Astronomy: Six Amazing Facts About Neptune and Its Moons

Lying at the very outskirts of our solar system, the ice giant Neptune has long represented both a challenge and a mystery to astronomers and space explorers alike.

The dark and cold planet, the furthest from our sun, is regularly belted by supersonic winds and has thus far been visited by just one human spacecraft.

Discovered using predictions made by Urbain Le Verrier and Johann Galle in September 1846, it was Le Verrier who suggested Neptune should be named after the Roman god of the sea.

In 1989, as it made its way out of the solar system, the Voyager 2 craft paid a flying visit to the ice giant, collecting both images and important data about this inhospitable world.

Here are six incredible facts about Neptune, its moons, and its rings.

1. How Far Is Neptune From The Sun and Earth?

The eighth planet in the solar system, Neptune is on average about 2.8 billion miles from our sun. That’s around 30 times the average distance from Earth to the sun.

It takes sunlight about 4 hours to travel to the ice giant, which takes 16 hours to rotate, and the planet is so dark that high noon on its icy surface would look like twilight here on Earth.

In fact, Neptune is so distant that it takes 165 earth years to complete one orbit of the sun. Since its discovery in 1846, we’ve seen it complete just one orbit, back in 2011. Occasionally Neptune’s orbit even takes it further away from the sun than dwarf planet Pluto.

Traveling at an average velocity of 42,000mph and launched in 1977, it took Voyager 2 around 12 years to travel from Earth to Neptune. It’s difficult to say how distant Neptune and Earth are, as they both follow their own elliptical orbit around the sun.

On February 18, there were approximately 2,866 million miles between the two planets.

2. How Big Is Neptune?

Neptune really puts the "giant" into "ice giant." The planet has a radius of about 15,300 miles and is four times wider than Earth, with 17 times its mass.

NASA says that if Earth was the size of a nickel then Neptune would be about as big as a basketball. This makes Neptune the fourth-largest planet in the solar system after Jupiter, Saturn and Uranus.

Neptune’s orbit and size aren’t the only monstrous things about it. Just like Earth, the ice giant’s axial tilt of 28 degrees means that it experiences seasons. But, because of the size of its orbit, each season lasts around 40 years.

3. What is Neptune Made Of And Why Is It Blue?

NASA says that an estimated 80 percent of Neptune’s mass is made up of a hot dense fluid of "icy" materials—water, methane, and ammonia—above a small, rocky core. The ice giant lacks a solid surface.

Some scientists suggest that there might be an ocean of super hot water under Neptune’s cold clouds, which are protected from boiling away by the incredibly high pressure that keeps them locked inside.

Avi Loeb, professor of science at Harvard University, told Newsweek: "Neptune has the strongest winds in the solar system, ripping through its atmosphere at over 1,000 miles per hour."

The reason Neptune takes the appearance of a blue marble is that methane in its upper atmosphere is a good absorber of red wavelengths of light, meaning the planet reflects mostly blue light back into space.

4. How Many Moons Does Neptune Have?

Despite its distance from the sun and frosty nature, Neptune is far from alone at the outskirts of the solar system. The planet shares space with at least 14 moons.

The first of these moons to be spotted, Triton, was discovered by William Lassell just 17 days after the discovery of the planet. Lassell made his name in the brewery business, using his wealth to finance telescopes.

As Neptune was named after the Roman god of the sea, its moons have been given the names of lesser sea gods and sea nymphs found in Greek mythology.

Neptune’s Three Largest Moons In Stats
Name: Triton
Circumference: 5282 miles
Mass: 2.1 x 1022 kg
Distance from Neptune: 220,438 miles

Special Features: By far the largest moon of Neptune, and one of the coldest objects in the solar system. Voyager 2 discovered that part of Triton’s surface resembles the rind of a cantaloupe.

The surface of this Neptunian moon is punctuated by ice volcanoes spewing a mix of liquid nitrogen, methane, and dust as high as 5 miles into the air. They instantly freeze and then snow back down to its surface.

Triton is the only moon in the solar system that has a retrograde orbit, meaning it orbits in the opposite direction to its parent planet’s spin.

Not only is Triton warming up, but the gravitational influence of Neptune is bringing it closer to the ice giant. Loeb said: "Triton will be disrupted in a few billion years by the gravity of Neptune, becoming a ring which might eventually accrete onto the planet."

Surface of Triton

The surface of the Neptunian moon of Triton covered with spewing volcanoes as pictured by Voyager 2. JPL-Caltech/NASA

Name: Proteus
Circumference: 820 miles
Mass: 5.0 x 1019 kg
Distance from Neptune: 73101 miles
Special Features: Neptune’s second-largest moon, Proteus has a non-spherical shape that astronomers believe comes from the fact the moon is right at the limit at which gravity molds planetary bodies.

Name: Nereid
Circumference: 621 miles
Mass: 3.0 x 1019 kg
Distance from Neptune: 3,426,127 miles
Special Features: The third-largest moon of Neptune and one of its outermost. Found in 1949, it was one of the last moons of Neptune to be discovered before Voyager’s trip to the ice giant in 1989.

The highly flattened, or eccentric, orbit of Nereid suggests it could actually be an asteroid that was captured by the gravitational influence of the ice giant.

5. Does Neptune Have Rings?

Neptune’s moons aren’t its only companions. The ice giant has five main rings of dust and debris that we are aware of. These rings, given the names Galle, Leverrier, Lassell, Arago and Adams, are believed to be fairly young and will not last particularly long.

The closest ring to the ice giant is Galle at 26,000 miles, while the most distant is Adams at 39,100 miles out.

In addition to these rings, Neptune’s ring system also has four prominent and peculiar clumps of dust that form ring arcs around the planet. Named Liberté (Liberty), Egalité (Equality), Fraternité (Fraternity), and Courage, these arcs are strange as the laws of motion suggest they should spread out rather than remain clumpy.

Researchers now believe it could be the gravitational influence of Neptune moon Galatea, located just inside the rings, that keeps them clumpy.

6. Why is Studying Neptune Important?

Loeb explained to Newsweek the importance of studying this distant ice giant, which while dissimilar to our home planet could teach us a great deal about planets that lie beyond the edge of the solar system; exoplanets.

Loeb said: "Understanding the extreme conditions on Neptune allows us to improve our models of planet formation and physical characteristics. This understanding is very helpful in interpreting data on planets around other stars, some of which are of the size and mass of Neptune.

"About 7.6 billion years from now, the sun will expand to a red giant and the habitable zone will move out from the Earth’s orbit to Neptune’s orbit. Could life develop briefly on that planet at that time?

"To figure it out, we need to understand better the chemistry of the planet."


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