Neptune is a truly fascinating world. But as it is, there is much that people don’t know about it. Perhaps it is because Neptune is the most distant planet from our Sun, or because so few exploratory missions have ventured that far out into our Solar System. But regardless of the reason, Neptune is a gas (and ice) giant that is full of wonder!

Below, we have compiled a list of 10 interesting facts about this planet. Some of them, you might already know. But others are sure to surprise and maybe even astound you. Enjoy!

1. Neptune is the Most Distant Planet:

This may sound like a pretty simple statement, but it’s actually rather complicated. When it was first discovered by in 1846, Neptune became the most distant planet in the Solar System. But then in 1930, Pluto was discovered, and Neptune became the second-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, which lasted until 1999. During that period, Neptune was again the most distant planet.

Then, at the XXVIth General Assembly of the International Astronomical Union – which took place between Aug 14th and 25th, 2006, in Prague – the issue of which was the most distant planet was visited once again. Confronted with the discovery of many Pluto-sized bodies in the Kuiper Belt – i.e. Eris, Haumea, Sedna and Makemake – and the ongoing case of Ceres, the IAU decided it was time to work out a clear definition of what a planet was.

In what would prove to be a very controversial decision, the IAU’s passed a resolution which defined a planet as “a celestial body orbiting a star that is massive enough to be rounded by its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite. More explicitly, it has to have sufficient mass to overcome its compressive strength and achieve hydrostatic equilibrium.”

As a result of this, Pluto was “demoted” from the status of planet and thereafter defined as a “dwarf planet” instead. And so, Neptune has once again become the most distant planet. At least for now…

2. Neptune is the Smallest of the Gas Giants:

With an equatorial radius of only 24,764 km, Neptune is smaller than all 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 has a much higher density than Uranus. In fact, at 1.638 g/cm3, Neptune is the densest gas giant in the Solar System.

3. Neptune’s Surface Gravity is Almost Earth-like:

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 (one g) 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 for the part where you wouldn’t stop sinking!

4. The Discovery of Neptune is Still a Controversy:

The first person to have seen Neptune was likely Galileo, who marked it as a star in one of his drawings. However, since he did not identify it as a planet, he is not credited with the discovery. That credit goes to French mathematician Urbain Le Verrier and the English mathematician John Couch Adams, both of whom predicted that a new planet – known as Planet X – 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 made the discovery first, and there are still defenders of each claim to this day. Today, the consensus among astronomers is that Le Verrier and Adams deserve equal credit for the discovery.

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 t0 move 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 -221.45 degrees Celsius (-366.6 °F). That’s almost three times the coldest temperature ever recorded here on Earth (-89.2°C; -129°F), which means that an unprotected human being would flash freeze in a second! Pluto gets colder, experiencing temperatures as low as 33 K (-240 °C/-400 °F). But then again, Pluto isn’t a planet any more (remember?)

7. Neptune has Rings:

When people think of ring systems, Saturn is usually the planet that comes to mind. But would it surprise you to know that Neptune has a ring system as well? Unfortunately, it is rather difficult to observe compared to Saturn’s bright, bold ring; which is why it is not so well-recognized. In total, Neptune has five rings, all of which are named after astronomers who made important discoveries about Neptune – Galle, Le Verrier, Lassell, Arago, and Adams.

These rings are composed of at least 20% dust (with some containing as much as 70%) which are micrometer-sized, similar to the particles that make up the rings of Jupiter. The rest of the ring materials consists of small rocks. The planet’s rings are difficult to see because they are dark, which is likely due to the presence of organic compounds that have been altered due to exposition to cosmic radiation. This is similar to the rings of Uranus, but very different than the icy rings around Saturn.

It’s believed that the rings of Neptune are relatively young – much younger than the age of the Solar System, and much younger than the age of Uranus’ rings. Consistent with the theory that Triton was a Kuiper Belt Object (KBO) that was seized by Neptune’s gravity (see below), they are believed to be the result of a collision between some of the planet’s original moons.

8. Neptune Probably Captured its Largest Moon, Triton:

Neptune’s largest Moon, Triton, circles Neptune in a retrograde orbit. That’s means that it orbits the planet backwards relative to Neptune’s other moons. This is seen as an indication that Neptune probably captured Triton – i.e. 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, Triton will likely will be torn apart by Neptune’s gravitational forces and become a magnificent ring around the planet. This ring will be pulled inward and crash into the planet. It is too bad that such an event will be happening so very long from now, because it would be amazing to watch!

9. Neptune has Only Been Visited up Close Once:

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, and also conduct a close flyby of Triton. Voyager 2 also viewed Neptune’s “Great Dark Spot“, the rotating storm system which has since disappeared, according to observations by the Hubble Space Telescope. Originally thought to be a large cloud itself, the information gathered by Voyager helped to shed light on the true nature of this phenomenon.

10. There are no Plans to Visit Neptune Again:

Voyager 2‘s amazing photographs of Neptune might be all we get for decades, as there are no firm plans to return to the Neptune system. However, a possible Flagship Mission has been envisioned by NASA to take place sometime during the late 2020s or early 2030s. For example, in 2003, NASA announced tentative plans to send a new Cassini-Huygens-style mission to Neptune, called the Neptune Orbiter.

Also described as a “Neptune Orbiter with Probes”, this spacecraft had a proposed launch date of 2016, and would arriving around Neptune by 2030. The proposed mission would go into orbit around the planet and study its weather, magnetosphere, ring system and moons. However, no information on this project has been forthcoming in recent years and it appears to have been scrapped.

Another, more recent proposal by NASA was for Argo – a flyby spacecraft that would be launched in 2019, which would visit Jupiter, Saturn, Neptune, and a Kuiper belt object. The focus would be on Neptune and its largest moon Triton, which would be investigated around 2029.

And these are just of the things that make Neptune such a fascinating planet, and one that is worthy of study. One can only hope that future missions will be launched to the outer Solar System that will be able to dig deeper into its many mysteries.

We have many interesting articles about Neptune here at Universe Today. Here is one about the Rings of Neptune, the Moons of Neptune, Who Discovered Neptune?, and Are There Oceans on Neptune?

If you’d like more information on Neptune, take a look at Hubblesite’s News Releases about Neptune, and here’s a link to NASA’s Solar System Exploration Guide to Neptune.

Astronomy Cast has some interesting episodes about Neptune. You can listen here, Episode 63: Neptune and Episode 199: The Voyager Program.