Pluto: 10 years on

The New Horizons fly-by of this ancient, icy world continues to surprise a decade later. So what have we learned? Ben Evans investigates

- Ben Evans

Ten years ago, Pluto emerged from the gloom at the Solar System's ragged edge - and a new world came in from the cold.

Five billion kilometres (3 billion miles) from Earth, on 14 July 2015, NASA's piano-sized New Horizons probe swept past the dwarf planet and glimpsed a crater-scarred landscape teeming with nitrogen glaciers, winding canyons, water-ice mountains and - maybe - a substantial ocean in liquid form.

The title 'planet' has proven a thorn in Pluto's side. Discovered by Clyde Tombaugh in 1930 and named by schoolgirl Venetia Burney for the Roman god of the underworld, generations of children knew Pluto from classroom mnemonics as the Sun's ninth planet.

In 1978, Jim Christy discovered Pluto's moon, Charon. The name honoured both the mythical ferryman of the dead and Christy's wife, Charlene. Charon is half Pluto's size and an eighth of its mass.

The two bodies’ barycentre of mass lies outside them: like figure skaters, they synchronously circle an empty patch of space every 6.4 days in tight tidal lockstep, each perpetually presenting the same hemisphere to the other.

Pluto's identity crisis

Pluto's planetary downfall began in 1992, when astronomers discovered the Kuiper Belt, a disc of millions of icy planetesimals beyond Neptune. It furnishes a treasure trove of pristine relics geochemically unaltered since the Solar System's infancy. With 3,000 Kuiper Belt objects officially known today, the detection of several large icy bodies after 2003 prompted the International Astronomical Union (IAU) to redefine what constituted a planet. In 2006, Pluto, now recognised as part of the Kuiper Belt, was recategorised as a 'dwarf planet'. Pluto went from the smallest planet to the biggest Kuiper Belt object.