NASA announced on July 20, 2011 that astronomers using the Hubble Space Telescope (HST) discovered a new moon orbiting Pluto. A team of astronomers led by Mark Showalter made this serendipitous discovery as part of an observing program to search for rings around Pluto similar to those around Saturn and the other giant planets.
The newly discovered moon joins the three previously known moons of Pluto: Charon, Hydra, and Nix. For the time being, astronomers designate the new moon as P4 to indicate that it is the fourth moon discovered orbiting Pluto. Eventually the International Astronomical Union (IAU) will approve an official name for P4.
With an estimated diameter between 8 and 21 miles P4 is Pluto’s smallest moon and is therefore too faint to appear in previous HST images of Pluto. Showalter’s team made the new images of Pluto to search for very faint rings around Pluto. They therefore used longer exposure times than the previous images, which allowed the HST cameras to image the faint moon.
The HST Wide Field Camera 3 first captured an image of P4 on June 28, 2011. A single image is however not sufficient to prove that the faint point of light is a new moon. It could easily be a distant faint background star. Subsequent images of Pluto on July 3 and 18 also showed the new moon orbiting Pluto. The probability that the three images of P4 were made by a background star (or stars) is vanishingly small.
Pluto’s largest and brightest moon, Charon, was first discovered on photographs taken with US Naval Observatory telescopes in 1978. These early images showed Charon as a small bump on Pluto and did not clearly resolve the moon. Shortly after the Hubble Space Telescope was launched, its cameras took photos that clearly resolved Pluto and Charon as two distinct objects. Hydra and Nix were discovered using the HST cameras in 2005.
The fact that astronomers discovered P4 while looking for rings rather than new moons provides a nice example of the role of serendipity in science. Many scientific discoveries are made by scientists who are looking for something other than their discovery.