Planet Neptune: Explore the Farthest Planet From the Sun!

~9 min

Neptune is known as the eighth and farthest planet from the Sun, which can't be seen with the naked eye from the Earth. This distant blue planet has faint rings, numerous moons, and no solid surface. In this article, we'll tell you more interesting details and curious facts about Neptune. Let's get started!


Neptune facts

  • Planet type: ice giant
  • Radius: 24,622 km (15,299.4 miles)
  • Mass: 1.02413×10^26 kg
  • Aphelion: 4.536 billion km (2.819 billion miles)
  • Perihelion: 4.459 billion km (2.771 billion miles)
  • Average distance from the Earth: 4.5 billion km (2.8 billion miles)
  • Surface temperature: −218 °C to −200 °C (−360 °F to −328 °F)
  • Solar day length: 0.6713 Earth day
  • Sidereal day length: 0.67125 Earth day
  • Year length: 164.8 Earth years
  • Age: 4.503 billion years
  • Named after: Roman god of the sea

When was Neptune discovered?

Neptune was the first planet discovered through mathematical calculations. One of the earliest recorded observations of the ice giant was made by Galileo Galilei, who spotted the planet with his primitive telescope in 1612-1613; however, the astronomer seems to have mistaken Neptune for a star.

In 1846, John Couch Adams, a British mathematician and astronomer, determined the position of Neptune, using only mathematics. Around the same time, the French astronomer Urbain Le Verrier calculated the planet's location independently of Adams. Le Verrier communicated his findings to the German astronomer Johann Gottfried Galle, who identified Neptune in the Berlin Observatory on September 23, 1846, increasing the number of known planets to eight.

How big is Neptune?

As the outermost planet from the Sun in our Solar System, Neptune has a very small apparent size and is challenging to observe from the Earth. Fortunately, the scientific data allow us to imagine the size of this remote planet.

Neptune's size

With a radius of 24,622 km (15,299.4 miles), Neptune is the fourth-largest planet in the Solar System and the smallest of gas giants. Its surface area covers 7.6 billion km^2 (2.9 billion mi^2), while to take a trip around the equator of the blue planet, you would have to cover a distance of 154,705 km (96,129 miles).

How many Earths fit in Neptune?

Neptune's radius is about four times that of our planet. If the Earth were the size of a coin, the ice giant would be as big as a baseball. Also, being the Solar System's third most massive planet, Neptune is more than seventeen times as massive as the Earth. Neptune's volume is about 57 times the volume of our planet: 57 Earths could fit inside the ice giant!

Neptune's orbit and rotation

Neptune is the eighth and farthest planet from the Sun. This distance creates the longest orbit of the eight planets. But unlike a Neptunian year, a day on Neptune is relatively short.

How long is a year on Neptune?

Neptune takes about 165 Earth years to complete one orbit around the Sun. As the axial tilt of Neptune is similar to those of Mars and our planet, the ice giant has seasons just like we experience on the Earth; each season lasts for about 40 years.

How long is a day on Neptune?

Neptune rotates faster than the Earth: one average Neptunian day lasts about 16 Earth hours. However, as the ice giant isn't a single solid object, its different parts rotate at different speeds. Neptune's equatorial zone takes about 18 hours to spin once, while the polar regions take about 12 hours to complete a rotation.

How far is Neptune?

As we've already mentioned above, Neptune is the farthest planet from the Sun. Sometimes the ice giant is even farther from our star than the dwarf planet Pluto!

How far is Neptune from the Sun?

Neptune lies at an average distance of 30 astronomical units or 4.5 billion km (2.8 billion miles) away from the Sun. However, sometimes the planet gets even farther than Pluto, whose highly eccentric orbit brings it inside Neptune's orbit for 20 years every 248 Earth years. The last time this switch happened was in 1979 and lasted until 1999.

How far is Neptune from Earth?

As Neptune and the Earth move through space, the distance between them is constantly shifting. When the planets are closest to each other, they lie at a distance of 4.3 billion km (2.7 billion miles). At its farthest, Neptune lies 4.7 billion (2.9 billion miles) km away from the Earth. Because of its extreme distance from our planet, Neptune became the last planet of the Solar System to be discovered.

How long does it take to get to Neptune?

The length of a trip to a planet depends on the planet's position and the spacecraft's route and speed. The only spacecraft to visit Neptune, Voyager 2, took a dozen years to reach the ice giant. NASA's New Horizons spacecraft passed through Neptune's orbit on its way to Pluto after eight years of travel.

