Pluto was not the first planet to be demoted, only the most famous
In the 1800s, Ceres was counted as a planet, and by around 1851 the tally had swollen to 23. Astronomers then invented the word asteroid and quietly reshuffled the list. Pluto's 2006 reclassification repeated that old move for a new crowd of icy worlds beyond Neptune.
A dwarf planet orbits the Sun directly and has enough mass for gravity to pull it round, yet not enough to dominate its orbit the way the eight classical planets do. Pluto is the model case. The International Astronomical Union and many astronomers have left such bodies off the planet roster since 2006, though plenty of planetary geologists still regard dwarf planets, and planetary-mass moons, as planets.
Pluto's own story explains the shift. Found in 1930, it was believed for almost half a century to outweigh Mercury. The 1978 discovery of its moon Charon allowed a proper mass measurement, and the answer was humbling: about a twentieth of Mercury's mass and a fifth of our Moon's, though still more than ten times heavier than Ceres. Its stretched, steeply tilted orbit also set it apart. In the 1990s astronomers began finding similar bodies in the Kuiper belt, and it became obvious that either many of them would join the planets or Pluto would have to leave.
The crunch came with Eris, discovered in January 2005 and briefly thought slightly bigger than Pluto; some press reports called it the tenth planet. At the IAU General Assembly in August 2006, a draft that would have added Charon, Eris and Ceres to the planets drew heavy objections. Two Uruguayan astronomers, Julio Angel Fernandez and Gonzalo Tancredi, offered an alternative: an intermediate category for bodies round enough but with uncleared orbits, which removed Pluto, Ceres and Eris from the planet list.
There is broad agreement that at least nine candidates qualify, among them Pluto, Eris, Haumea and Ceres, with Salacia a borderline tenth. Spacecraft have visited only Pluto and Ceres, and in 2015 the New Horizons and Dawn missions showed both could be geologically active. Seven of the others have moons, which lets astronomers weigh them and estimate density. Sedna has neither a visit nor a known moon, so its mass remains hard to pin down.
Source: Dwarf planet
