The fifth moon is the one designated as P5 in the picture. NASA’s astronomers found it using the Hubble Space Telescope. According to NASA’s website, ‘the moon is estimated to be irregular in shape and 6 to 15 miles across. It is in a 58,000-mile-diameter circular orbit around Pluto that is assumed to be co-planar with the other satellites in the system.’
Photo credit: NASA; ESA; M. Showalter, SETI Institute.