It's been a decade since astronomers last spied a moon orbiting Neptune. Now, the Hubble Space Telescope has spotted a new satellite circling our solar system's most far-flung planet.
Dubbed S/2004 N 1 (for now), the moon measures just 12 miles across, making it the smallest known moon in the Neptunian system. Its diminutive size may explain why it's taken astronomers so long to spot. It's also nearly coal black in color and, consequently, reflects very little light (NASA estimates it's roughly 100-million times dimmer than the faintest star you can pick out with your naked eye ), which could also explain why it's managed to elude detection for as long as it has.
Mark Showalter of the SETI Institute in Mountain View, Calif., found the moon July 1, while studying the faint arcs, or segments of rings, around Neptune. "The moons and arcs orbit very quickly, so we had to devise a way to follow their motion in order to bring out the details of the system," he said. "It's the same reason a sports photographer tracks a running athlete — the athlete stays in focus, but the background blurs."
The method involved tracking the movement of a white dot that appears over and over again in more than 150 archival Neptune photographs taken by Hubble from 2004 to 2009.
On a whim, Showalter looked far beyond the ring segments and noticed the white dot about 65,400 miles from Neptune, located between the orbits of the Neptunian moons Larissa and Proteus. The dot is S/2004 N 1. Showalter plotted a circular orbit for the moon, which completes one revolution around Neptune every 23 hours.