Neptune has both regular and irregular moons: Here's all about them
After Pluto was demoted to a dwarf planet, Neptune became Earth’s farthest neighbour in the solar system. While the rest of the planets stand out for their unique characteristics, Neptune has always piqued people’s interest, courtesy its 14 moons. These lunar bodies are divided into two categories: Regular moons and irregular moons. All of Neptune’s moons derive their respective names from Greek mythology. They are: Naiad, Thalassa, Despina, Galatea, Larissa, Hippocamp, Proteus, Triton, Nereid, Halimede, Sao, Laomedeia, Psamathe and Neso. This arrangement has been done keeping in mind their closeness to their planet.
Regular vs. irregular moons
As already mentioned, Neptune’s 14 moons are divided into two categories, regular moons and irregular moons. While the first seven moons namely Naiad, Thalassa, Despina, Galatea, Larissa, Hippocamp, and Proteus are known as regular or inner moons, the rest seven moons, aka, Triton, Nereid, Halimede, Sao, Laomedeia, Psamathe and Neso are considered to be irregular moons.
While the regular moons are elongated and have circular and prograde (moving in a forward direction) orbits along the equatorial plane of Neptune, the irregular moons have eccentric orbits that are not only distant from Neptune but also have retrograde (directed backwards) pathways. However, Triton is an exception. Although it is considered to be an irregular moon courtesy its inclined and retrograde orbit, it is still close to Neptune and has a circular pathway.
It is presumed that the regular moons are closely associated with Neptune’s five dusty rings and they often orbit around it faster than Neptune’s own rotation. As a result, it is expected that these moons will break apart or collide with Neptune, one by one, sooner or later. Scientists have also found out that the irregular moons, though far away, are vastly affected by Neptune’s centre of gravity and are ex situ.
Discovery of Neptune’s moons
Neptune’s largest moon Triton was also the first one to be discovered on 10th October 1846, just 17 days after Neptune’s own discovery. The discoverer was William Lassell. More than a century later in 1949, Neptune’s second moon Nereid was discovered by Gerard P. Kuiper. Decades later on 24th May 1981, Larissa was discovered accidentally by a group of astrophysicists, comprising of Harold J. Reitsema, Larry A. Lebofsky, William B. Hubbard and David J. Tholen. The next discovery of Neptune’s moons Naiad, Thalassa, Despine, Galatea, and Proteus happened in 1989 when Voyager 2 spacecraft was sent by NASA to explore the space. The same spacecraft was also responsible for discovering the rest of the moons (except Hippocamp) with the help of its ground-based telescopes throughout 2001. The 14th moon Hippocamp was discovered on 15th July 2013 by the Hubble Space Telescope.