John Wheeler: Pluto is no longer a planet because of a necessary definition
Since Pluto's discovery, too many similar objects similar to Pluto have been found.
FARGO — A conversation with a friend recently revolved, so to speak, around the planet Pluto no longer being called a planet. Pluto was discovered by Clyde Tombaugh in 1930 by going back and forth between star field slides taken on different nights. Anything moving from plate to plate could not be a star, and so Pluto was found. Since then, better telescopes and better techniques have found many other smallish objects in orbit around the sun.
In 2006, the International Astronomical Union declared that to be a planet, an object must be in orbit around the sun, it must be approximately round due to having enough mass to have assumed hydrostatic equilibrium, and it must have swept its orbital path of other objects of similar size except its own moons by virtue of being gravitationally dominant. Pluto fails at the third of these, and so is considered a dwarf planet, along with numerous other recently discovered similar objects.