How Many Moons Does Neptune Have?

Neptune is the eighth planet from the sun, and was discovered in 1846. Neptune takes 164 years to orbit around the Sun once. Neptune is the furthest away planet in our solar system, and has 14 moons.

Neptune’s largest moon is called Triton, and is named after the Greek god of the ocean (Neptune is the name for the Roman god of the ocean). Triton was discovered by William Lassell, an English astronomer. Triton is a little bit smaller than Earth’s moon, and is covered in many active volcanoes. These volcanoes erupt freezing cold nitrogen all over Triton’s surface.

Triton is a particularly strange moon, as it has a retrograde orbit: this means that it orbits Neptune backwards (in the opposite direction to Neptune’s rotation). A retrograde orbit is a strong clue to scientists that Triton was captured by Neptune’s gravitational pull. One theory is that Triton hit Neptune, and lost all of its momentum when it bounced off; another theory is that maybe Triton smashed into one of Neptune’s other moons, and then couldn’t escape Neptune’s gravity.

Neptune’s second moon to be discovered is called Nereid. It was found in 1949 by Gerard Kuiper, an American astronomer who was born in the Netherlands. In Greek mythology, the Nereids are sea nymphs, and are helpful to sailors that are fighting dangerous storms. Nereid is the outermost of Neptune’s moons. It reflects a reasonable proportion (14%) of the sunlight that strikes it, which makes it slightly brighter than Earth’s moon. Nereid has the most eccentric orbit of any moon in the solar system – sometimes it is as far away as 9.66 million km from Neptune, while at other times it is as close as 1.37 million km.

In 1981, a third moon was discovered, named Larissa. Unfortunately, soon after discovering Larissa, astronomers lost it! Larissa is named after one of the Nereids. Larissa is a small moon, and is irregularly shaped and covered in craters. Larissa’s orbit is slowly spiralling inwards towards Neptune – this means that Larissa may eventually crash into Neptune’s atmosphere, or Neptune’s tidal forces may pull Larissa apart! Larissa orbits Neptune once every 13 hours and 20 minutes.

In 1989, Voyager 2 observed Neptune, passing close to 5000 km above Neptune’s north pole. When Voyager 2 flew past Neptune, it saw Larissa again. At this point, Larissa was deemed to be “officially” discovered. Voyager 2 also discovered that Neptune had 5 additional moons. These were named Naiad, Thalassa, Despina, Galatea, and Proteus. Astronomers then discovered 5 more moons in 2002 and 2003, and named them Halimede, Sao, Psamathe, Laomedeia, and Neso. Many of these other moons are also named after sea nymphs (Nereids), while others are named after other Greek river or sea gods and goddesses.

In July 2013, it was discovered that Neptune had another moon, bringing the total count of Neptune’s moons to 14. This moon was discovered by Mark Schowalter, a Senior Research Scientist at the SETI Institute (Search for Extraterrestrial Intelligence). This moon is so new, that it doesn’t even have a proper name yet – instead, it is called S/2004 N 1. This new moon is tiny when compared to Triton – while Triton is 2705 km in diameter, S/2004 N 1 is only 20km in diameter! It orbits around Neptune once every 23 hours. Voyager 2 remains the only spacecraft to have visited Neptune, but perhaps if more are sent up, more moons will be discovered!