Article: From its discovery in 1846 until the subsequent discovery of Pluto in 1930, Neptune was the farthest known planet. When Pluto was discovered it was considered a planet, and Neptune thus became the penultimate known planet, except for a 20-year period between 1979 and 1999 when Pluto's elliptical orbit brought it closer to the Sun than Neptune. The discovery of the Kuiper belt in 1992 led many astronomers to debate whether Pluto should be considered a planet or as part of the Kuiper belt. In 2006, the International Astronomical Union defined the word "planet" for the first time, reclassifying Pluto as a "dwarf planet" and making Neptune once again the outermost known planet in the Solar System.

Now answer this question: What discovery made astronomer's debate Pluto's status as a planet?
the Kuiper belt