It Was There All Along

Hubble Space Telescope Images Reveal Neptune’s Moon

Neptune is the most distant planet in our solar system, which makes its systems of moons and rings a challenge to observe from Earth. These things are small and dim, even in Hubble Space Telescope images and data.

On July 1, Dr. Mark Showalter, a scientist at the SETI Institute in California, spotted something in Hubble images that hadn’t been seen before circling around the giant planet. Or least, it hadn’t been seen by human eyes. It was a small moon orbiting the planet about once every 23 hours. He was actually studying faint arcs (segments) of rings. “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.”

It turns out that this little moon, now dubbed S/2004 N1, was in images taken by Hubble between 2004 and 2009, but it’s extremely small and dim. It’s likely no one was looking directly for that moon, so it remained hidden in the data archives, waiting for Dr. Showalter to find it.

This finding is a great example of archival discoveries—that is, findings made as astronomers go back through earlier observations. They do this for a variety of reasons. Sometimes it’s to check and see if the object ever appeared to observers before. Or, as in the case of a supernova explosion, to see what the object that exploded looked like in earlier observations. Or, sometimes, they do it to chart change over time (for example, changes on the surface of Mars).

Prior observations of astronomical objects through the world’s panoply of telescopes (both ground-based, orbiting, and in situ (at planets) give us a good feel for how the planets, moons, rings, and distant objects like stars, nebulae, and galaxies change over time. The universe is a constantly evolving place, and it will always provide us with new objects and events to discover and study.