There was no concept of a dwarf planet till 2006. There were planets, satellites, comets and asteroids, meteors and of course stars including the sun among others. In 2006, the concept of dwarf planet was developed and adopted by the International Astronomical Union. The idea and the objective were to classify a few objects in the solar system that are not as big to be planets but also not satellites, comets or asteroids. It must be noted that the concept of dwarf planet is not universally accepted. There are many who reject the definition and hence the classification of Pluto as a dwarf planet.
Dwarf Planet: Definition
There is a list of criteria that qualifies an object/rock in space as a dwarf planet. The body must orbit the sun, it should have its own gravity and should have a stable form, one that has attained hydrostatic equilibrium. The object while having enough gravity of its own should have failed to clear the neighborhood as it rotates/revolves and along its orbital path. Also, it must not be a satellite that orbits a planet.
The most notable incident that started the naming and subsequent debate pertaining to dwarf planet was the de-recognition of Pluto as a planet. Till 2008, Pluto was a planet but according to the definition of dwarf planet, it became one. There are four other dwarf planets that have been recognized and studied to an extent: Eris, Ceres, Makemake & Haumea.
Pluto did not have a cleared neighborhood, it was not too big in size for a planet and it was observed that there was no defined atmosphere for the object to be called a planet. However, latest evidences from the New Horizon’s mission of NASA may lead to re-recognize Pluto as a planet. There is evidence of cloud formation in the atmosphere of Pluto which will be debated in time to come.
Pluto is considered to be the biggest dwarf planet along with Eris. Ceres is a dwarf planet in the asteroid belt, which is between the orbits of Mars and Jupiter. Ceres was discovered long ago but it was regarded as a planet upon discovery and later was classified as an asteroid. It was classified as a dwarf planet after the concept was defined. Eris is considered by many as the largest dwarf planet. It was discovered in 2005 and was mistaken as the tenth planet. Eris and Makemake may have their orbits farther away from that of Pluto.