The world was introduced to dwarf planets in 2006, when petite Pluto was stripped of its planet status and reclassified as a dwarf planet. The International Astronomical Union (IAU) currently recognizes two other dwarf planets, Eris and Ceres.
What differentiates a dwarf planet from a planet? For the most part, they are identical, but there's one key difference: A dwarf planet hasn't "cleared the neighborhood" around its orbit, which means it has not become gravitationally dominant and it shares its orbital space with other bodies of a similar size. (Astronomers and other experts are debating this definition.)
Is Pluto a Dwarf Planet?
Because it has not cleared the neighborhood around its orbit, Pluto is considered a dwarf planet. It orbits in a disc-like zone beyond the orbit of Neptune called the Kuiper belt, a distant region populated with frozen bodies left over from the solar system's formation. The dwarf planet is a whopping 3.7 billion miles (5.9 billion kilometers) from the sun, and its average temperature hovers around -356 degrees Fahrenheit (-215 degrees Celsius).
Pluto's surface is composed of a mixture of frozen nitrogen, methane, and carbon monoxide ices. The dwarf planet also has polar caps and regions of frozen methane and nitrogen.
Pluto has three known moons, Hydra, Nix, and Charon. With a diameter of about 737 miles (1,186 kilometers), Charon is the largest of Pluto's moons. The duo's gravity puts them in a synchronous orbit, which means they face each other with the same side all the time.
In January 2006, NASA launched its New Horizons spacecraft. It swung past Jupiter for a gravity boost and scientific studies in February 2007, conducted a six-month-long reconnaissance flyby study of Pluto and its moons in summer 2015, and culminated with Pluto's closest approach on July 14, 2015. As part of an extended mission, the spacecraft is heading farther into the Kuiper Belt to examine another of the ancient, icy mini-worlds in that vast region, at least a billion miles beyond Neptune’s orbit.
New Horizons also found Pluto to have blue skies and water ice.
Also considered by many to be an asteroid, Ceres, like Pluto, was also renamed as a dwarf planet in 2006. Ceres was discovered by Italian astronomer Giuseppe Piazzi in 1801.
Ceres's shape resembles a flattened sphere with a diameter of about 590 miles (950 kilometers). It is by far the largest and most massive known body in the asteroid belt, and it contains about one-third of the estimated total mass of all asteroids in the belt.
Ceres is made up of a rocky inner core surrounded by a mantle of water-ice. A thin, dusty, outer crust covers the dwarf planet named after the Roman goddess of grain.