This enhanced colour image of Pluto and its moon, Charon, was taken by NASA’s New Horizons spacecraft in July 2015.
Pluto was discovered and classified as a planet in 1930, when astronomer Clyde Tombaugh of the Lowell Observatory examined photographic plates of the sky and noticed a tiny dot against the backdrop of stars. The name “Pluto” was proposed by 11-year-old English schoolgirl, Venetia Burney.
But, in July 2005, astronomers found the distant object, Eris, which was thought to be larger than Pluto. The discovery prompted the awkward question: if Pluto is a planet, shouldn’t Eris be considered a planet too? And what about all the other icy objects out in the Kuiper belt? In August 2006, the International Astronomical Union (IAU) ruled that Pluto should be demoted to the status of ‘dwarf planet’.
Pluto is smaller than the Earth’s moon, with an orbit that is so eccentric that it crosses the orbit of Neptune and actually gets closer to the sun than Neptune for 20 of its 248-year-long trip. Nevertheless, it is a complex world of oceans, mountains and glaciers. Scientists have found evidence of ammonia on its surface, making it likely that an underground ocean exists. The presence of ammonia is exciting, because it is a significant precursor to prebiotic chemistry, including the formation of amino acids and a whole bunch of other biologically interesting chemicals.
Pluto should be swiftly reinstated as a planet. Not least because we can then revive the highly useful mnemonic for remembering the order of the planets in our solar system: My Very Educated Mother Just Showed Us Nine Planets!