One year on Neptune lasts about 165 Earth years.

Neptune is the farthest known major planet, about 30 times Earth's distance from the Sun, or roughly 4.5 billion kilometers. Planets farther out move more slowly in their orbits and have much longer paths to travel. Combine the two and Neptune needs about 164.8 Earth years to go around once.

That leads to a neat milestone. The planet was first seen through a telescope on 23 September 1846. In July 2011, it returned to roughly the same position in its orbit, completing its first Neptunian year since humans knew it existed. Nobody alive at its discovery could have seen that anniversary.

Neptune's axis is tilted, like Earth's, so it has seasons, but each one lasts about four decades.

Neptune was found by mathematics before anyone recognised it in a telescope. Astronomers noticed that Uranus was drifting slightly from its predicted path, as though pulled by an unseen planet. The French mathematician Urbain Le Verrier calculated where that planet should be, and in 1846 Johann Galle at the Berlin Observatory found it within about one degree of his prediction.

Only one spacecraft has visited Neptune. Voyager 2 flew past in August 1989, revealing a dynamic, deep blue world with a giant storm called the Great Dark Spot and winds faster than on any other planet, reaching around 2,000 kilometers per hour. It also photographed Triton, Neptune's largest moon, with geysers of nitrogen gas on its icy surface.

Because each Neptunian season lasts decades, astronomers have observed only part of one full seasonal cycle. Telescopes such as Hubble and Webb now track changes in its clouds and brightness year by year.

Some studies have linked changes in Neptune's cloud cover to the Sun's 11 year activity cycle, a sign that even distant planets respond to the Sun.

Orbital periods grow rapidly with distance. Mercury, closest to the Sun, completes a year in just 88 days. Earth takes 365 days, Jupiter about 12 years, Saturn about 29 and Uranus about 84. Neptune's 165 years tops them all among the planets, and distant dwarf planets such as Pluto take even longer, about 248 years.

This relationship was first described by Johannes Kepler in the early 1600s. His third law links a planet's orbital period to its distance from the Sun, and Neptune fits it beautifully.

To picture the scale, imagine someone born on Neptune's discovery day in 1846. They would have needed to live to 165 just to celebrate a first Neptunian birthday in 2011. In that time on Earth, people went from horse drawn carriages to smartphones, crewed Moon landings and a spacecraft visit to Neptune itself. Pluto, which was once counted as a planet, has not yet completed even one orbit since it was discovered in 1930.