Question

why do scientists find it hard to classify pluto?

Answer 1

The IAU also created a newer classification, "dwarf planet," which is an object that meets planetary criteria except that it has not cleared debris from its orbital neighborhood. This definition meant that Pluto — considered a planet at the time — was demoted and reclassified as a dwarf planet.

Answer 2

Explanation:

The main development that led to Pluto's reclassification was the discovery, over the past few decades, that it's part of a cloud of debris called the Kuiper belt — a region of the solar system, out past Neptune, that's filled with thousands of chunks of rock and ice (including Eris, which is actually more massive)