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Top Ten Facts About Neptune
10. Neptune was discovered via mathematical calculation—not direct observation.
Sometimes the brightest "stars" we see in the night sky are actually planets, but Neptune is much too far away to be seen without aid of a telescope. For this reason it remained undiscovered until rather recently, and only because of Uranus.
Uranus was discovered quite accidentally in 1781. Extended observations showed that there were periodic fluctuations in its orbit. These fluctuations could be accounted for if there was another planet farther away. Using mathematical calculations, astronomers Urbain Le Verrier of France and John Couch Adams of Britain were able to independently determine where this new planet should be found. In 1846, Johann Galle confirmed these calculations by observing Neptune.
9. The methane in its atmosphere accounts for its blue color.
Similar to looking at Venus, when we look at Neptune what we see is not the surface—rather it's the light reflected off its clouds. Neptune's atmosphere is similar to the other outer planets, being composed primarily of hydrogen and helium. It also contains small amounts of methane, which is why it is blue. You may wonder why Uranus is more of a dull blue with a greenish tint instead of the same bright blue seen on Neptune, since they each get their tint from the methane in their atmospheres. The answer is that scientists aren't really sure, and are still looking into that very question. There must be some other factor at play; we just don't know what it is yet.
Uranus and Neptune are unique among our outer planets in that they are ice giants—not gas giants, as previously thought. Interestingly, most of the exoplanets we have discovered are ice giants as well. This is strong incentive to revisit Neptune someday, as it's an example of an extrasolar-like planet that's much more easily reachable.
8. The average temperature at surface-level on Neptune is -328 degrees Fahrenheit.
Neptune is set apart from Uranus in a curious way. Though it's half again as far away from the Sun as Uranus—2.8 billion miles versus 1.79—it's about the same temperature. The amount of sunlight that reaches Neptune is obviously much feebler, and yet it is giving off between two and three times the amount of heat that it receives from the Sun. To account for this, there must be some sort of internal heat being generated. Little more is known at this point, though some speculation has been made regarding the source.
Some believe this may be a natural process stemming from the formation of our solar system. The planets were hot and are still in the process of cooling off. There may also be radioactive decay contributing to Neptune's internal heat. Its core is estimated to be roughly the same temperature as the surface of the Sun, so there's definitely something going on!
7. A day on Neptune lasts 16 hours.
Since Neptune is about four times wider in diameter than the Earth, you'd think it would take a lot more time to make a complete rotation. However, Neptune is spinning much more quickly than we are. Its day is only 16 hours long. This rapid spinning leads to a banding appearance in its atmosphere similar to what's seen on the other outer planets (most notably Jupiter).
6. Neptune is home to the fastest winds in our solar system.
The speed of Neptune's rotation also causes extremely high winds—up to 1,250 miles per hour! That's five times the top speed of the most formidable hurricanes seen on Earth. The cold temperatures in Neptune's atmosphere may serve to reduce the friction, so wind rushes around more quickly. As you may imagine, these extreme winds can lead to very violent storms. In 1989, Voyager 2 captured an image of Neptune's "Great Spot" (pictured at the right), which was a giant storm measuring about 8,000 by 4,000 miles in size. It may sound similar to Jupiter's massive storm, the Great Red Spot, and in essence it is—however Jupiter's storm has lasted for decades. Neptune's, however, wasn't visible when we tried to view it with the Hubble Space Telescope just three short years later.
5. A single year on Neptune lasts 165 Earth years.
Neptune is very far from the Sun—about 2,798 million miles away. Its orbit is incredibly huge, so of course it takes a long time for the planet to travel all the way around the Sun. To make a complete orbit takes Neptune 165 Earth years.
Its distance from the Sun affects the duration of its year in another way as well. The two factors determining gravitational pull are mass and distance. Neptune is pretty massive, but since it's so far away from the Sun it isn't as strongly affected by the Sun's gravitational pull. Therefore, it travels more slowly as it orbits. Slower travel plus a lot of space to cover make for a very long orbitational period.
4. Neptune has 13 confirmed moons.
Only seventeen days after Neptune was discovered, William Lassell discovered its largest moon: Triton. Its other moons are smaller and darker, so they couldn't be detected before we had more advanced instruments at our disposal. Twelve more moons have been confirmed to orbit Neptune since Lassell discovered the first. It is interesting that they are named after lesser sea gods in Greek mythology, considering Neptune is the Roman god of the sea.
Thanks to the Voyager 2 flyby in 1989, we know several super cool things about Triton. First, we have discovered active ice volcanoes on Triton. According to NASA, the volcanoes are believed to "spout what is probably a mixture of liquid nitrogen, methane and dust, which instantly freezes and then snows back down to the surface." Second, it has its own atmosphere. It's made up largely of nitrogen, like ours, though it's much thinner. Third, it orbits the planet in the opposite direction of all the other moons, out of sync with Neptune's rotational direction. This suggests that it is most likely an asteroid that was pulled in and trapped in orbit by Neptune's gravitational pull. The best part is that Triton will likely eventually be crushed by Neptune's gravity and may even form another ring around the planet.
In addition to the thirteen confirmed, Neptune has a provisional moon which was discovered in 2013. It was discovered by Mark Showalter as he was going through photos taken by the Hubble Space Telescope. One of the images, shown at the right, showed an additional faint dot. It has yet to be confirmed an official moon.
3. If you weigh 100 lbs on Earth, you would weigh 113 pounds on Neptune.
Weight is a measure of gravity's pull on something, so it can change based on location. The factors affecting gravity are mass and distance. On Earth, we experience the gravity that we do because of how far we are from the center of the Earth and the amount of mass that it's composed of.
Neptune is over 17 times as massive as the Earth. Its gravitational pull at surface-level is greater than Uranus' even though Neptune is slightly smaller. This is because though it's smaller, it's also denser—there's more material packed in. In fact, it's the densest of all the outer planets. The large mass it's made up of accounts for its greater gravitational pull on objects at surface-level.
2. Neptune has six rings.
Though Neptune was discovered in 1846, we didn't know it had rings until very recently—1989. The Voyager 2 flyby sent back pictures which clearly showed them. Just like the rings of the other jovian planets, Neptune's rings are made up of objects as small as dust or as large as big boulders. They may have been formed similarly to how our planets formed around our Sun. Neptune's gravity may have trapped a lot of gas and dust in its orbit which accreted to form larger particles. Many rings are the result of a moon being smashed, either from a collision or the parent planet's gravity.
1. Voyager 2 is the only spacecraft that has visited Neptune.
Until Voyager 2 flew past Neptune in 1989 and sent back photos and measurements, we knew exceptionally little about the planet. Just a few decades ago it was believed to essentially be a smaller version of a Jupiter- or Saturn-like planet, but Uranus and Neptune are both decidedly different in composition. Instead of being gas giants like Jupiter and Saturn, they are ice giants. Interestingly, most of the exoplanets we have discovered are ice giants as well. This is even more incentive to revisit our farthest planet, as it's an example of an extrasolar-like planet that's much more easily reachable.
Even today we really don't know all that much about the farthest known planets in our solar system. This is the primary reason that many scientists would like to make another trip there. There's much we could learn, but unfortunately it's still very expensive—we're talking several billion dollars. The US definitely doesn't exactly have that kind of money to throw around, but perhaps in the future several countries will team up for a joint mission to our outermost planets.