Pluto was considered as one of the nine planets in the year 1930. Although it was a distant and frigid oddball, it was still regarded as one of the planets in the solar system. However, it was the year 2006 when the International Astronomical Union (IAU) reclassified it and placed it under the category of dwarf planets. It made Pluto distinct from the other eight true planets of the solar system.
As per IAU, a real planet is the one which fulfills the three essential criteria. These are-
- It must circle the sun and no other object which leads moon to come out of the league.
- It must be big enough to be rounded into a sphere or spheroid by its gravity, but not so large that its innards host the fusion reactions that power stars.
- It must have cleared its neighbors of other orbiting bodies.
Since Pluto does not fulfill the third criterion of clearing it neighbors it was supposed to become something other than a true planet and the name given was- a dwarf planet. The ring of icy bodies beyond Neptune which is called as Kuiper Belt is far from clear which made Pluto out of the race.
Although many people and scientists related to the field accepted the fact, some objected to it and considered the IAU’s definition of planet flawed. Alan Stren, the principal investigator of NASA's New Horizons mission ridicule the criteria put forward by IAU on various grounds and support the claim of Pluto being a true planet entirely. Stern has been a vocal proponent of Pluto's planethood and has argued that the IAU's decision stemmed at least partly from a very nonscientific desire to keep the solar system's planetary stable down to a "manageable" number.
Alan Stern along with David Grinspoon, a planetary scientist came up with a book named "Chasing New Horizons: Inside the Epic First Mission to Pluto” that got published in which Grinspoon and Stern took aim at the IAU's "hastily drawn" and "flawed" planet definition, reserving special ire for the "clearing your neighborhood" requirement.
Recently The Washington Post published a "Perspectives" piece the two scientists wrote titled, "Yes, Pluto Is a Planet." In the book, Alan and David have brought a simple definition of planets which they also presented at a planetary science conference held in Texas.
With so many contradictory facts and features, the existence of Pluto as a true planet or dwarf one is still under research. We never know whether IAU will ever accept that Pluto is a true planet all over again or not.