The official definition of a planet is a modern set of criteria established by the scientific community to accurately categorize objects in our Solar System. This formal classification became necessary due to rapid technological progress, which led to the discovery of many large, icy bodies in distant space. The International Astronomical Union (IAU), the global authority for celestial nomenclature, adopted this new standard in 2006. This definition provides a clear, three-part test that separates the eight classical planets from all other objects orbiting the Sun.
The Need for a Scientific Standard
For centuries, the term “planet” was loosely applied to any celestial body that appeared to “wander” across the night sky. This simple classification became unsustainable as telescopes improved and astronomers began discovering more distant, massive objects. The need for a formal standard became particularly pressing with the discovery of objects such as Eris, which was found to be even more massive than Pluto.
These discoveries demonstrated that the outer Solar System contained a vast population of large, icy worlds. Astronomers were forced to either accept potentially hundreds of planets or create a precise set of rules. In August 2006, the IAU addressed this issue by adopting Resolution 5A, which formally defined a planet using specific physical and dynamic characteristics. This resolution provided the foundation for a consistent classification system reflecting current knowledge of solar system formation and dynamics.
The First Two Criteria: Orbit and Mass
The first two requirements for planethood establish the fundamental physical characteristics an object must possess. The first criterion is straightforward: a body must be in orbit around the Sun. This requirement immediately distinguishes planets from moons, which orbit another planet, and from rogue objects like interstellar asteroids.
The second criterion specifies that the celestial body must have sufficient mass for its self-gravity to overcome rigid body forces, resulting in a shape of hydrostatic equilibrium. This means the object must be massive enough that its own gravity pulls all its material inward, forcing it into a nearly round or spherical shape. This inward gravitational pull is balanced by the outward pressure of the body’s internal material. This characteristic distinguishes large, rounded objects from smaller, irregularly shaped objects, like many asteroids and comets, which lack the gravity to achieve this balance.
Gravitational Dominance of the Orbital Path
The third and most distinguishing criterion is that the celestial body must have “cleared the neighborhood” around its orbit. This requirement is dynamic, relating to the object’s gravitational influence over its orbital zone rather than just its physical shape. To satisfy this condition, the planet must be the gravitationally dominant body in its orbital path.
“Clearing the neighborhood” does not mean the orbit is completely empty of other objects, as every planet shares its path with minor bodies like asteroids and comets. Instead, the planet’s mass and gravity must be powerful enough to have either accreted, captured, or ejected most other comparably sized objects from its orbital zone over the age of the Solar System. The planet must control the gravitational destiny of nearly all other material in its region.
This concept of gravitational dominance is measured by comparing the planet’s mass to the combined mass of all other material that shares its orbital path. For a planet, this ratio is extremely high, demonstrating that it has successfully swept up or scattered the majority of the original planet-forming debris. Objects that fail this test share their orbital path with a significant population of other large bodies, signaling a lack of supreme gravitational control.
The Resulting Categories of Solar System Bodies
The application of these three criteria results in three distinct categories of objects within our Solar System, as defined by the IAU. The first category is the “planet,” which meets all three requirements, including clearing its orbital neighborhood, and includes the eight classical planets from Mercury to Neptune.
The second category is the “dwarf planet,” which meets the first two criteria but fails the third test of gravitational dominance. Pluto is the most famous example because it resides in the crowded Kuiper Belt and has not gravitationally dominated its orbital path. Other recognized dwarf planets include Ceres, Eris, Haumea, and Makemake.
The third and broadest category is “Small Solar System Bodies” (SSSBs). This group includes all other objects that orbit the Sun but fail to meet the second criterion of achieving hydrostatic equilibrium. This diverse category encompasses the vast majority of minor celestial bodies, such as comets, most asteroids, and smaller objects found in the Kuiper Belt.