by whom?
Clyde Tombaugh
Pluto
Pluto was discovered by Clyde Tombaugh in 1930 
what is the definition of a planet? A planet is a body that orbits the Sun, is massive enough for its own gravity to make it round and has "cleared its neighbourhood" of smaller objects approaching its orbit how were they described during antiquity? Ancient Greek astronomers employed the term "asteres planetai" meaning "wandering stars", to describe those starlike lights in the heavens that moved over the course of the year and during the middle ages? Medieval and Renaissance writers generally accepted the idea of seven planets what is one controversy that is going on regarding this? A number of Pluto-as-planet proponents have circulated a petition among astronomers to alter the definition when was this controversial entity discovered? 1930
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Pluto [SEP] Introduction
Pluto (minor planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodies beyond the orbit of Neptune. It was the first and the largest Kuiper belt object to be discovered. Pluto was discovered by Clyde Tombaugh in 1930 and declared to be the ninth planet from the Sun. After 1992, its status as a planet was questioned following the discovery of several objects of similar size in the Kuiper belt. In 2005, Eris, a dwarf planet in the scattered disc which is 27% more massive than Pluto, was discovered. This led the International Astronomical Union (IAU) to define the term "planet" formally in 2006, during their 26th General Assembly.