An astronomer’s whim and sharp eye have led to the discovery of a city-sized moon orbiting Neptune. Designated S/2004 N 1, the roughly 20-kilometer-wide satellite is the 14th known moon to circle the solar system’s outermost planet.

Mark Showalter, an astronomer at the SETI Institute in Mountain View, Calif., found the satellite while sifting through images taken by the Hubble Space Telescope. His plan had been to study the rocks and ice within Neptune’s faint rings but he decided to expand his search farther from the planet. The analysis turned up a tiny white dot about 105,000 kilometers away from the blue-green gas giant. Showalter plotted the moon’s orbit by tracking the dot’s position in more than 150 photos of Neptune taken by Hubble between 2004 and 2009.

The moon is so small and dark that it escaped detection by Voyager 2, the NASA probe that passed within 5,000 kilometers of Neptune in 1989. Hubble managed to spot the moon from a million times farther away. But it took Showalter, who now has six moon discoveries to his name, to uncover it.