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? Pluto would need to have an atmosphere. Pluto would need to clear the neighborhood of its orbit

Pluto would need to clear the neighborhood of its orbit

Explanation:

Pluto was originally recognized as a planet before it was later reclassified as a dwarf planet. The reason for its reclassification was because it failed to achieve on of the classification required to be termed a full-planet; it has not "cleared the neighborhood" around its orbit. "Clearing the neighborhood" means that a planetary body has to be the dominant body in their orbit around the sun.