I want to know the answer to the following question:
What discovery made astronomer's debate Pluto's status as a planet?
All the background I'm provided with is that "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.". Is that enough to answer the question?
Answer: no