Pluto might not be a planet, but it just got another moon.
Technically a 'dwarf planet', after its status was downgraded in 2006, Pluto is now thought to have five moons of its own, Nasa said.
Discovered with the Hubble Space Telescope, the new moon is just six to 15 miles across.
Provisionally named 'P5', the moon is by far the smallest orbiting Pluto - which itself is only 0.6% as massive as the Earth.
Nasa said the discovery will help it navigate the space near Pluto when the New Horizons spacecraft makes a long-awaited 'fly by' of the dwarf planet in 2015.
"The discovery of so many small moons indirectly tells us that there must be lots of small particles lurking unseen in the Pluto system,"said Harold Weaver of the Johns Hopkins University Applied Physics Laboratory.
Pluto's largest moon, Charon, was discovered in 1978, while two others were discovered in 2006 and a fourth in 2011.
It is thought that Pluto and its moons are the collective debris resulting from a collision between two large bodies billions of years ago.
"The moons form a series of neatly nested orbits, a bit like Russian dolls," said team lead Mark Showalter of the SETI Institute.
The new moon has an orbit of around 29,000 miles, which is about eight times smaller than the orbiting distance of Earth's moon.
The International Astronomical Union stipulates that that Pluto's moons have to receive mythological names associated with the underworld.
The bodies Pluto, Nix, Hydra and Charon already do - and eventually P4 and P5 will join them.