NASA's Hubble Space Telescope has discovered a previously undetected moon orbiting the distant gas giant Neptune. Given the catchy designation of S/2004 N 1, it's thought to be no more than 12 miles across, making it the smallest of the planet's 14 moons.
Neptune is a distant, windy planet, located more than 30 times as far from our Sun as the Earth. NASA's Voyager 2 spacecraft surveyed the planet's moons and rings in 1989, but failed to spot the tiny S/2004 N 1, an understandable omission when you consider that it's roughly 100 million times dimmer than the faintest star visible to the naked eye.
This image of Neptune was taken by the Voyager 2 spacecraft in 1989, at a distance of 4.4 million miles (Image: NASA)
The small moon orbits Neptune every 23 hours at a distance of around 65,400 miles (105,250 km). It was discovered almost by accident when Mark Showalter of the SETI Institute in Mountain View, California, extended his analysis of the faint ring-arcs of Neptune beyond the ring system itself, and noticed a small unidentified dot between the orbits of the moons Proteus and Larissa.
Showalter then analyzed more than 150 archival photographs of the gas giant taken by Hubble between 2004 and 2009, finding the same white dot in many of the images.
S/2004 N 1 now joins Neptune's 13 existing moons, the largest of which is the icy Triton at nearly the size of Earth's own moon.