The primary finding that made astronomers begin to doubt that Pluto was a planet was the discovery of other similar-sized celestial objects in the Kuiper Belt, particularly Eris, which is similar in size to Pluto. This led to questions about what constitutes a planet. Additionally, Pluto's orbit is highly elliptical and crosses the orbit of Neptune, which added to the complexity of classifying it as a planet. Ultimately, in 2006, the International Astronomical Union (IAU) redefined the criteria for what constitutes a planet, and Pluto did not meet these criteria, leading to its reclassification as a "dwarf planet."
What finding made astronomers begin to doubt that Pluto was a planet?
They discovered that it traveled around the Sun in an oval-shaped orbit.
They found that although it was the farthest planet from the Sun, it had a rocky surface.
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