The term plutoid was introduced by the International Astronomical Union (IAU) in 2008 as a formal classification for a specific group of celestial objects beyond the orbit of Neptune. This new designation was a direct consequence of the 2006 reclassification of Pluto from a planet to a dwarf planet. Astronomers realized the outer solar system contained many large, icy bodies similar to Pluto that required a distinct category. The plutoid classification helps distinguish these distant, rounded, ice-rich objects from other celestial bodies, such as rocky asteroids found closer to the Sun.
Defining the Plutoid: A Subset of Dwarf Planets
A plutoid is, by definition, a type of dwarf planet that fulfills all the established criteria for that class of object. The first requirement is that the body must be in direct orbit around the Sun, meaning it is not a moon or satellite of another object.
The second criterion dictates that the body must possess sufficient mass for its gravity to overcome material forces. This gravitational pull forces the object into hydrostatic equilibrium, resulting in a nearly round or spherical form. This self-rounding process differentiates dwarf planets from smaller, irregularly shaped bodies like most asteroids and comets.
The third criterion prevents it from being classified as a full-fledged planet. A dwarf planet has not gravitationally cleared the neighborhood around its orbit, meaning its orbital path is shared with or crossed by other celestial debris. This lack of gravitational dominance established the dwarf planet category as distinct from the eight major planets.
The term plutoid is a sub-category that inherits these conditions but adds a crucial geographical constraint. All plutoids must first meet the full definition of a dwarf planet, establishing their orbit around the Sun, their nearly spherical shape, and their shared orbital path.
The Crucial Orbital Requirement
The factor that separates a plutoid from other dwarf planets is its specific location within the solar system. A plutoid must orbit the Sun at a distance greater than that of the planet Neptune. This means that every plutoid is a Trans-Neptunian Object (TNO) located in the cold, distant regions beyond the ice giant.
This orbital requirement was established to create a classification for the large, icy worlds found primarily in the Kuiper Belt and the Scattered Disc. The Kuiper Belt is a vast, doughnut-shaped ring of small, frozen bodies extending from Neptune’s orbit outward, and it is the region where these plutoids primarily reside.
This geographical constraint immediately excludes the dwarf planet Ceres, which is the largest object in the main asteroid belt located between the orbits of Mars and Jupiter. Although Ceres satisfies the other three criteria—it orbits the Sun, is nearly round, and has not cleared its orbital zone—its inner solar system location means it is not a plutoid.
The plutoid designation groups together large, icy bodies that represent a distinct population of the outer solar system. These objects are remnants from the solar system’s formation and possess compositions rich in frozen volatiles like methane, nitrogen, and water ice. The orbital requirement identifies the largest, gravitationally-rounded members of the Trans-Neptunian population.
Current Classified Plutoids
The IAU currently recognizes four celestial bodies as confirmed plutoids, representing the outer solar system’s largest residents.
- Pluto is the prototype of the category. It possesses a thin atmosphere of nitrogen, methane, and carbon monoxide, along with a complex system of five known moons, including its large companion, Charon. Its surface is characterized by vast plains of nitrogen ice and towering water-ice mountains.
- Eris is the second object, whose 2005 discovery was the catalyst for the reclassification debate. Eris is the most massive known dwarf planet and is found in the Scattered Disc, with a highly eccentric orbit far beyond the main Kuiper Belt. Its surface is extremely reflective and covered in frozen methane, suggesting a composition similar to Pluto’s but with a higher density.
- Makemake is the third-brightest object in the Kuiper Belt, after Pluto. It is notable for its reddish-brown color, thought to be caused by complex organic molecules called tholins on its surface. Its surface is coated with frozen methane, ethane, and nitrogen, and it possesses a single small moon.
- The final confirmed plutoid is Haumea, distinguished by its highly elongated, ellipsoidal shape, resembling a rugby ball. This unusual shape results from its extremely rapid rotation, completing a turn on its axis in less than four hours. Haumea is also notable for having two moons, Hiʻiaka and Namaka, and a faint ring system.