On July 1, 2013, the Hubble Space Telescope spotted a previously undetected moon orbiting the planet Neptune. The moon, measuring only 19 kilometres (12 miles) across, was discovered by astronomer Mark Showalter of the SETI Institute using photographs taken by Hubble. Appearing as a moving white dot in the images, Showalter was able to pin down the moon after going through 150 images between the years 2004 and 2009. Showalter discovered that it orbited the planet Neptune every 23 hours at an altitude of 104,000 kilometres (65,400 miles).
The moon, currently designated S/20044 N1, is extremely far away and dim, being 100 million times fainter than the faintest star that can be seen by the human eye. This explains in part why the moon remained undetected for so long; that and because its orbit was beyond Neptune's small ring system, known as 'arcs'. The moon's orbit is between those of the moons Larissa and Proteus.
“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 … It’s the same reason a sports photographer tracks a running athlete—the athlete stays in focus, but the background blurs,” said SETI astronomer Mark Showalter in a NASA statement.
The origins of S/20044 N1 is still unknown at the moment, as are those of the various other smaller moons and bodies orbiting Neptune. However, it is believed that Triton, Neptune's largest moon, may play a role in it. It is often believed that Triton was actually a planetoid or similar object from the Kuiper Belt. The Kuiper Belt is a formation—similar to the asteroid belt but more massive—at the edge of the solar system, and composed primarily of frozen water, ammonia and methane. It is thought that Neptune's gravitational pull captured Triton, whose arrival may or may not have torn apart the existing moons and rings, creating Neptune's current set of orbiting bodies. The new moon discovery brings the total moon count of the planet Neptune to 14.
The location of the newly discovered moon compared to the others. (Hubblesite)
Neptune and Voyager 2
Neptune is the eighth and farthest planet in our solar system, having been bestowed the latter title after Pluto was demoted to planetoid in 2006. The planet is the fourth largest by diameter and third largest by mass (Uranus has less mass than Neptune despite being slightly bigger), and is the densest gaseous giant in our solar system. Like the other gaseous planets, Jupiter, Saturn, and Uranus, Neptune has no solid surface and is instead composed of massive volumes of gases with an incredibly hot and dense core. The planets, also called Jovian planets (after Jupiter, the first to be discovered), are many times the mass and size of terrestrial planets such as the Earth. For example, Neptune is 17 times the mass of the Earth, Uranus is 15 times, Saturn is 95 times, and Jupiter is a whopping 317 times more massive.
A scale model of the planets in our solar system. (NASA)