Feature

Why we only just discovered Neptune's 14th moon

NASA's Hubble has spotted an unusual bright spot circling the giant planet

Even with NASA's powerful (if temporarily hobbled) new instruments like the Kepler Space Telescope allowing us to peer farther into the universe's distant corners at exotic new worlds, it's the trusty old Hubble, launched in 1990, that is proving there's still plenty of undiscovered stuff left in our own solar system.

Take Neptune, the blue-green giant nestled between Uranus and Pluto. Mark Showalter of the SETI institute in Mountain View, Calif., recently discovered a brand-new, previously unseen moon circling the planet, making it Neptune's 14th to date.

It's not very big: S/2004 N 1 (yes, it's a mouthful) is just 12 miles across, or about the size of a modest metropolitan city. Its unassuming size may explain why we had a hard time spotting it in the first place; it even escaped the eye of NASA's Voyager 2 spacecraft, which flew right past it in 1989.

Showalter himself barely noticed the moon — which is 100 million times fainter than the faintest star in the night sky we can see with our naked eye — as he was studying Hubble images of Neptune's arcs on July 1 of this year.

Hidden in Neptune's rings, Showalter noticed an unusually bright dot about 65,400 miles away from the planet. To figure out exactly what it was he had to track it, Showalter pored over 150 archival Neptune photographs. "The moons ... 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."

In several photographs the white dot resurfaced, again and again and again.

By connecting the dots, Showalter could trace the new moon's orbital path as it circled Neptune. In the end, each revolution was found to take about 23 hours.

It just goes to show you that the Hubble, even after a long two decades, is still capable of showing us a new thing or two.

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