It started when Mark Showalter followed a whim. On July 1, the SETI Institute astronomer was studying -- as one does, when one is a SETI Institute astronomer -- archival pictures from the Hubble Space Telescope. The images were of Neptune, and Showalter was analyzing the faint arcs, or segments of rings, that surround the ice giant. Here was the whim: Showalter had a thought that he should look beyond the ring segments ... and, when he did so, he discovered a tiny, white dot about 65,400 miles from the planet. The dot he spotted was located between the orbits of the Neptunian moons Larissa and Proteus. And Showalter noticed that the dot appeared repeatedly in more than 150 archival photographs of Neptune taken by Hubble between 2004 and 2009.
Here, now, is why whims can be worth following: the dot, it turns out, is another moon for Neptune -- the planet's fourteenth that we've discovered. It's named, for the moment, S/2004 N 1. And the little thing is tiny: Showalter estimates it to be, at the most, 12 miles across, making it the smallest moon we know of in the Neptunian system. It is in fact so small, and so dim, that it's about 100 million times fainter than the faintest star that can be seen with the naked eye -- a body so miniscule that it escaped even the eagle eyes of NASA's Voyager 2 spacecraft, which flew past Neptune in 1989, surveying the planet's system of moons and rings.