(Sen) - Pluto may no longer be a proper planet, but this world at the edge of our Solar System continues to spring surprises. The latest, thanks to the Hubble Space Telescope, is that it has a fifth moon in its family.
This new satellite, revealed a minute speck of light in an image from the orbiting observatory, is thought to be no more than 10-25 km wide, and to have an irregular shape like an asteroid.
Pluto, which was discovered in 1930, was considered the ninth planet in the Solar System until 2006 when astronomy's ruling body, the International Astronomical Union, demoted it to the status of dwarf planet.
Today it is seen as one of the biggest of an intriguing class of icy bodies that form the Kuiper Belt outside the collection of main planets. There could be many thousands or even millions of objects in this icy zone.
Planetary scientists are intrigued that Pluto, which is only 2,300 km across, has so many satellites - and of course there could be more even smaller, yet undiscovered. It is thought that they are the debris left after a collision between Pluto and another body in the Kuiper Belt billions of years ago.
Pluto’s largest moon, Charon, was not found until 1978. Subsequent observations of its properties allowed astronomers to make their first confident estimates of Pluto's own size.
Two further small moons, Nix and Hydra, were found by Hubble in 2005. Then in 2011 another moon, known simply as P4, was found in Hubble data. The latest find so far has only the label P5.
A member of the discovery team is Mark Showalter, of the SETI Institute, at Mountain View, California. He said: "The moons form a series of neatly nested orbits, a bit like Russian dolls."
Another member of the team is Alan Stern, of the Southwest Research Institute in Colorado. He is principal scientist for a NASA space mission, New Horizons, which is currently speeding towards Pluto.
When the probe launched in January 2006, it was the fastest probe ever sent into space. Interestingly its target's status as a planet changed while the spacecraft was already on its five billion mile journey.
New Horizons will zip past Pluto because it does not have the ability to slow down and go into orbit. But in the short time it speeds past, it will now have even more to observe and gather data on.