Yesterday the US space agency NASA announced the discovery of a fourth moon orbiting around the tiny dwarf planet of Pluto, out at the far edge of the Solar System.
It was already known that Pluto has three moons, the largest – Charon – is thought to be around 1000 km in diameter. The others, Nix and Hydra, are relatively small bodies thought to be some 50 to 100 km in diameter.
Discovered by the Hubble Space Telescope, the new moon is even smaller than Nix or Hydra, and is thought to be a mere 30 km or so across. Provisionally called P4, the astronomer who discovered the moon is given the honour of naming it officially. The name chosen is generally that of a mythological figure.
Speaking to reporters, Mark Showalter, who led the observation team in California, said: “I find it remarkable that Hubble’s cameras enabled us to see such a tiny object so clearly from a distance of more than three billion miles (five billion kilometres).”
Traditionally thought of as the ninth planet, the furthest out from the Sun, Pluto was controversially demoted from full planet status in 2007 when a team of astronomers reclassified it as a “dwarf planet”. The dwarf planet orbits at an average of 9.5 billion kilometers from the Sun. By comparison the Earth orbits the Sun at an average distance of 150 million kilometers.
Little of the light and heat of the sun reach so far out into space, and Pluto is believed to be a barren ball of ice and rock where temperatures are not far above absolute zero.