India, Sept. 19 -- Twenty years ago-on August 24, 2006-some 2,500 members of the International Astronomical Union in its general assembly in Prague, Czech Republic voted on a definition that earthlings lacked: "What is a planet?". Resolution 5A, voted on by just 424 eligible members, put forth three conditions that define a celestial body as a planet: An orbit around the Sun; sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape; and an orbital path clear of other celestial objects.

Then came Resolution 6A, which saw "237 votes in favour, 157 against and 17 abstentions", to declare the first casualty of the new definition. Pluto, failing on the third condition...