Exploring Blue Planet Uranus

Uranus
Uranus was first studied up close by the Voyager 2 spacecraft in 1986. These images are in visible light (left) and near-infrared (right) light. It appears featureless and blue due to its mixture of methane gas in the hydrogen/helium atmosphere, along with some ammonia and other gases. The details seen in infrared light show faint cloud bands around the southern pole and atmospheric "hood" as seen by Voyager 2. NASA

In the pantheon of the planets, Uranus is a gas giant that lies well beyond Saturn in the outer solar system. Until 1986, it was studied from Earth, through telescopes that revealed very little about its true character. That changed when the Voyager 2 spacecraft swept past and captured the first close-up images and data of Uranus, its moons, and rings. 

Discovery of Uranus

Uranus (pronounced either ū·rā′·nəs or ūr′·ə·nəs), is visible to the naked eye, even though it is so distant.

  However, because it is so distant from us it moves much more slowly across the sky than the other planets visible from Earth. As a result, it wasn't identified as a planet until 1781. That's when Sir William Herschel observed it many times through his telescope and came to the conclusion that it was an object orbiting the Sun. Curiously, Herschel initially insisted that this newly re-discovered object was a comet, although he often mentioned that it could be more similar to objects like Jupiter or the ringed planet Saturn.

Naming the "New" Seventh Planet from the Sun

Herschel initially named his discovery Georgium Sidus (literally "George's Star," but taken as George's Planet) in honor of Britain's newly minted King George III. Unsurprisingly, however, this name was not met with a very warm reception beyond Britain. Therefore, other names were proposed, including Herschel, in honor of its discoverer.

Another suggestion was Neptune, which of course ended up getting used later.

The name Uranus was suggested by Johann Elert Bode and is the Latin translation of the Greek God Ouranos. The idea was from mythology, where Saturn was the father of Jupiter. So, the next world out would be the father of Saturn: Uranus.

This line of thinking was well-received by the international astronomy community, and in 1850, was the officially recognized name for the planet.

Orbit and Rotation

So, what kind of world is Uranus?  From Earth, astronomers could tell the planet has a not-insignificant eccentricity in its orbit, making it 150 million miles closer to the Sun at some times than others. On average Uranus is about 1.8 billion miles from the Sun, orbiting the center of our solar system every 84 Earth years.

The interior of Uranus (that is, the surface area below the atmosphere) rotates every 17 Earth-hours or so. The thick atmosphere is wracked with intense high-level winds that blow around the planet in as little as 14 hours. 

A unique feature of the faint-blue world is the fact that it has a highly tilted orbit. At nearly 98 degrees with respect to the orbital plane, the planet appears to at times "roll" around in its orbit.

Structure

Determining the structure of planets is a tricky business since astronomers can't just drill deep inside and see what comes out. They have to take measurements of what elements are present, typically using techniques such as reflection spectra, then using information such as its size and mass to estimate how much (and in what states) the various elements exist.

 

While not all models agree on the details, the general consensus is that Uranus has about 14.5 Earth masses, and its material is arranged in three distinct layers:

The central region is thought to be a rocky core. It only has about four percent of the planet's total mass the rocky core, so it's fairly small, compared to the rest of the planet. 

Above the core lies the mantel. It contains more than ninety percent of Uranus's total mass and makes up the majority of the planet. The primary molecules found in this region include water, ammonia, and methane (among others) in a semi-ice-liquid state.

Finally, the atmosphere shrouds the rest of the planet like a blanket. It contains the rest of Uranus's mass and is the least dense part of the planet. It consists primarily of elemental hydrogen and helium.

Rings

Everyone knows about the rings of Saturn, but actually, all the outer four gas giant planets all have rings. Uranus was the second one discovered to have such phenomena.

Like the brilliant rings of Saturn, those around Uranus are tiny individual particles of dark ice and dust. The material in these rings may have one been the building blocks of a nearby moon destroyed by impacts from asteroids, or perhaps even by gravitational interactions from the planet itself. In the distant past, such a moon may have wandered too close to its parent planet and been torn apart by the strong gravitational pull. In a few million years, the rings could be completely gone as their particles plunge into the planet or fly out to space.