Pluto s P4 . ... Credit Copyright: NASA , ESA , Mark Showalter ( SETI Institute ) Explanation: 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. ... 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. ... NASA Official: Jay Norris. ...