Pluto held the title of the ninth planet in our solar system for 76 years following its discovery in 1930. This tradition ended abruptly in August 2006 when the International Astronomical Union (IAU) voted to reclassify the distant world. This decision was based on rigorous scientific criteria that had previously been undefined. Understanding these reasons reveals why Pluto no longer fits among the eight major planets.
Setting the Standard: The International Astronomical Union’s Planet Criteria
The IAU’s 2006 resolution established the first formal definition of a “planet” for objects within our solar system, requiring a celestial body to satisfy three distinct conditions. The first condition is that the object must orbit the Sun, which Pluto satisfies, circling the Sun every 248 Earth years. The second criterion demands the object possess sufficient mass for its gravity to pull it into a nearly round shape (hydrostatic equilibrium). Pluto’s spherical structure confirms it meets this condition as well. The third criterion, which ultimately changed Pluto’s status, requires the body to have “cleared the neighborhood” around its orbit, a condition the other eight planets successfully fulfill.
The Failure to Clear the Orbital Neighborhood
The third criterion refers to a planet’s gravitational dominance within its orbital path. A true planet must be the primary gravitational influence in its zone, having either absorbed or ejected smaller objects through gravitational scattering. The eight major planets are massive enough to have achieved this dominance; for example, Earth’s mass is approximately 1.7 million times greater than the combined mass of all other objects sharing its orbital path. Pluto does not exhibit this level of gravitational control, as its mass is significantly smaller than the total mass of other objects in its orbital region. Estimates suggest Pluto makes up only about 0.07 times the mass of all other bodies within its zone, revealing a profound lack of orbital dominance.
Context: Membership in the Kuiper Belt
Pluto failed to clear its orbit because of its location in the outer solar system. It resides within the Kuiper Belt, a massive, distant, and densely populated region extending beyond Neptune. This zone is filled with thousands of icy bodies and minor planets, known as Kuiper Belt Objects (KBOs). Pluto is scientifically understood to be one of the largest members of this population, not a singular, isolated planet. This context was highlighted by the 2005 discovery of Eris, a trans-Neptunian object confirmed to be more massive than Pluto. The existence of other similarly sized bodies forced the IAU to establish a clear, formal scientific definition.
The Official Status: Defining the Dwarf Planet
The IAU created the “dwarf planet” classification for objects that meet most, but not all, of the planetary criteria. A dwarf planet orbits the Sun and is massive enough to be nearly round. The crucial distinction is that a dwarf planet has not cleared the neighborhood around its orbit, the criterion Pluto fails to satisfy. Pluto was formally reclassified as the prototype of this new group, establishing that planets and dwarf planets are two separate classes of celestial bodies. Pluto now shares this classification with objects like Eris, Ceres, Makemake, and Haumea, officially removing it from the roster of the eight classical planets.