Discovery

Neptune is too far away to see with the naked eye. So how did we discover it?
After 20 years of calculations, 2 different teams of astronomers made a pretty good guess based on some unusual movement from Uranus.
In 1845 Johann Gottfried Galle used these calculations and found a bright, blue spot nearly in the exact location predicted, making Neptune the ONLY planet to be found on purpose.
After some debate, the planet was named after the Roman God of the sea due to its rich, blue color and keeping with the other planets bearing Roman names.

Did you know that Neptune was the only planet found entirely on purpose? In the early 19th century, after the planet that was to be called Uranus was discovered by accident, astronomers were still grappling with discrepancies in its orbit. It wasn’t moving like it was supposed to! It’s very difficult to study the mechanics of something so far away, and so for twenty years, starting in 1821, astronomers came up with equations and tables to explain the movements of Uranus. The problem is that Neptune is too far away to see, so they had to guess. In 1845, two separate astronomers made guesses that were nearly identical. British mathematician John Couch Adams and French mathematician Urbain Le Verrier sent these guesses to astronomers with powerful telescopes to look out into space and see if there was anything there. German astronomer Johann Gottfried Galle took Le Verrier’s equations and found a spot within 1 degree of the predicted area! That spot turned out to be the planet that we now call Neptune. This means that Neptune is the only planet found entirely through mathematical prediction!

These astronomers, as well as the countries they were from, fought harshly over the right to name the planet. Because there was no global consensus on Uranus’ name either, it got thrown into the fight. Galle, who discovered the planet, proposed Janus, but Le Verrier wanted it named after him, with Uranus named Herschel after its discoverer. While France supported this most other countries were not very fond of the choice. Several different people proposed Neptune as a name, both for the planet’s rich blue color as well as continuing with the nomenclature used for the rest of the solar system- Roman mythology. Neptune is the Roman god of the seas, fitting for this cold, icy, stormy planet. In fact, most countries and cultures use the name of the god or mythological ruler of the seas as the title of our most distant sibling.

Exploration

So how do we know anything at all about a planet so far away?
No one has ever traveled to Neptune, but Voyager 2, launched in 1977, spent a whole day taking pictures and measurements, unveiling a host of new information including unseen moons and viscous weather systems.
The Hubble Space Telescope was able to zoom in, giving us even better clarity and understanding of this distant neighbor in our Solar System.
The Webb Telescope (launched 12/25/21) is giving us even deeper insights to the make-up of the planet and its moons, pointing us to the mysteries of our own origins.

What we know of Neptune comes almost entirely from the interstellar equivalent of a fast-food drive-through window. Despite its name, Voyager 2 was launched sixteen days before its twin on August 20, 1977, and after spending 12 years in space photographing Jupiter, Saturn, and Uranus it reached Neptune with just enough time for a single days worth of pictures and tests, including a gravity assisted flyby of Triton, its largest moon.

The closest Voyager got to the planet was 3000 miles away from its cloud tops. In just a few hours it determined that Neptune had a weather system far more active than expected, and with winds that whip by at speeds higher than 1,200 miles an hour, it is the windiest locale in the solar system. It also discovered six new moons (Despina, Galatea, Larissa, Proteus, Naiad and Thalassa) and four new rings! Talk about a productive day! It also discovered a swirling storm large enough to swallow the entire Earth, that scientists called the Great Dark Spot.

WHAT WE KNOW

Neptune is the densest of the gas giants, made up of mostly fluid forms of water, methane, and ammonia.
Neptune’s core is solid and may have a super hot ocean of water, held in place due to the enormous pressures in the atmosphere.
Neptune tilts at 28 degrees, giving it seasons like Earth and Mars. However, each season lasts over 40 Earth-years!
Triton is the only moon in the solar system that orbits in the opposite direction of its planet’s spin. This ‘retrograde motion’ creates many questions that scientists hope to answer through more study.

So how do we know anything at all about a planet so far away?

No one has ever traveled to Neptune, but Voyager 2, launched in 1977, spent a whole day taking pictures and measurements, unveiling a host of new information including unseen moons and viscous weather systems.The Hubble Space Telescope was able to zoom in, giving us even better clarity and understanding of this distant neighbor in our Solar System.

The Webb Telescope (launched 12/25/21) is giving us even deeper insights to the make-up of the planet and its moons, pointing us to the mysteries of our own origins.

What we know of Neptune comes almost entirely from the interstellar equivalent of a fast-food drive-through window. Despite its name, Voyager 2 was launched sixteen days before its twin on August 20, 1977, and after spending 12 years in space photographing Jupiter, Saturn, and Uranus it reached Neptune with just enough time for a single days worth of pictures and tests, including a gravity assisted flyby of Triton, its largest moon.

The closest Voyager got to the planet was 3000 miles away from its cloud tops. In just a few hours it determined that Neptune had a weather system far more active than expected, and with winds that whip by at speeds higher than 1,200 miles an hour, it is the windiest locale in the solar system. It also discovered six new moons (Despina, Galatea, Larissa, Proteus, Naiad and Thalassa) and four new rings! Talk about a productive day! It also discovered a swirling storm large enough to swallow the entire Earth, that scientists called the Great Dark Spot.

ICY STRUCTURE

Most of Neptune’s mass is made up of a hot dense fluid of "icy" materials – water, methane, and ammonia – above a small, rocky core. Of the giant planets, Neptune is the densest. It is one of our Solar System’s two “Ice Giants” along with Uranus.
Scientists think there might be an ocean of super hot water under Neptune's cold clouds. It does not boil away because incredibly high pressure keeps it locked inside.

28.3° AXIAL TILT

Neptune has a tilt of 28 degrees, which is similar to Earth and Mars. This means it has seasons like Earth, but because of its long orbit (165 Earth years) each season lasts over forty years.

While Neptune is the most distant planet from the Sun, dwarf planet Pluto has such an eccentric orbit that every 250 years it comes closer to the sun than Neptune for 20 years.

14 MOONS

There are 14 known moons orbiting Neptune. Its largest moon, Triton, was discovered on October 10, 1846, by William Lassell, just 17 days after Galle discovered the planet. Because Neptune was named for the Roman god of the sea, its moons are named for various lesser sea gods and nymphs in Greek mythology.

Triton is the only large moon in the solar system that has a retrograde orbit (that means it circles its planet in a direction opposite to the rotation) which makes scientists think that it may have been an independent object that Neptune captured. Triton is extremely cold, with surface temperatures around -391 degrees Fahrenheit. Despite the cold, Voyager 2 discovered geysers spewing icy material upward more than 5 miles. Triton's has a thin atmosphere, also discovered by Voyager, and it is growing warmer, with no known explanation.

Last updated February 23, 2023. Content written by Trevor Macduff.