Missions to Neptune

Only one spacecraft, Voyager 2, has visited Neptune. This space probe was launched in 1977 to study outer planets. Having visited Jupiter, Saturn, and Uranus, it headed towards Neptune. Voyager 2 reached the blue planet in August 1989, passing about 4,800 km (2,983 miles) above its north pole. The spacecraft studied Neptune's atmosphere, magnetosphere, rings, and moons and took amazing images of the ice giant. At the moment, there are no approved future missions to visit this distant planet.

What is Neptune made of?

Along with Uranus, Neptune is one of two ice giants in our Solar System. Also, it's the densest of all the gas giants.

Formation of Neptune

Like the rest of the Solar System's planets, Neptune formed about 4.5 billion years ago. According to scientists, the blue planet formed closer to the Sun than it is now and settled into its current position in the outer Solar System about 4 billion years ago.

Neptune's structure

At the heart of the planet, there is a solid core made of silicates, nickel, and iron, which is approximately 1.2 times the size of the Earth. Neptune's core is surrounded by a hot fluid of “icy” materials such as water, methane, and ammonia, which, in its turn, is covered by a layer of clouds.

What is the surface of Neptune like?

The ice giant does not have a solid surface. The Neptunian atmosphere is made up predominantly of hydrogen and helium with a trace of methane. Neptune is the windiest planet in the Solar System: the winds reach speeds of about 2,100 km/h (1,300 mi/h). As Neptune lies at a great distance from the Sun, its outer atmosphere is one of the coldest places in the Solar System.

Neptune's Great Dark Spot

Neptune's Great Dark Spot was a huge storm in the southern hemisphere of the planet at the time of the Voyager 2 flyby in 1989. The winds in the storm were the strongest ever recorded on any Solar System's planet. By 1994, the Great Dark Spot had disappeared completely; however, a very similar spot appeared in Neptune's northern hemisphere in 2016.

Neptune's moons

Like the other giant planets, Neptune has a large satellite system. All the moons of the ice giant were named after minor water deities in Greek and Roman mythology.

How many moons does Neptune have?

Neptune has 14 known moons. The first Neptune's moon to be discovered was Triton: it was first observed by William Lassell seventeen days after the discovery of the blue planet in 1846. One more Neptune's natural satellite was discovered no sooner than 1949 by Gerard P. Kuiper, who named this moon Nereid. The third moon to be found was Larissa, first observed in 1981 by a group of astronomers. About a decade later, in 1989, Voyager 2 confirmed the discovery of Larissa and spotted five inner moons: Naiad, Thalassa, Despina, Galatea, and Proteus. In 2001, five outer moons — Halimede, Sao, Psamathe, Laomedeia, and Neso — were found. A small moon named Hippocamp was discovered in 2013.

What is Neptune's largest moon?

With a diameter of 2,700 km (1,680 miles), Triton is the largest moon of Neptune and the seventh-largest moon in the Solar System. It makes up more than 99.5% of all the mass in orbit around Neptune, including the other known Neptunian moons and the planet's rings. As it shares many similarities with Pluto, Triton is thought to be an independent object (probably, a dwarf planet), captured by Neptune's gravity from the Kuiper belt.

Neptune's rings

Like other gas giants — Jupiter, Saturn, and Uranus — Neptune can boast a ring system. The rings of the outermost major planet were discovered in 1984 and eventually imaged by the Voyager 2 in 1989.

How many rings does Neptune have?

Neptune has five main rings named after the astronomers who studied this blue planet: Galle, Leverrier, Lassell, Arago, and Adams. Also, in the outermost ring, Adams, there are four prominent clumps of dust known as arcs: Fraternité, Égalité, Liberté, and Courage. Despite the laws of motion predicting that the arcs should distribute the material uniformly throughout the rings, they are stable structures; scientists believe the arcs might be stabilized by the gravitational effects of Neptune's moon Galatea.

What are the rings of Neptune made of?

Neptune's rings are dark, reddish, and variable in size and density. Most of them are tenuous and faint. Scientists do not understand the composition of the rings of Neptune in detail; they probably consist of ice and radiation-processed organic compounds. The rings are thought to be relatively young and might have been formed by Neptune's destroyed moon.

