The definition of a planet has been a subject of evolving scientific understanding, especially with the discovery of numerous celestial bodies in the outer Solar System. Modern astronomy necessitated a precise, standardized definition to classify the diverse objects being found beyond the orbit of Neptune. This formal classification is based on three specific criteria that every celestial body must satisfy to be officially recognized as a planet in our Solar System.
The International Astronomical Union’s 2006 Resolution
The need for a formal definition became pressing in the early 2000s when astronomers began discovering large objects in the Kuiper Belt, such as Eris, which appeared to be comparable in size or even larger than Pluto. If these new discoveries were also to be considered planets, the number of planets in the Solar System would rapidly increase, creating an unmanageable and arbitrary classification system. The global authority responsible for celestial nomenclature, the International Astronomical Union (IAU), was tasked with resolving this dilemma.
In August 2006, at the IAU General Assembly in Prague, astronomers voted on Resolution 5A, which established the three conditions for a celestial body to be designated a planet. This resolution fundamentally changed the understanding of the Solar System’s structure and introduced new categories for objects that did not meet all the requirements. The definition created a clear distinction between the eight recognized planets and all other objects orbiting the Sun.
The First Requirement: Orbiting Our Star
The first criterion for a planet is that it must be in orbit around the Sun. This condition immediately separates planets from other major types of celestial bodies. Objects that orbit a planet, such as Earth’s Moon, are classified as natural satellites or moons, regardless of their size or shape.
This requirement ensures that planets are primary bodies within a star system. It also excludes objects that might be passing through our Solar System from interstellar space, which are not gravitationally bound to the Sun. Furthermore, the body must not be massive enough itself to sustain the nuclear fusion reactions that would classify it as a star.
The Second Requirement: Achieving a Spherical Shape
The second criterion dictates that a celestial body must possess sufficient mass for its self-gravity to overcome rigid body forces, compelling it into a state of hydrostatic equilibrium. This physical balance gives planets their nearly round shape. Hydrostatic equilibrium is reached when the immense, inward pull of the object’s own gravity is balanced by the outward pressure of the material within its structure.
For smaller objects, the internal strength of the rock and ice material maintains an irregular, potato-like shape, preventing gravity from uniformly distributing the mass. Once an object reaches a certain mass threshold, its gravity becomes powerful enough to crush and reshape the material into the lowest energy configuration. This threshold is dependent on the composition; icy bodies can achieve this spherical shape at diameters around 400 kilometers, while rockier bodies require a larger size, closer to 600 kilometers.
The Third Requirement: Dominating the Orbital Path
The third condition for planet status is that the body must have “cleared the neighborhood” around its orbit. This means the planet must be the gravitationally dominant body within its orbital zone, having either accreted, scattered, or controlled the motion of most other objects that share its orbital region. A planet’s immense mass ensures that its gravitational influence has either ejected smaller bodies from its path or pulled them into itself or into a stable orbit as a satellite.
This criterion is based on dynamical dominance, where a planet’s mass is overwhelmingly larger than the total mass of all other objects in its orbital zone. The eight recognized planets meet this standard, as the combined mass of all other non-satellite objects in their respective orbits is negligible in comparison. If a celestial body’s orbit is shared by other objects of comparable size, or if it is merely one of a large population of similar bodies that cross its path, it fails this requirement. Pluto, for instance, shares its orbital region with many sizable Kuiper Belt objects, demonstrating a lack of gravitational supremacy.
Classifications for Objects That Do Not Qualify
Celestial bodies that satisfy some, but not all, of the planet criteria fall into other specific classifications established by the IAU. An object that meets the first two requirements—it orbits the Sun and has achieved a nearly round shape from hydrostatic equilibrium—but fails the third criterion of clearing its orbital neighborhood is designated a “dwarf planet”. Pluto is the most famous example of a dwarf planet, as it is round and orbits the Sun, but is part of the densely populated Kuiper Belt.
The IAU created a third classification, the “Small Solar System Body” (SSSB), for all other objects orbiting the Sun that are not planets or dwarf planets. These bodies typically fail the second criterion, meaning they do not have enough mass for their gravity to pull them into a spherical shape. This category includes the vast majority of asteroids, comets, and most Trans-Neptunian Objects.