Shelly was surprised to learn the dwarf planet Pluto used to be considered a planet. What would have had to be true for Pluto to remain categorized as a planet?

Under the International Astronomical Union’s definition, a planet must orbit the Sun, be nearly round, and clear the neighborhood around its orbit. Pluto meets the first two criteria, but it shares its orbital region with many other objects. It would have needed to clear that neighborhood to remain classified as a planet, so the correct choice is clearing its orbital neighborhood.

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