Astronomers Vote on the Solar System. Pluto Loses.

Nothing happened to Pluto. Humanity changed the paperwork.

Pluto had been a planet for 76 years.

Then, on a Thursday afternoon in Prague, astronomers held up yellow voting cards.

On 24 August 2006, members of the International Astronomical Union gathered at their General Assembly to settle a question that sounded philosophical but had become surprisingly practical:

What, exactly, is a planet?

The problem had grown as astronomers discovered more objects in the outer Solar System. Pluto, classified as the ninth planet since its discovery in 1930, no longer looked quite so comfortably unique.

The first proposed solution was expansive.

On 16 August, the IAU published a draft definition under which the Solar System might contain twelve planets: the familiar nine plus Ceres, Pluto's companion Charon and the distant object then called 2003 UB313.

That proposal did not survive the meeting.

Eight days later, astronomers assembled for the final vote. The adopted definition said that a Solar System planet must orbit the Sun, have enough gravity to become approximately round, and have cleared the neighbourhood around its orbit.

Under that definition, Pluto met the first two criteria but not the third.

IAU members voted by raising yellow cards.

The result did not destroy Pluto, move Pluto or alter Pluto's orbit by a millimetre.

It changed the noun.

Pluto became a dwarf planet, while Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune remained planets.

The Solar System now officially contained eight.

The sequence gives the episode its particular WTF quality. During the same international meeting, astronomers went from seriously proposing a twelve-planet Solar System to approving an eight-planet Solar System.

Pluto had spent 76 years in schoolbooks, models and mnemonic devices as Planet Number Nine. Its physical circumstances had not suddenly changed.

The definition had.

And on Thursday afternoon in Prague, that was enough.

The Solar System lost a planet without losing an object.