Clearing the neighborhood
The last work of becoming a planet is lonely. You spent the final stretch sweeping your orbit clean, swallowing or flinging away every rival that shared your lane around the star. When your path was yours alone, the work was done. That clearing is, by definition, what makes a planet a planet.
Three rules for a world
The people who will one day look up at you settled on three rules: a planet orbits its star, is round under its own gravity, and has cleared its neighborhood. You pass all three. The smaller bodies that pass only the first two get a gentler name, dwarf planet. You are the full thing.