparing the physical dimensions of objects in the solar system, Jupiter’s moon Ganymede and Saturn’s moon Titan are immense, with diameters that surpass one of our solar system’s recognized planets. Ganymede, the largest moon, has a diameter of approximately 5,270 kilometers.
This makes Ganymede physically larger than the planet Mercury, which measures about 4,880 kilometers across. Titan is a close second, with a diameter of roughly 5,150 kilometers, also exceeding Mercury’s size. Both of these moons are also significantly larger than the largest known dwarf planet, Pluto.
However, the physical size comparison is not a perfect measure of mass, as density also plays a role in total gravitational pull. Despite being larger in volume, both Ganymede and Titan are less massive than Mercury, which is composed of denser materials. Ganymede is estimated to have only 45% of Mercury’s total mass, indicating a lower overall density.
The Critical Distinction: Defining a Planet
The reason Ganymede and Titan are not classified as planets, despite their imposing size, comes down to their orbital dynamics and the formal criteria established by the International Astronomical Union (IAU) in 2006. The IAU defined a planet based on three specific conditions a celestial body must meet.
The first two conditions relate to the body itself: it must be in orbit around the Sun, and it must have sufficient mass to assume a nearly round shape, a state known as hydrostatic equilibrium. This criterion explains why large moons like Ganymede and Titan appear spherical.
However, it is the third and most decisive criterion that excludes all moons from planetary status: A planet must have “cleared the neighborhood” around its orbit. This means the object must be gravitationally dominant in its orbital zone, having either absorbed or flung away most other material of a comparable size.
The eight recognized planets have all achieved this gravitational dominance. For instance, Jupiter is so massive that it controls the orbital path of everything nearby, including its own large moons. Moons fail this third test because they orbit a planet, not the Sun directly, and they are not the gravitationally dominant object in their region. They are satellites whose path is controlled by their parent planet.
The Dwarf Planet Exception
While a moon cannot be larger than a “planet” under the current IAU definition, a moon can certainly be larger than a dwarf planet. The category of dwarf planet was created for objects that satisfy the first two criteria—orbiting the Sun and being nearly round—but fail the third one of clearing their orbital path. Pluto is the most famous example of a dwarf planet, existing within a zone of similar objects called the Kuiper Belt.
The largest moons in the solar system are indeed physically larger and more massive than all known dwarf planets. Ganymede, Titan, Callisto, Io, Europa, Triton, and even Earth’s Moon are all more massive than Pluto and Eris, which is the most massive known dwarf planet.
This comparison confirms that a moon can be bigger than a class of objects that contains the word “planet” in its name. The seven largest moons are not only larger than Pluto, but also more massive, meaning they contain more total material. The existence of the dwarf planet category provides the definitive answer to the core question, showing that a moon can surpass a recognized planetary-class object in both size and mass.