Upcoming events

July 25: Neptune near the Moon; lunar occultation of Neptune

  • Occultation start: 12:30 GMT (8:30 a.m. EDT)
  • Occultation end: 16:22 GMT
  • Close approach time: 14:31 GMT (10:31 a.m. EDT)
  • Close approach distance: 0°30'

On July 25, the 82%-illuminated Moon and Neptune (mag 7.9) will meet in the constellation Pisces. The planet will rise at about midnight and will be visible in binoculars. Also, a lunar occultation of Neptune will be observable over the Pacific Ocean and North America.

August 21: Neptune near the Moon; lunar occultation of Neptune

  • Occultation start: 19:59 GMT (3:59 p.m. EDT)
  • Occultation end: 23:37 GMT
  • Close approach time: 21:54 GMT (5:54 p.m. EDT)
  • Close approach distance: 0°37'

On August 21, the 97%-illuminated Moon and Neptune (mag 7.9) will meet in the constellation Pisces. The planet will rise in the evening and will be visible in binoculars. Also, a lunar occultation of Neptune will be observable over Africa, Europe, and Asia.

September 21, 2024: Neptune opposition

Neptune will reach opposition on September 21, 2024, at 00:08 GMT (September 20, 08:08 p.m. EDT). The planet will shine with a magnitude of 7.8 in the constellation Pisces. You can spot Neptune right after sunset, rising in the opposite direction of the Sun. It will reach its highest point around midnight and remain visible until dawn. However, even at its brightest, Neptune may be challenging to see without a telescope. So, grab one if you want to enjoy a clear view of this distant planet.

December 8: Neptune ends retrograde motion

On December 8, Neptune will end its retrograde motion and start moving prograde. Neptune goes retrograde about every 12 months; its retrograde period lasts about 5 months. Learn more about retrograde motion from our dedicated article.

March 19, 2025: Neptune at solar conjunction

On March 19, Neptune will be at its closest point to the Sun in the sky. The planet will be only 1°15' from the Sun, which means that Neptune will be completely unobservable for several weeks, lost in the Sun's glare. In astronomy, solar conjunction is a celestial configuration in which an object has an elongation of nearly 0°, meaning it is at the same celestial longitude as the Sun. Learn more about this phenomenon in our dedicated article.

July 4, 2025: Neptune enters retrograde motion

On July 4, Neptune will begin its retrograde motion, which means it will appear to be moving “backward” in the sky. The planet’s retrograde will last until December 10, when it will come back to its normal (prograde) motion. Neptune goes retrograde about every 12 months; its retrograde period lasts about 5 months. Learn more about retrograde motion from our dedicated article.

December 10, 2025: Neptune ends retrograde motion

On December 10, Neptune will end its retrograde motion and start moving prograde. Neptune goes retrograde about every 12 months; its retrograde period lasts about 5 months. Learn more about retrograde motion from our dedicated article.


What color is Neptune?

Neptune's methane atmosphere absorbs the red light from the Sun and reflects the blue light into space. As a result, the planet has its vivid blue color and cool, calm veneer.

How cold is Neptune?

Lying at a great distance from the Sun, Neptune is one of the coldest places in the Solar System. The average temperature on Neptune is about −214 °C(−353 °F). The ice giant's hottest place is its south pole, where the temperature is about −200 °C(−328 °F).

Why won't Pluto collide with Neptune?

Pluto's orbit doesn't lie in the same plane as the eight planets; the orbits of the dwarf planet and Neptune are steeply inclined to one another. Also, Neptune and Pluto are in a 3:2 orbital resonance, which prevents the planets from coming close to each other.

How did Neptune get its name?

Shortly after the discovery, Le Verrier proposed the name Neptune for the new planet. However, later he attempted to name the ice giant Le Verrier, after himself, which met with disapproval outside France. In December 1846, Friedrich Georg Wilhelm von Struve, the director of Pulkovo Observatory in Saint Petersburg, came out for the name Neptune, and soon it became internationally accepted.

Did you know?

  • Neptune's surface gravity is the second-largest in the Solar System, surpassed only by Jupiter.
  • Neptune's largest moon, Triton, is the only large natural satellite in the Solar System that follows a retrograde orbit, moving in the opposite direction of its planet's rotation.
  • Triton is one of the few geologically active natural satellites in our Solar System: it has active geysers erupting sublimated nitrogen gas.
  • In 2011, Neptune completed its first 165-year orbit since its discovery in 1846.
  • Neptune cannot support life as we know it.

We hope that you’ve discovered something interesting and new about Neptune, the eighth and farthest planet from the Sun. If you liked the article, share it on social media and watch our cartoon about Neptune.

We wish you clear skies and happy observations!