Neptune – The Real Blue Planet

Photograph of Neptune in true colour by Voyager 2 in 1989.
Photograph of Neptune in true colour by Voyager 2 in 1989. Credit: NASA
Share this:

Neptune, the icy-giant in our Solar System is a wonderful blue world with supersonic winds, fourteen moons and even sporting wispy rings.

 

 

The Solar System of which our planet- Earth- is a part is a real wonder. It is not particularly special in terms of the physical properties of the cosmic bodies within it. All the planets orbit a non-descript, average star in a non-descript arm of the Milky Way Galaxy.

But our Solar System is special because on one planet, life abounds and one creature – us – has obtained enough consciousness to look up at the sky and wonder, what is out there?

We know that there are eight planets that orbit our star, as well as countless rocky asteroids and icy comets that circle the Sun from very far away.

The nearest planets to the Sun are the rocky ones (Mercury, Venus, Earth, and Mars), followed by Jupiter and Saturn, the gas-giants. And then we have the ice-giants that almost touch the Kuiper Belt (that holds the icy-comets mentioned earlier).

One of these ice-giants is Neptune and the other Uranus. Neptune is the eight and farthest planet from the Sun and the fourth largest in the Solar System. This world, whipped by supersonic winds, is the only planet in the solar system not visible to the naked eye.

This dark, cold world is almost 30 times as far from the Sun as Earth and this means that its orbit is very long. So much so that it was only in 2011 that Neptune completed its first 165-year orbit since its discovery in 1846. And because of this distance, high noon on Neptune would seem like dim twilight to us. The warm light we see here on Earth is roughly 900 times as bright as sunlight on Neptune.

Photograph of Neptune in true colour by Voyager 2 in 1989.

Photograph of Neptune in true colour by Voyager 2 in 1989. Credit: NASA

Discovery

Neptune’s existence was first predicted by two mathematicians, Urbain Le Verrier in France and John Couch Adams in England. By analysing discrepancies in the orbit of Uranus, they independently calculated the position of a yet-to-be-discovered planet. These calculations led German astronomer Johann Gottfried Galle to turn his telescope towards the predicted region and, on September 23, 1846, he spotted the elusive Neptune.

However, centuries before Neptune’s official discovery, Galileo’s meticulous telescope observations of December 28, 1612 and January 27, 1613, unknowingly documented the planet’s position, though he believed it to be a fixed star near Jupiter, which is why Galileo is not credited with its discovery.

Neptune’s Namesake: A Dive into Mythology

Roman mythology provided the inspiration for Neptune’s name, as suggested by Le Verrier. The Roman god of the sea, Neptune, was the counterpart of the Greek god Poseidon. This choice was fitting, considering the planet’s deep blue hue and immense distance, reminiscent of the vast and mysterious ocean depths. It was a symbolic nod to the unknown, a world waiting to be explored.

Size and Distance: A Planet of Titanic Proportions

Neptune is a true giant, boasting a diameter nearly four times that of Earth. With a mass 17 times Earth’s, it ranks as the fourth-largest planet in our solar system and the third-most massive. Despite its sheer size, Neptune’s distance from the Sun makes it a lonely outpost. At an average distance of 4.5 billion km, it receives only about 40% of the sunlight that reaches Earth. This translates to a frigid surface temperature of around -214°C, making it one of the coldest planets in our solar system.

A size comparison of Neptune and Earth. Credit: NASA

Orbit and Rotation: A Whirling Dance in the Darkness

Neptune’s journey around the Sun is a slow one. It takes the planet a whopping 165 Earth years to complete a single orbit, meaning a year on Neptune is the equivalent of 165 years on Earth! Its rotation is equally leisurely, with a single day on Neptune lasting about 16 Earth hours. This sluggish spin contributes to the planet’s distinctive banded appearance, as atmospheric features have ample time to develop and persist.

Neptune (red arc) completes one orbit around the Sun (centre) for every 164.79 orbits of Earth. The light blue object represents Uranus.

A Symphony of Moons

Neptune isn’t alone in its cosmic dance. It is accompanied by a retinue of 14 known moons, each named after figures from Greek and Roman mythology. The largest and most famous of these is Triton, a frozen world that stands out for its retrograde orbit, meaning it orbits Neptune in the opposite direction of the planet’s rotation. This suggests that it may once have been an independent object that Neptune captured. Triton is also the only moon in our solar system known to have active geysers erupting with ice and nitrogen, which were spotted by the Voyager 2 spacecraft, the only human-made spacecraft to reach Neptune. Triton was discovered on Oct. 10, 1846, by William Lassell, just 17 days after Johann Gottfried Galle discovered the planet.

An annotated picture of Neptune's many moons as captured by the James Webb Space Telescope. The bright blue diffraction star is Triton, Neptune's largest moon.

An annotated picture of Neptune’s many moons as captured by the James Webb Space Telescope. The bright blue diffraction star is Triton, Neptune’s largest moon. Credit: NASA

 

A time-lapse video of Neptune and its moons

Ringing in the Mysteries

Adding to Neptune’s allure are its faint, dusty rings. Unlike the bright and prominent rings of Saturn, Neptune’s rings are wispy and barely visible even from powerful telescopes. They are composed of microscopic particles of ice and dust, and their existence hints at a complex and dynamic interplay of forces within the Neptunian system.

