How well do you know your astronomical neighborhood? Let's go on a trip through our solar system and explore the weird, the wild, and the beautiful all around us. Today, we are learning all about Neptune.
Where is it?
Neptune is the eighth planet of our solar system, sitting 2.8 miles from the sun and 2.5 miles from Earth. More than 30 times as far from the Sun as Earth, Neptune is the only planet in our solar system not visible to the naked eye.
Neptune is so far from the Sun that high noon on the big blue planet would seem like dim twilight to us. The warm light we see here on our home planet is roughly 900 times as bright as sunlight on Neptune.
Who “discovered” it?
Science is frequently amazing. But, from time to time, it’s good to take a step back and let some things fill us with awe.
To wit, the discovery of Neptune was the first done not by optics, but by mathematics.
Basically, astronomers studied Uranus’ orbit and theorized that there must be another planet nearby. That’s because of gravitational fluctuations, that, when they calculated it through Newtonain physics, suggested there was an undiscovered planet in the solar system.
Based on these calculations made by astronomer Urbain Le Verrier, fellow stargazer Johann Gottfried Galle used the Fraunhofer telescope at the Berlin Observatory and made the first observations of the new planet, only 1 degree from its calculated position. That fateful night happened on Sept. 23-24, 1846.
Is there a more pure expression of the scientific method than that? A theory is tested and proven with a new discovery.
It’s not often that the results of that test are the discovery of an entire planet in our neighborhood.
What’s up with the name?
With Neptune, we are back to the Roman pantheon. Neptune is the Roman god of the sea. His Greek counterpart is Poseidon. In both traditions, Neptune was also a god of horses. Interestingly, one of his enduring symbols was the trident, a three-pronged spear used for spear fishing.
Johann Gottfried Galle
Mosaic of Neptune (Regional Archeological Museum Antonio Salinas, Palermo)
Like Uranus, we have more detailed discussions about the naming of Neptune because it was discovered less than 200 years ago. Galle, who had been the first to observe it with a telescope, wanted to name it after Le Verrier.
LeVerrier himself first pushed the name Neptune, but quickly reverted to naming it after himself. Of course, nothing is simple and the scientific community debated it at the time, before most settled on Neptune as the official name by the end of 1846.
Because Neptune is the god of the sea, and the planet itself is icy blue, this felt fitting to astronomers of the day.
What’s it made of?
With a radius of 15,299.4 miles (24,622 kilometers), Neptune is about four times wider than Earth. Neptune's atmosphere is made up mostly of hydrogen and helium with just a little bit of methane.
Neptune's neighbor Uranus is a blue-green color due to such atmospheric methane, but Neptune is a more vivid, brighter blue, so there must be an unknown component that causes the more intense color.
Neptune is one of two ice giants in the outer solar system (the other is Uranus). Most (80% 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.
Neptune does not have a solid surface. Its atmosphere (made up mostly of hydrogen, helium, and methane) extends to great depths, gradually merging into water and other melted ices over a heavier, solid core with about the same mass as Earth.
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.
It seems contradictory to view an ice giant like Neptune as having a superheated ocean under those blue clouds. But then again, that’s what makes our solar system so fascinating. We are constantly learning new and unexpected facts about our nearest neighbors.
Can we live there?
Doubtful. Much like fellow ice giant Uranus, Neptune would prove a challenging environment. The density of the atmosphere would put anyone on it’s sludgy surface at a great disadvantage. Then, there’s the wind.
Neptune is our solar system's windiest world. Despite its great distance and low energy input from the Sun, Neptune's winds can be three times stronger than Jupiter's and nine times stronger than Earth's.
These winds whip clouds of frozen methane across the planet at speeds of more than 1,200 miles per hour (2,000 kilometers per hour). Even Earth's most powerful winds hit only about 250 miles per hour (400 kilometers per hour).
How long is a year there? What about a day?
One day on Neptune takes about 16 hours (the time it takes for Neptune to rotate or spin once). And Neptune makes a complete orbit around the Sun (a year in Neptunian time) in about 165 Earth years (60,190 Earth days).
Sometimes Neptune is even farther from the Sun than dwarf planet Pluto. Pluto's highly eccentric, oval-shaped orbit brings it inside Neptune's orbit for a 20-year period every 248 Earth years. This switch, in which Pluto is closer to the Sun than Neptune, happened most recently from 1979 to 1999. Pluto can never crash into Neptune, though, because for every three laps Neptune takes around the Sun, Pluto makes two. This repeating pattern prevents close approaches of the two bodies.
Has NASA sent any missions there?
Voyager 2 is the only spacecraft to fly by Neptune. No spacecraft has orbited this distant planet to study it at length and up close.
Voyager 2’s encounter with Neptune capped a 4.3 billion-mile (7 billion-kilometer) journey when, on Aug. 25, 1989, at 03:56 UT, it flew about 2,980 miles (4,800 kilometers) over the cloud tops of the giant planet, the closest of its four flybys. It was the first human-made object to fly by the planet. Its 10 instruments were still in working order at the time.
During the encounter, the spacecraft discovered six new moons (Proteus, Larissa, Despina, Galatea, Thalassa, and Naiad) and four new rings.
Neptune Small Dark Spot D2
The planet itself was found to be more active than previously believed, with 680-mile (1,100-kilometer) per hour winds. Hydrogen was found to be the most common atmospheric element, although the abundant methane gave the planet its blue appearance.
Images revealed details of the three major features in the planetary clouds—the Lesser Dark Spot, the Great Dark Spot, and Scooter.
Voyager photographed two-thirds of Neptune’s largest moon Triton, revealing the coldest known planetary body in the solar system and a nitrogen ice “volcano” on its surface. Spectacular images of its southern hemisphere showed a strange, pitted cantaloupe-type terrain.
Can I see it from here?
As we mentioned above, Neptune is the only planet in our solar system not visible to the naked eye.
Are there any pretty pictures of it?
Of course! There are many amazing shots of Neptune, though maybe not as many as some of the other planets that have seen more exploratory missions. Here are our favorites.