Twenty years after Pluto was reclassified as a dwarf planet, Caltech astronomer Mike Brown, whose discovery of Eris helped prompt the 2006 debate, continues to defend the decision. In a Scientific American retrospective, Brown argues that a planet must not only be round and orbit the Sun but also clear its orbital neighbourhood. He says restoring Pluto would require classifying numerous moons and Kuiper Belt objects as planets, making consistency difficult. For Brown, Pluto remains fascinating — but not a planet.
