The celestial bodies we observe, such as planets and moons, have a scientific classification that is more intricate than a simple distinction based on size or brightness. Understanding the difference requires focusing on gravitational relationships and orbital paths. The formal classification relies on specific, universally applied criteria that dictate whether a celestial body is a primary orbiter of a star or a secondary orbiter of a larger body. This system helps scientists categorize the vast array of objects found throughout the solar system.
The Official Definition of a Planet
The modern definition of a planet was formally established in 2006 by the International Astronomical Union (IAU), creating three distinct criteria that an object must satisfy to earn the title. The first requirement is that the body must be in orbit around the Sun, rather than orbiting another non-stellar body. Secondly, the celestial body must possess sufficient mass for its own gravity to overcome rigid body forces, resulting in a nearly round shape (hydrostatic equilibrium).
The third, and most specific, criterion is that the object must have “cleared the neighborhood” around its orbit. This concept, also referred to as dynamical dominance, means that the object must be the single gravitationally dominant body in its orbital zone. A planet’s gravity must have either pulled in, scattered, or influenced all other comparable-sized objects in the vicinity of its orbital path.
While a planet may share its orbital space with smaller bodies, the planet itself must overwhelmingly outweigh the combined mass of all other objects in that region by a significant margin. For instance, a body that meets the first two criteria but shares its orbital zone with a population of similar-sized objects, like Pluto in the Kuiper belt, is classified as a dwarf planet. The inability to achieve this orbital dominance is the primary factor distinguishing the eight recognized planets from the dwarf planets.
The Definition of a Natural Satellite
A moon is scientifically known as a natural satellite, and its classification is based entirely on its orbital relationship to a larger celestial body. A natural satellite is an astronomical body that orbits a planet, a dwarf planet, or a smaller Solar System body, rather than having a primary orbit around the Sun. This difference in what the body orbits is the clearest distinction from a planet.
The formation of these moons varies widely. Some are believed to have formed from the same circumplanetary disk of material that created their parent planet, resulting in a regular orbit. Other moons, particularly those with irregular orbits, are thought to be captured asteroids or comets pulled into the gravitational field of a planet. Our solar system contains hundreds of these natural satellites.
The parent body that a moon orbits is called its primary, and the moon is held in orbit by the primary’s gravitational pull. The size of a natural satellite can vary dramatically, from small, irregularly shaped objects to massive spheres that rival the size of planets. Regardless of its physical characteristics, any object whose primary orbit is around a larger non-stellar body is classified as a natural satellite.
Size, Shape, and Exceptions to the Rules
Physical attributes like size and shape alone are insufficient to differentiate between a planet and a moon; orbital definitions are necessary for classification. Ganymede (orbiting Jupiter) and Titan (orbiting Saturn) are both larger in diameter than the planet Mercury. Despite their immense size, which meets the hydrostatic equilibrium requirement, they are classified as moons because their orbital path is around a planet and not the Sun.
The composition of these large moons also differs significantly from terrestrial planets. Ganymede is composed of water ice and rock, giving it a much lower density than the metal-rich Mercury. Ganymede’s mass is only about 45% of Mercury’s, despite its larger diameter, illustrating that sheer volume does not equate to planetary status. If Ganymede were placed in orbit around the Sun, it would be classified as a planet, provided it could clear its orbital zone.
Conversely, dwarf planets demonstrate that meeting the size and shape criteria is not enough to be a planet. Dwarf planets like Pluto are large enough to be nearly round, satisfying the second IAU criterion, but they fail the third test of clearing their orbital neighborhood. A moon’s orbit around a planet disqualifies it, while a dwarf planet’s failure to gravitationally dominate its own orbit around the Sun prevents it from being grouped with the major planets.