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Top Ten Facts About Uranus
10. Uranus was the first planet to be discovered by telescope.
If you look up at the night sky, you probably won't be able to see Uranus without the aide of at least binoculars—and even then you'll need to know right where to look. For this reason, it remained undiscovered for a very long time. In 1781, William Herschel spotted Uranus via telescope while looking for binary star systems. He saw an object obscuring his view, though he didn't initially know it was a planet (he thought it was a star or comet). He continued his observations and saw that it moved noticeably against the background of stars; it was not a star itself. Further observations showed that it moved much too slowly to be a comet either, and thus he concluded it must be a planet. He wanted to name it after England's King George III, but it was eventually decided by others to be called Uranus. All of the other planets (except Earth) are named after Roman gods, but Uranus is the Greek king of the gods and sky.
While Herschel is credited with its discovery, it was actually recorded by John Flamsteed about a century earlier. He isn't given credit, however, because he always believed it to be a star; he dubbed it 34 Tauri.
9. Its average temperature is -320 degrees Fahrenheit.
Uranus is very, very far from the Sun—about 19 times as far away as we are, which puts it at about 1,784 million miles away. That's more than twice as far away as the next closest planet, Saturn. As you can probably imagine, Uranus is very cold. It's almost exactly tied with Neptune for being the coldest planet in our solar system. (This makes Neptune extra interesting, since it should be much colder as the farthest planet from the Sun--learn more about Neptune here!)
To look at it, you probably wouldn't guess that it features extremely bizarre, tempestuous weather patterns. It seems peaceful, calm, and really kind of dull. However, it's super windy (with wind speeds of up to 540 mph) and has storms on a massive scale, that could easily encompass the whole Earth. Its weather is also affected by its axial tilt.
8. It rotates on its side and backwards.
Uranus is one of only two planets in our solar system to feature retrograde rotation, along with Venus. They spin clockwise, while all the other planets (including our own) spin counter-clockwise. Further deviating from the norm, Uranus rotates almost completely on its side. Likely a result of a major collision, this has some especially interesting effects. First, each pole of Uranus spends about 20 years in constant sunlight followed by 20 years in constant darkness. As you can imagine, this leads to some bizarre seasons of the same length.
The weather on Earth is primarily solar-driven, and Uranus is much the same since there's not a lot of internally generated energy emitted by the planet. Since its so much farther away one might expect its weather to be a little "diluted." If we were to visit the surface-level of Uranus, however, we'd quickly be swept away in the mega-winds of incredibly huge storms.
7. A day on Uranus lasts only 17 hours.
Since Uranus 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, Uranus is spinning much more quickly than we are. Its day is only 17 hours long, though the entirety of each day is typically spent in either complete darkness or sunshine the whole time, as mentioned above. The rapid spinning is a major factor in the high wind speeds featured on the planet.
6. The composition of its atmosphere gives it its blue-green color.
When we look at Uranus, what we're really seeing is light reflected off the tops of its clouds. Its atmosphere is composed primarily of hydrogen and helium, but it also has a small amount of methane. This is what makes it blue-green.
Uranus and Neptune are the ice giants of our solar system. The body of Uranus is made up of an icy mixture of water, methane, and ammonia. It's also believed to feature an ocean of very hot water. How could that be, in such a frigid world? The collision which caused Uranus's axial tilt may have been caused by a bombardment of comets, which would account for the presence of water and the heated nature thereof. Collisions on a large scale generate a tremendous amount of heat from the friction of the bodies hitting each other (friction is the same thing that provides enough heat to produce a fire by rubbing two sticks together). Collisions plus the high pressure featured on Uranus could create a lot of heat. This hot ocean is presumed to surround a small rocky core.
5. Uranus has a small ring system.
Though Uranus was discovered centuries ago, we didn't know it had rings until very recently—1977. The ring system was discovered when Uranus was passing in front of a distant star. The star kept winking before it was blocked by Uranus. This phenomenon was repeated when it came into view on the other side again. Scientists discovered that this was because Uranus's rings were blocking the star.
At this time they identified only five or six of the rings, but we now know there are at least thirteen. The closest one to the planet is also the largest by a wide margin. It is about 2500 kilometers wide, whereas the others are an average of about 6.
The inner rings are dark and thin, while the outer rings are brighter. Just like the rings of the other jovian planets, Uranus' 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. Uranus'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.
4. Uranus has 27 moons.
In 1787, just six years after he discovered the planet, Herschel discovered its largest two moons: Oberon and Titania. They are both named after characters in William Shakespeare's A Midsummer Night's Dream. The additional 25 moons that have been discovered since are also named after Shakespeare's characters, with a couple taking their names from characters in the works of Alexander Pope. Many were discovered in the 1986 Voyager 2 flyby, and some were more recently found by advanced telescopes like the Hubble.
The inner moons are believed to be made up of water ice and rock. Voyager 2 focused on these inner moons, so we have discovered more information about them than the planet's outer moons. Unfortunately, the composition of those outer moons remains largely unclear. Astronomers think these are likely asteroids that closely crossed paths with Uranus and became locked in orbit by the planet's gravitational pull.
It's interesting to note that the rings and moons orbit around Uranus's equator. Models show that if there had been a single collision leading to Uranus's axial tilt, the moons and rings would have been expected to have a polar orbit. Models have also shown that with two collisions occurring while the moons and rings were still forming, they would likely take on an equatorial orbit (more on that here).
3. Though Uranus is 14.5x massive as Earth, its gravitational pull at surface level is 11% less than ours.
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. Uranus is more massive, but much less dense. That means that its surface level is way farther from the planet's center of gravity, accounting for its weaker gravitational pull. A person who weighs 150 pounds on Earth would weigh about 134 pounds on Uranus.
2. A Uranian year is equivalent to 84 Earth years.
Uranus is very far from the Sun—about 1,784 million miles away (compared to Earth's distance of 93 million miles from the Sun). 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 Uranus 84 Earth years.
Its distance from the Sun affects the duration of its year in another way as well. In the previous fact, the two factors determining gravitational pull were mentioned: mass and distance. Uranus 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 orbital period.
1. Voyager 2 is the only mission that's visited Uranus—so far.
Until Voyager 2 flew past Uranus in 1986 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. As previously mentioned, they are ice giants instead of gas giants like Jupiter and Saturn. Interestingly, most of the exoplanets we have discovered are ice giants as well—not gas giants. This is even more incentive to revisit Uranus, 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 planets in our solar system. This is the primary reason that many scientists would like to make another trip there. The European Space Agency was very close to beginning a new mission to Uranus, called Uranus Pathfinder, but unfortunately it was rejected in one of the final stages of selection.
Another bizarre fact about Uranus is that its magnetic field is a little wonky. We typically expect them to be pretty in line with the planet's rotational axis, but Uranus' magnetic field is 60 degrees off. If our magnetic field were the off that much, the north magnetic pole would be as far south as New Delhi, India! If we could visit Uranus, we might be able to learn more about magnetic fields—especially how they form.
There's much we could learn, but the costs of such a journey are astronomical—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.
It's pronounced YUR-in-iss...not yur-AY-niss. You're welcome.