The 2006 debate surrounding
Pluto and what constitutes a planet led to Ceres being considered for reclassification as a planet. A proposal before the International Astronomical Union for the
definition of a planet would have defined a planet as "a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid-body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet". Had this resolution been adopted, it would have made Ceres the fifth planet in order from the Sun.
This never happened, however, and on 24 August 2006 a modified definition was adopted, carrying the additional requirement that a planet must have "
cleared the neighborhood around its orbit".
By this definition, Ceres is not a planet because it does not dominate its orbit, sharing it as it does with the thousands of other asteroids in the
asteroid belt and constituting only about a third of the mass of the belt. Bodies that met the first proposed definition but not the second, such as Ceres, were instead classified as
dwarf planets.
Ceres is the largest object in the asteroid belt.
[13] It is sometimes assumed that Ceres has been reclassified as a dwarf planet, and that it is therefore no longer considered an asteroid. For example, a news update at Space.com spoke of "Pallas, the largest asteroid, and Ceres, the dwarf planet formerly classified as an asteroid",
[62] whereas an IAU question-and-answer posting states, "Ceres is (or now we can say it was) the largest asteroid", though it then speaks of "other asteroids" crossing Ceres' path and otherwise implies that Ceres is still considered an asteroid.The
Minor Planet Center notes that such bodies may have dual designations.
The 2006 IAU decision that classified Ceres as a dwarf planet never addressed whether it is or is not an asteroid. Indeed, the IAU has never defined the word 'asteroid' at all, having preferred the term '
minor planet' until 2006, and preferring the terms '
small Solar System body' and 'dwarf planet' after 2006. Lang (2011) comments "the [IAU has] added a new designation to Ceres, classifying it as a dwarf planet. ...
By [its] definition,
Eris,
Haumea,
Makemake and Pluto, as well as the largest asteroid, 1 Ceres, are all dwarf planets", and describes it elsewhere as "the dwarf planet–asteroid 1 Ceres".
[65] NASA continues to refer to Ceres as an asteroid,
[66] as do various academic textbooks.
Ceres (dwarf planet) - Wikipedia