Pluto: Demoted Just Before Its Hidden Ocean Came to Light

On July 14, 2015, the New Horizons spacecraft flew past Pluto at a distance of about 12,500 kilometres. The images sent back to Earth showed a world that was not frozen solid and silent: mountains several kilometres high, a nitrogen plain thousands of kilometres wide, long fractures, and a bright heart-shaped region. Beneath that ice, geological data also point to liquid water.

Only nine years before that flyby, the International Astronomical Union had removed Pluto from the list of nine planets and placed it among dwarf planets. The decision, made on August 24, 2006, was explained by Pluto’s failure to clear its orbital neighbourhood. But the timing creates an uncomfortable question: the public was told to see Pluto as a small secondary object just as humanity was about to see a world with an atmosphere, active geology, and possibly an ocean below its surface.