A tiny, previously unknown moon circling Neptune has been spotted by astronomers using the Hubble telescope.

The moon, which is currently known as S/2004 N1, was found on July 1 by Mark Showalter of the SETI Institute in Mountain View, Calif., NASA announced Monday.

It is less than 20 kilometres wide and its orbit is 105,000 kilometres from Neptune, between those of Larissa and Proteus, two of Neptune's other 14 known moons. It circles Neptune once every 23 hours.

The tiny moon is so small and dim that it was even missed by NASA's Voyager 2 spacecraft when it flew by and observed Neptune, its moons and its rings in 1989.

According to NASA, the moon is roughly 100 million times fainter than the faintest star that can be seen with the naked eye. Showalter noticed the moon as a faint white dot while tracking the planet's fast-moving ring segments, which are located between other moons, between 42,000 and 63,000 kilometres from Neptune.

"We had to devise a way to follow their motion in order to bring out the details of the system," Showalter said in a statement.

"It's the same reason a sports photographer tracks a running athlete — the athlete stays in focus, but the background blurs." After realizing what he had found, Showalter located the same white dot in Hubble images taken between 2004 and 2009, allowing him to figure out the moon's orbit.

Showalter plans to write a formal report about the discovery for publication in a scientific journal, but told CBC News in an email that "it will probably take a few months."

Neptune is officially the eighth and furthest planet from our sun, located 30 times further away than the Earth. It is a frigid gas giant 17 times the mass of the Earth.