Pluto: Diary of an ex-planet

Updated and expanded following the spectacular 2015 flyby of Pluto by NASA's New Horizons space probe, this Kindle Short Read (12,000 words) tells the engaging story of this fascinating little world on the edges of the Solar System: from its discovery to its controversial demotion, and the renewed interest of planetary scientists as its secrets were finally unlocked.

Did you know?

*From its discovery in 1930 up until 2006, Pluto was considered to be the Solar System’s ninth and outermost planet, but it was not the first ‘planet’ to be demoted. At the time of their discovery in the early nineteenth century, the large asteroids Ceres, Pallas, Juno, and Vesta were considered to be planets. But many more asteroids were discovered in the middle of the century, and the four were eventually reclassified as asteroids.

*Pluto is far smaller than any of the eight currently recognised planets: its diameter of 2,376.6 km (1476.75 miles) is less than half that of the smallest currently recognised planet, Mercury (4,879.4 km (3031.9 miles)). Pluto’s mass of 1.303 ± 0.003Ч1022 kg is less than 1/25th that of Mercury.

*Pluto’s axial tilt is higher than that of any of the eight recognised planets. At 122°32', it exceeds even the 97°46' of Uranus, so Pluto effectively rotates on its side. For comparison, Earth’s axial tilt is 23°26'.

*Pluto’s total of five moons might be small in comparison to Saturn’s 82 and Jupiter’s 79, but it is nevertheless impressive considering that Mercury, Venus, Earth, and Mars can muster a grand total of just three moons between them.

*Pluto’s major moon, Charon, is 1,212.0 ± 1.0 km (753.1 miles) in diameter, and it is larger in comparison to its primary than any other known moon. Pluto and Charon are 19,571 km (12,161 miles) apart centre to centre, and orbit around a barycentre (mutual centre of gravity) once every 6 days and nine hours. The pair are tidally locked, meaning that both keep the same side facing one another. From one side of Pluto, Charon hangs motionless in the sky, never rising nor setting; from the other side, it is never seen at all. Prior to 2006, the Pluto-Charon system was often described as a binary planet.

*Depending on Pluto’s distance from the Sun, light takes 4 hours (when at perihelion) and 6 hours 50 minutes (when at aphelion) to make the trip. For comparison, it takes light just over eight minutes to reach Earth from the Sun.

*To boldly go: A small portion of Clyde Tombaugh’s ashes were carried aboard the New Horizons space probe, which became the first and so far only spacecraft to visit Pluto. To date, only a handful of human remains have been sent into space, and no others have travelled any further than the Moon.

*Gustav Holst was an early Pluto-sceptic. His seven-movement The Planets: Suite was composed during World War I, and it included all the then-known planets apart from Earth. Pluto was discovered four years before Holst’s death, but he declined to compose an additional movement. In 2000, Manchester’s Hallй Orchestra commissioned the English composer Colin Matthews to write an eighth movement, which he called ‘Pluto, the Renewer’. The movement was dedicated to Holst’s daughter, composer Imogen Holst, who died in 1984.

*The Walt Disney cartoon dog Pluto the Pup was named after Pluto and not, as is sometimes believed, the other way round.

*The radioactive element plutonium (Pu), which can be used in the manufacture of nuclear weapons, is named after Pluto. It followed Neptunium (Np), named after Neptune, and Uranium (U), named after Uranus.