Pluto

The little world with a heart that lost its planet title

Pluto was counted as the 9th planet for 76 years, but in 2006 scientists called it a 'dwarf planet' — it's too small, smaller than our Moon! Pluto is so far from the Sun that the Sun looks like an ordinary star from there, and a year lasts 248 Earth years. In 2015 the New Horizons probe flew past Pluto and sent back a picture of a giant icy 'heart' on its surface. Pluto's moon Charon is so big that the two spin together as if dancing.

Pluto (radius 1,188 km, two-thirds of the Moon) is the largest known Kuiper Belt object, discovered by Clyde Tombaugh in 1930. In 2006 the International Astronomical Union defined 'planet' (it must have cleared its orbit of other bodies) — Pluto became a dwarf planet (with Eris, Ceres, Haumea and Makemake). Its orbit is elongated (30–49 AU) and tilted 17°; from 1979 to 1999 it was closer to the Sun than Neptune. Surface: nitrogen, methane and carbon monoxide ices at −230 °C; Sputnik Planitia is a 1,000 km nitrogen glacier (the left lobe of the 'heart'), with cryovolcanoes and a thin atmosphere with blue haze. Charon (radius 606 km) is half Pluto's size, and the system's centre of mass lies outside Pluto — a 'double dwarf planet'; both keep one face to each other, orbiting in 6.4 days. Four more small moons (Styx, Nix, Kerberos, Hydra). New Horizons (2006–2015, 9.5 years, 5 billion km) made the first and only visit.

Fun facts

• Since its discovery (1930) Pluto has not yet completed one orbit of the Sun — that happens in 2178.
• Pluto's name was suggested by an 11-year-old English girl, Venetia Burney.
• Pluto and Charon always face each other — as if dancing hand in hand.