Nix and Hydra
were first introduced to human eyes in Hubble
Space Telescope images from May 2005,
as Pluto's second and third known moons.
Now Hubble images have revealed a fourth satellite for the
icy, dwarf planet.
Provisionally designated P4, it completes an orbit of Pluto
in about 31 days.
smallest and dimmest known moon,
P4 is estimated to be 13 to 34 kilometers across.
The newly discovered satellite was first spotted
in Hubble observations from June 28, and later confirmed
in a follow-up on July 3 and July 18.
These two panels are composites of both the short and long exposures
that include brighter Pluto itself along
with Pluto's largest moon Charon.
Camera noise and image artifacts also show up
in the long exposure segments.
The Hubble observations were made while searching for
around the distant world in support of NASA's
New Horizons mission, set to
fly by the Pluto system in 2015.