A fifth moon for Pluto

Several observations made with the Hubble Space Telescope have enabled astronomers to discover a fifth moon around the dwarf planet Pluto. This new natural satellite seems to be no more than 24km wide.

L’un des 9 clichés du télescope spatial Hubble qui a permis de découvrir la cinquième lune de Pluton (indiquée par P5). Celle-ci orbite à 42.000 km de distance de la planète naine. Credit: NASA/ESA/M. Showalter (SET Institute)

Since its discovery in 1930 by the American astronomer Clyde Tombaugh (1906-1997), Pluto has constantly intrigued scientific writers. First, it was estimated to be intermediate in size between Mercury (4,878km in diameter) and Mars (6,792km). But then, what had been considered to be our Solar System’s ninth and most distant planet turned out to be much smaller with a diameter of 2,306km, even smaller than the Moon around our Earth (3,474km). Later, thanks in particular to infrared spectroscopy, 98% of its surface was determined to be covered with frozen nitrogen (temperature ?218°C to ?240°C).

5 moons discovered in 34 years, from 1978 to 2012
In 1978, Pluto was given a moon, Charon, identified by James Christy of the U.S. Naval Observatory, a step forward in our difficult attempts to learn more about this strange planet, so distant that significant details elude even the most powerful telescopes. With a diameter of 1,207km, this moon dubbed Charon was half its planet’s size. In 2005, photographs of the ninth planet taken by the Hubble Space Telescope revealed the existence of another two, much smaller natural satellites: Nix (46km in diameter) and Hydra (61km). The following year, in 2006, the International Astronomical Union excluded Pluto as a true planet, defining it as a “dwarf planet” (a planet not having sufficiently “cleaned” its orbit). In 2011, Hubble identified a fourth moon, dubbed P4 (for Pluto-4) until an official name is coined. It is estimated to have a diameter between 14 and 40km. After these first two discoveries, a third was made from 26 June to 9 July 2012, when Hubble made 9 photos of Pluto showing the presence of a fifth moon, apparently 24km wide and irregular in shape. For the moment, it is numbered S/2012 (134340) 1 or, more simply P5 for Pluto-5. No doubt this latest discovery is of great interest to NASA’s New Horizons project, which should cross paths with Pluto in July 2015, within a distance as close as 10,000km (the first probe ever to fly over Pluto). The dwarf planet’s moons will be one of the mission’s scientific goals.

The New Horizons probe (artist’s rendition) will pass through the Plutonian system in July 2015. A first! Credit: NASA