Interesting Facts About Neptune

Neptune and its Great Dark Spot. Image credit: NASA/JPL

1. Neptune is the most distant planet
This sounds simple, but it’s actually pretty complicated. When it was discovered by in 1846, Neptune became the most distant planet in the Solar System. But then in 1930, Pluto was discovered, and it became the most distant planet. But Pluto’s orbit is very elliptical; and so there are periods when Pluto actually orbits closer to the Sun than Neptune. The last time this happened was in 1979, lasting until 1999. During that period, Neptune was the most distant planet. And then, in 2006, the International Astronomical Union decided that Pluto isn’t a planet any more. And so, Neptune became the most distant planet; for now…

2. Neptune is the smallest of the gas giants
With an equatorial radius of only 24,764 km, Neptune is smaller than the other gas giants in the Solar System: Jupiter, Saturn and Uranus. But here’s the funny thing, Neptune is actually more massive than Uranus by about 18%. Since it’s smaller but more massive, Neptune is much more dense than Uranus.

3. Neptune’s surface gravity is almost Earthlike
Neptune is a ball of gas and ice, probably with a rocky core. There’s no way you could actually stand on the surface of Neptune without just sinking in. However, if you could stand on the surface of Neptune, you would notice something amazing. The force of gravity pulling you down is almost exactly the same as the force of gravity you feel walking here on Earth. The gravity of Neptune is only 17% stronger than Earth gravity. That’s actually the closest to Earth gravity in the Solar System. Neptune has 17 times the mass of Earth, but also has almost 4 times larger. This means its greater mass is spread out over a larger volume, and down at the surface, the pull of gravity would be almost identical. Except you’d sink right in…

4. The discovery of Neptune is still a controversy
The first person to probably see Neptune was Galileo, who marked it as a star in one of his drawings. He didn’t realize what he was looking at, so that doesn’t count. The French mathematician Urbain Le Verrier and the English mathematician John Couch Adams both made predictions that a new planet would be discovered in a specific region of the sky. When astronomer Johann Gottfried Galle actually found the planet in 1846, both mathematicians took credit for the discovery. English and French astronomers battled over who actually made the discovery, and there are still defenders of each claim to this day. Most astronomers consider that Le Verrier and Adams shared the discovery, and don’t worry about it any more.

5. Neptune has the strongest winds in the Solar System
Think a hurricane is scary? Imagine a hurricane with winds that go up to 2,100 km/hour. As you can probably imagine, scientists are puzzled how an icy cold planet like Neptune can get its cloud tops moving so fast. One idea is that the cold temperatures and the flow of fluid gasses in the planet’s atmosphere might reduce friction to the point that it’s easy to generate winds that move so quickly.

6. Neptune is the coldest planet in the Solar System
At the top of its clouds, temperatures on Neptune can dip down to 51.7 Kelvin, or -221.4 degrees Celsius. That’s cold! Pluto gets colder, but then, Pluto isn’t a planet any more.

7. Neptune’s moon Triton is even colder
There are many cold places in the Solar System, but one of the coldest is the surface of Neptune’s moon Triton. This is the largest of Neptune’s 13 moons, and the only one with enough mass and gravity to pull itself into a sphere. In fact, it’s the 7th largest moon in the Solar System. Temperatures on the surface of Triton can dip down to only 38 Kelvin or – 235 degrees Celsius. But even though it’s incredibly cold, the surface of Triton is very active. During its 1989 flyby, NASA’s Voyager 2 spacecraft observed volcanoes or geysers erupting liquid nitrogen.

8. Neptune probably captured Triton
Neptune’s largest Moon, Triton, orbits in a retrograde orbit. that’s means that it orbits around the planet backwards compared to the other moons that orbit Neptune. This means that Neptune probably captured Triton; the moon didn’t form in place like the rest of Neptune’s moons. Triton is locked into a synchronous rotation with Neptune and is slowly spiraling inward towards the planet. At some point, billions of years from now, it’ll be torn apart by Neptune’s gravitational forces and become a magnificent ring around the planet. And then the ring will be pulled inward to crash into the planet. It would be amazing to watch.

9. Neptune has only been visited once up close
The only spacecraft that has ever visited Neptune was NASA’s Voyager 2 spacecraft, which visited the planet during its Grand Tour of the Solar System. Voyager 2 made its Neptune flyby on August 25, 1989, passing within 3,000 km of the planet’s north pole. This was the closest approach to any object that Voyager 2 made since it was launched from Earth. During its flyby, Voyager 2 studied Neptune’s atmosphere, its rings, magnetosphere. It also made observations of Neptune’s moons.

10. There are no plans to visit Neptune again
Voyager 2′s amazing photographs of Neptune might be all we get for decades. There are no firm plans to return to Neptune. There are tentative plans from NASA to send a new mission to Neptune called the Neptune Orbiter. This spacecraft would launch in 2016 and take about 14 years to get to Neptune, arriving around 2030. It would go into orbit around the planet and study its weather, magnetosphere, ring system and moons. Let’s hope they actually launch it.

Want more information about Neptune? Here’s NASA’s World Book on Neptune, and here’s NASA’s Solar System Exploration Guide.

We have recorded an entire episode of Astronomy Cast just about Neptune. You can listen to it here, Episode 63: Neptune.

Reference:
NASA