Neptune's Newest Moon

Astronomers have spotted a tiny object circling Neptune. This find, designated S/2004 N 1, brings the planet's moon count to 14.

Spacecraft and powerful ground-based telescopes have swollen the census of moons known to be orbiting the outer planets. However, perhaps owing to its great distance, Neptune has been missing from astronomers' "new moon" proclamations for a decade.

That's changed with the announcement that a tiny, previously unseen moonlet has been found. A team of observers led by Mark Showalter (SETI Institute) recorded it repeatedly in Hubble Space Telescope images of Neptune taken between 2004 and 2009. But the little body sat unnoticed for years, owing to its tiny size and rapid motion around the planet. "This moon never sits still long enough to have its picture taken," Showalter says.

He chanced upon it on July 1st, while researching the planet's faint rings and ring arcs. "I was working with some older images from 2009 that I thought I was finished with," Showalter explains. The arcs circle Neptune quickly, in just hours, so to make them stand out, he'd offset each image in the Hubble sequence to account for the orbital motion. "Oh, I should look farther out," he thought, and when he did (again adjusting for orbital motion), a "fairly obvious dot" turned up.

Showalter quickly found the moonlet in several other sets of Hubble images taken in 2004 and 2005. He also checked images from Voyager 2, which flew past Neptune in 1989, but it's not there. In fact, with an estimated magnitude of 26.5, S/2004 N 1 barely registered in the individual HST images. If its surface is dark, like those of other nearby satellites, then the new find has a diameter of no more than 10 to 12 miles (16 to 20 km) — the smallest of Neptune's 14 known moons.

Ironically, these images have been in the public domain for years. "Anyone could have discovered this," Showalter admits.

According CBET 3586, issued earlier today by the IAU's Central Bureau for Astronomical Telegrams, S/2004 N 1 appears to be traveling in a near-circular orbit very close to Neptune's equatorial plane. Based on its positions recorded during the past 5 years, the object must circle the planet every 22^h 28.1^m and therefore must average 65,420 miles (105,283 km) from Neptune's center. That puts it between the small inner moons Larissa and Proteus.

The known census of planetary moons now stands at 181 (if you count those of dwarf planets Pluto, Eris, and Haumea). To see how the new discovery fits in among its Neptunian neighbors, see Sky & Telescope's online guide to planetary satellites.