This Voyager 2 image, taken in 1989, was the first to show Neptune's rings in detail. Credit: NASA/JPL

This Voyager 2 image, taken in 1989, was the first to show Neptune’s rings in detail. Credit: NASA/JPL

Neptune's rings and moons viewed in infrared by the James Webb Space Telescope

Neptune’s rings and moons viewed in infrared by the James Webb Space Telescope. Credit: NASA

Birth of an ice-giant

Neptune was born 4.5 billion years ago, coalescing from swirling dust and gas, at which time it was likely closer to the Sun than its current icy home – just like Uranus. Over time, it migrated outward, becoming the distant king of the solar system’s cold realm.

As a layered marvel, ighty percent or more of the planet’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. And under that frozen surface, scientists suspect an ocean of super-hot water, which does not boil away because incredibly high pressure keeps it locked inside.

Four images taken a few hours apart with the NASA/ESA Hubble Space Telescope's Wide Field Camera 3. Near infrared radiation data has been used as red channel.

Four images taken a few hours apart with the NASA/ESA Hubble Space Telescope’s Wide Field Camera 3. Near infrared radiation data has been used as red channel. Credit: NASA

Neptune has no solid surface. Its gas-rich atmosphere (made up mostly of hydrogen, helium, and methane) extends to great depths, and blends seamlessly into deeper layers of melted ices, eventually reaching a rocky core as massive as Earth.

Neptune – cloud cover over three decades (1994-2023). False color image based on data from WFPC2 and WFC3 instruments of the Hubble Space Telescope. Credit: NASA

Neptune – cloud cover over three decades (1994-2023). False color image based on data from WFPC2 and WFC3 instruments of the Hubble Space Telescope. Credit: NASA

 

Physical and chemical composition of Neptune's interior

Physical and chemical composition of Neptune’s interior. Credit: NASA

The air on this ice planet is a hydrogen-helium cocktail with a little bit of methane. This methane gives Uranus its blue-green hue, but Neptune’s secret ingredient paints a deeper, more vibrant blue. What is it? We do not know!

Combined colour and near-infrared image of Neptune, showing bands of methane in its atmosphere, and four of its moons, Proteus, Larissa, Galatea, and Despina

Combined colour and near-infrared image of Neptune, showing bands of methane in its atmosphere, and four of its moons, Proteus, Larissa, Galatea, and Despina. Credit: NASA

Neptune is our solar system’s windiest world. Even though it is so far away from the Sun, its winds reach hurricane-like speeds, three times stronger than Jupiter’s and a staggering nine times stronger than Earth’s mightiest gusts. These super storms fling frozen methane clouds across the planet at supersonic speeds (more than 1,200 miles per hour or 2,000 kilometres per hour). To compare, Earth’s most powerful winds hit only about 250 miles per hour (400 km per hour).

Bands of high-altitude clouds cast shadows on Neptune's lower cloud deck. The colour is exaggerated to show the clouds more clearly.

Bands of high-altitude clouds cast shadows on Neptune’s lower cloud deck. The colour is exaggerated to show the clouds more clearly. Credit: NASA

In 1989, Neptune’s southern hemisphere housed a colossal storm, the “Great Dark Spot,” big enough to swallow Earth whole. This soon vanished, leaving behind a legacy of ever-evolving weather patterns but new storms have appeared on different parts of the planet.

The Great Dark Spot in an enhanced colour image by Voyager 2.

The Great Dark Spot in an enhanced colour image by Voyager 2. Credit: NASA

The Northern Great Dark Spot and a smaller companion storm imaged by Hubble in 2020

The Northern Great Dark Spot and a smaller companion storm imaged by Hubble in 2020. Credit: NASA

The appearance of a Northern Great Dark Spot in 2018 is evidence of a huge storm brewing.

The appearance of a Northern Great Dark Spot in 2018 is evidence of a huge storm brewing. Credit: NASA

The Great Dark Spot (top), Scooter (middle white cloud),[97] and the Small Dark Spot (bottom), with contrast exaggerated.

The Great Dark Spot (top), Scooter (middle white cloud), and the Small Dark Spot (bottom), with contrast exaggerated. Credit: NASA

Unlike Earth, Neptune’s magnetic field isn’t neatly aligned with its rotation (tipped over by about 47 degrees compared with the planet’s rotation axis). This tilt creates a dynamic and complex magnetosphere, 27 times stronger than Earth’s, forever in flux as the planet spins. Like Uranus, whose magnetic axis is tilted about 60 degrees from the axis of rotation, Neptune’s magnetosphere undergoes wild variations during each rotation because of this misalignment.

The study of this icey, windswept world continues to unravel the secrets of the outer solar system. Spacecraft like Voyager 2 have provided invaluable data and stunning images of the planet. The advent of the Hubble Space Telescope, large ground-based telescopes with adaptive optics and now the James Webb Space Telescope has allowed for additional detailed observations from afar.

Neptune's shrinking vortex

Neptune’s shrinking vortex. Credit: NASA

But much remains unknown. From the source of its internal heat to the possibility of hidden oceans beneath its icy surface, Neptune beckons us to solve its mysteries.

All images by NASA.

 

 

 

Become a Patron!

 

Buy me a coffee

 

I am a Chartered Environmentalist from the Royal Society for the Environment, UK and co-owner of DoLocal Digital Marketing Agency Ltd, with a Master of Environmental Management from Yale University, an MBA in Finance, and a Bachelor of Science in Physics and Mathematics. I am passionate about science, history and environment and love to create content on these topics.

Free Email Updates
We respect your privacy.