The question of what constitutes the “10th planet” of our solar system is rooted in a historical search for distant worlds. The current scientific answer is straightforward: there is no officially recognized 10th planet, and the solar system contains only eight major planets. The debate over a tenth world led to a precise, formal definition of “planet” that clarified the status of many large, icy bodies. This reclassification settled the planetary count and established a new category for objects that orbit the Sun but do not meet all the requirements of a true planet.
The Search for Planet X
The idea of an undiscovered major world originated in the 19th century with the theoretical search for a massive, unseen object dubbed “Planet X.” Astronomers observed slight irregularities, or perturbations, in the orbits of Uranus and Neptune that could not be fully explained by the gravity of known objects. American astronomer Percival Lowell initiated a systematic search at the start of the 20th century to find the body he believed was gravitationally nudging the outer planets.
This search eventually led to the 1930 discovery of Pluto by Clyde Tombaugh, which was initially hailed as the long-sought Planet X. However, later, more precise calculations of Neptune’s mass, combined with Pluto’s small size, showed that Pluto could not exert the necessary gravitational influence to cause the observed orbital discrepancies.
The Objects That Sparked the Debate
The modern debate over a potential 10th planet was ignited by the discovery of large, icy bodies orbiting beyond Neptune in the Kuiper Belt. The pivotal moment came in 2005 with the discovery of the object initially designated 2003 UB\(_{313}\) by a team led by astronomer Mike Brown. This object, later named Eris, was significant because early measurements indicated it was at least as large as, if not slightly larger than, Pluto itself.
The existence of Eris, a world similar to Pluto but with a greater distance, forced the astronomical community to face a choice. If Pluto was the ninth planet, Eris should logically be the tenth, and the likely existence of other similar-sized Trans-Neptunian Objects (TNOs) meant the planetary count could quickly balloon. Discoveries of TNOs like Makemake and Haumea further highlighted the need for a clear standard for planethood. Eris’s size and mass—it is about 27% more massive than Pluto—made an arbitrary increase in the number of planets untenable.
Establishing the Modern Definition of a Planet
The escalating planetary crisis necessitated a formal resolution, which the International Astronomical Union (IAU) addressed at its 2006 General Assembly in Prague. The IAU established three criteria for a celestial body to be officially classified as a planet. The first two criteria were uncontroversial: the object must orbit the Sun, and it must have sufficient mass for its own gravity to pull it into a nearly round shape (hydrostatic equilibrium).
The third and most contentious criterion required that the object must have “cleared the neighborhood” around its orbit. This means a planet must be gravitationally dominant enough to have either swept up or ejected all other significant debris and smaller bodies from its orbital path. The eight major planets easily meet this standard, as they are vastly more massive than any other objects that share their orbital zones.
Objects like Eris and Pluto fulfill the first two criteria, as they orbit the Sun and are massive enough to be spherical. However, they fail the third criterion because they orbit within the crowded Kuiper Belt, a vast ring of thousands of other Trans-Neptunian Objects. Their orbits are shared with, and not gravitationally dominant over, this population of other bodies. This failure to clear their orbital neighborhood was the reason the IAU definition reduced the number of planets from nine to eight.
The Dwarf Planet Classification
The 2006 IAU resolution created a new category for objects that met the first two conditions of planethood but failed the third. These objects, including the candidates for the “10th planet,” were assigned the classification of “Dwarf Planet.” This designation recognizes their planetary characteristics, such as their spherical shape, without granting them the full status of a major planet.
The IAU currently recognizes five celestial bodies as official dwarf planets. Ceres is the only one located in the inner solar system, residing in the main asteroid belt between Mars and Jupiter. The other four recognized dwarf planets—Pluto, Haumea, Makemake, and Eris—are all Trans-Neptunian Objects located in the distant Kuiper Belt or the scattered disk beyond Neptune. The reclassification of these worlds formally ended the debate over a potential 10th planet, establishing a stable, eight-planet solar system.