Nonet that became an octet in 2006

PLANETS

Think of the celestial bodies orbiting our sun—one was reclassified as a "dwarf" in the mid-2000s, leaving eight behind.

This clue plays on the reclassification of Pluto in 2006, when astronomers downgraded it from a planet to a dwarf planet. "Nonet" means a group of nine, referring to the original nine planets, while "octet" (a group of eight) hints at the remaining planets after Pluto’s status changed. The answer "PLANETS" fits because it’s the collective term for the solar system’s primary bodies—now officially eight.