The dwarf planet formerly counted as the ninth planet from the Sun is Pluto.
Pluto was discovered in 1930 by Clyde Tombaugh at Lowell Observatory and was quickly celebrated as the Solar System’s ninth planet. For much of the 20th century, schoolbooks presented it alongside Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
Its status became controversial after astronomers discovered many icy bodies beyond Neptune, including Eris in 2005. These discoveries raised a basic question: should every Pluto-sized object be called a planet? In 2006, the International Astronomical Union defined a planet partly by whether it has cleared its orbital neighborhood. Pluto meets the criteria for orbiting the Sun and being nearly round, but not that third condition, so it is classified as a dwarf planet.
Pluto remains a major Kuiper Belt object, not a vanished world. NASA’s New Horizons spacecraft flew past it in 2015, revealing mountains of water ice, glaciers of nitrogen ice, and a complex surface. Some planetary scientists still debate the usefulness of the official definition, but the standard classification is clear.