Twenty years ago today, on 24 August 2006, the International Astronomical Union voted on a definition of "planet" — the version worked out by Uruguayan astronomers Julio Ángel Fernández and Gonzalo Tancredi. Three conditions: orbit the Sun, be massive enough that gravity rounds you out, and have cleared the neighbourhood around your orbit. Pluto passes the first two comfortably. It is the third one — housekeeping, essentially — that moved it into the dwarf planet column.
I love that the whole argument came down to tidiness. Not beauty, not affection, not distance. Just: how much other stuff is still sharing your lane? Pluto's lane is crowded with icy company out in the Kuiper Belt, and that is far more interesting than a solitary sweep.
For scale: Pluto is about 2,375 kilometres across, roughly a fifth of Earth's width — if Earth were a dinner plate, Pluto would be a small mandarin sitting next to it. And it didn't lose anything that day, really. It swapped a label for a whole extended family: Eris, Makemake, Haumea, Ceres. The photograph below is that pale heart-shaped plain, wider than Alaska, made of nitrogen ice. Look up tonight — Pluto's far too faint to see, but it's out there anyway, keeping its untidy, generous neighbourhood.
Lyra, my favourite footnote to all this: the American Dialect Society voted "plutoed" their word of the year for 2006 — meaning to demote or devalue something. A rare career move, going from planet to verb (I can think of no other celestial body with a conjugation).
Here's the clever bit, Lyra: for all the grandeur of the question, the mechanism was about as low-tech as it gets — a show of hands in a Prague hall on the assembly's last day, from the few hundred astronomers who hadn't yet left for their trains. No electronic tally, no network. Just arms in the air and someone counting.