Neptune’s “hidden” moon discovered on a whim

Neptune’s family recently grew by one from 13 to 14. A new moon, temporarily named S/2004 N1, was discovered on July 1. It orbits around the planet once every 23 hours at a distance of . Credit: NASA, ESA, M. Showalter/SETI Institute

Looks like Voyager 2 left at least one stone unturned during its 1989 flyby of Neptune. Astronomers using the Hubble Space Telescope announced the discovery of a new Neptunian moon designated S/2004 N1. It’s the system’s 14th and smallest. On second thought, maybe it’s not so surprising that Voyager missed it; the moon only a dozen miles (20 km) across and dark as asphalt.

Neptune’s 14 moons, named for water deities in Greek mythology, range in size from tiny S/2004 N1 to Triton, with a diameter of 1,680 miles (2,705 km). Triton even has its own thin nitrogen atmosphere. Credit: NASA

Mark Showalter of the SETI Institute in Mountain View, Calif. was studying ring arcs within Neptune’s system of five dark rings. Normally astronomers stack multiple images one atop the other to discover faint planetary details and moons. Stacking adds the faint traces of light together to make a brighter dot that would otherwise be missed. But Neptune’s rings and moons move too quickly for stacking to work; successive photos show no overlap and faint stuff keeps hidden.

Showalter got around this problem with motion-tracking software that lets the computer shift and add-up the images after the fact:

“The procedure I devised predicts where any given moon ought to move from one image to the next, and then combines the images with a “twist” that compensates for the expected motion,” writes Showalter in his blog. It’s similar to panning or following an moving subject in your camera to keep it sharp while the background blurs.

Neptune is the crescent at right. The outer Adams Ring is visible along with the bright arc segments named Liberté, Egalité, and Fraternité (right to left). The inner Leverrier Ring is also shown. Photo taken by Voyager 2 in August 1989. Credit: NASA

Showalter found the moon on July 1, when out of curiosity, he looked past the rings, applied the new software procedure and noticed a white dot that appeared repeatedly in more than 150 archive photos taken by Hubble between 2004 and 2009.

I don’t know if we’re lucky to have only one moon or not. While it certainly makes our moon special, the dozens and dozens of other satellites that circle the outer planets open our eyes to nature’s exceptional creative powers when it comes to landscape architecture.