The Moon is the largest and brightest object in our night sky. Its substantial size often causes confusion regarding its classification, as it is larger than the dwarf planet Pluto and is the fifth-largest moon in the entire solar system. Despite its spherical shape and impressive scale, the Moon is officially classified by astronomers as a natural satellite, and not as a planet. This distinction is rooted in the formal scientific framework used to categorize celestial bodies orbiting our Sun.
The Modern Definition of a Planet
The definitive classification of solar system objects was formalized by the International Astronomical Union (IAU) in August 2006, following a contentious debate among astronomers. This resolution established three specific criteria that a celestial body must satisfy to be officially recognized as a planet. The first of these criteria is that the object must be in orbit directly around the Sun, not around another planet. This orbital path distinguishes planets from moons and other satellites.
The second requirement is that the object must possess sufficient mass for its own gravity to overcome the rigid forces within its structure. This gravitational compression forces the body into a state of hydrostatic equilibrium, which results in a nearly round shape. The third and final criterion is that the object must have “cleared the neighborhood” around its orbit, meaning it must be the gravitationally dominant body in its orbital path.
To “clear the neighborhood” means that the planet’s gravitational influence has either absorbed, scattered, or captured most other smaller objects that share its orbital zone. Any non-satellite body that meets the first two criteria but fails the third is categorized as a dwarf planet, which explains the reclassification of Pluto. The IAU’s 2006 definition provided a clear, if controversial, set of rules for classifying the major bodies in our solar system.
Criteria the Moon Satisfies
The Moon satisfies one of the IAU’s three criteria, which is a major reason for the common public confusion about its classification. It is massive enough that its self-gravity has pulled it into a nearly spherical, or ellipsoidal, shape, meeting the hydrostatic equilibrium requirement. The Moon is the second-densest moon known in the solar system, only slightly behind Io.
The Moon is larger than all known dwarf planets, including Pluto and Eris. Its diameter is about 3,474 kilometers, which is more than a quarter of Earth’s diameter. This size and resulting shape are associated with planetary status, but this single characteristic is not sufficient for planethood.
Orbits and the Satellite Distinction
The primary reason the Moon is not a planet is its failure to meet the first orbital requirement. A planet must orbit the Sun directly, whereas the Moon is gravitationally bound to and orbits the Earth. This relationship immediately classifies the Moon as a natural satellite of Earth. This distinction is necessary for organizing the solar system, as a body orbiting a planet cannot simultaneously be considered a planet itself under the current IAU framework.
The relationship between the two bodies is complex, however, due to the Moon’s relatively large mass compared to Earth. The Moon is so massive that the system’s center of mass, known as the barycenter, is located about 4,671 kilometers from Earth’s center. This location is approximately three-quarters of the way to Earth’s surface, meaning the barycenter is still located inside the Earth. If the barycenter were outside the Earth, the system would more closely resemble a true “double planet,” but the Moon’s status as a satellite remains official.
Both Earth and the Moon orbit this common barycenter, and it is this point that follows the clean elliptical path around the Sun. The Earth itself “wobbles” around the barycenter as it travels, which can make the system appear like two partners dancing around a central point. However, the Moon is still clearly subordinate to Earth’s gravitational pull, and it does not meet the third planetary criterion of dynamically dominating its own orbit around the Sun.