The secrets of Pluto’s largest moon

Nobody knew it was there until 1978. For almost 50 years after Pluto’s discovery in 1930, the dwarf planet had no known companions out there on the edge of the Solar System. Today, of course, in the wake of the New Horizons mission and a myriad of discoveries since the first inklings of the Kuiper Belt came in 1992, we know it positively teems out there. Charon is one of a system of five moons, and Pluto is the largest member of a huge collection of objects orbiting beyond Neptune. The ongoing hunt for a large planet in the extreme reaches of the Solar System has so far come to nothing, and this is the domain of the small, with Pluto’s reclassification as a dwarf planet just the first in a whole raft of triumphantly tiny accolades.

Pluto’s largest moon, however, has some remarkable features of its own, despite only having a diameter of 1,212 kilometres (753 miles) – about 10.5 per cent that of Earth’s. One-eighth the mass of its co-orbitee Pluto, and half the diameter, it’s tidally locked to the larger body, but large enough that the two orbit a centre of mass between them. The International Astronomical Union’s general assembly considered a proposal in 2006 to reclassify the pair as a double planet, but despite it being spherical, it wasn’t clear Charon was in hydrostatic equilibrium, a state in which the force of gravity is balanced by outward pressure from the body. This state is necessary to give it dwarf planet status.