But the process for redefining planet was deeply flawed and widely criticized even by those who accepted the outcome. At the 2006 IAU conference, which was held in Prague, the few scientists remaining at the very end of the week-long meeting (less than 4 percent of the world’s astronomers and even a smaller percentage of the world’s planetary scientists) ratified a hastily drawn definition that contains obvious flaws. For one thing, it defines a planet as an object orbiting around our sun – thereby disqualifying the planets around other stars, ignoring the exoplanet revolution, and decreeing that essentially all the planets in the universe are not, in fact, planets.
Even within our solar system, the IAU scientists defined “planet” in a strange way, declaring that if an orbiting world has “cleared its zone,” or thrown its weight around enough to eject all other nearby objects, it is a planet. Otherwise it is not. This criterion is imprecise and leaves many borderline cases, but what’s worse is that they chose a definition that discounts the actual physical properties of a potential planet, electing instead to define “planet” in terms of the other objects that are – or are not – orbiting nearby. This leads to many bizarre and absurd conclusions. For example, it would mean that Earth was not a planet for its first 500 million years of history, because it orbited among a swarm of debris until that time, and also that if you took Earth today and moved it somewhere else, say out to the asteroid belt, it would cease being a planet.
To add insult to injury, they amended their convoluted definition with the vindictive and linguistically paradoxical statement that “a dwarf planet is not a planet.” This seemingly served no purpose but to satisfy those motivated by a desire – for whatever reason – to ensure that Pluto was “demoted” by the new definition.
By and large, astronomers ignore the new definition of “planet” every time they discuss all of the exciting discoveries of planets orbiting other stars. And those of us who actually study planets for a living also discuss dwarf planets without adding an asterisk. But it gets old having to address the misconceptions among the public who think that because Pluto was “demoted” (not exactly a neutral term) that it must be more like a lumpy little asteroid than the complex and vibrant planet it is. It is this confusion among students and the public – fostered by journalists and textbook authors who mistakenly accepted the authority of the IAU as the final word – that makes this worth addressing.
Source: Yes, Pluto is a planet – SFGate