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 <answer>the Kuiper belt</answer> in 1992 led many astronomers to debate whether Pluto should be considered a planet or as part of <answer>the Kuiper belt</answer>. 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.
What discovery made astronomer's debate Pluto's status as a planet? 
the Kuiper belt