Large moon of Pluto, contains a tenth of its mass, making it a double dwarf planet. Its surface is completely covered in water ice, possibly from geysers. This could imply that it has subsurface liquid water. Ammonia may be helping to keep this water liquid near the surface. Has only occulted a star twice, once in 1980, and again in 2005. Has 10% less rock than Pluto, suggesting it formed in a collision and coalesced with outer material, similar to Earth's moon. The impactors could have been each about half the size of the moon.