Planets: Uranus Facts

Planets: Saturn
March 23, 2019
Solar System
March 21, 2019

URANUS The Little Blue Planet


Uranus is the seventh planet from the Sun. While being visible to the naked eye, it was not recognised as a planet due to its dimness and slow orbit. Uranus became the first planet discovered with the use of a telescope. Uranus is tipped over on its side with an axial tilt of 98 degrees. It is often described as “rolling around the Sun on its side.”


Uranus was officially discovered by Sir William Herschel in 1781

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It is too dim to have been seen by the ancients. At first Herschel thought it was a comet, but several years later it was confirmed as a planet. Herscal tried to have his discovery named “Georgian Sidus” after King George III. The name Uranus was suggested by astronomer Johann Bode. The name comes from the ancient Greek deity Ouranos.



Uranus turns on its axis once every 17 hours, 14 minutes.


The planet rotates in a retrograde direction, opposite to the way Earth and most other planets turn.

Uranus makes one trip around the Sun every 84 Earth years.

During some parts of its orbit one or the other of its poles point directly at the Sun and get about 42 years of direct sunlight. The rest of the time they are in darkness.

Uranus is often referred to as an “ice giant” planet.


Like the other gas giants, it has a hydrogen upper layer, which has helium mixed in. Below that is an icy “mantle, which surrounds a rock and ice core. The upper atmosphere is made of water, ammonia and the methane ice crystals that give the planet its pale blue colour.

Uranus hits the coldest temperatures of any planet.

With minimum atmospheric temperature of -224°C Uranus is nearly coldest planet in the solar system. While Neptune doesn’t get as cold as Uranus it is on average colder. The upper atmosphere of Uranus is covered by a methane haze which hides the storms that take place in the cloud decks.

Uranus has two sets of very thin dark coloured rings.

The ring particles are small, ranging from a dust-sized particles to small boulders. There are eleven inner rings and two outer rings. They probably formed when one or more of Uranus’s moons were broken up in an impact. The first rings were discovered in 1977 with the two outer rings being discovered in Hubble

Space Telescope images between 2003 and 2005.



Uranus’ moons are named after characters created by William Shakespeare and Alexander Pope. These include Oberon, Titania and Miranda. All are frozen worlds with dark surfaces. Some are ice and rock mixtures. The most interesting Uranian moon is Miranda; it has ice canyons, terraces, and other strange-looking surface areas.

Only one spacecraft has flown by Uranus.

In 1986, the Voyager 2 spacecraft swept past the planet at a distance of 81,500 km. It returned the first close-up images of the planet, its moons, and rings. The seventh planet from the Sun with the third largest diameter in our solar system, Uranus is very cold and windy.

The ice giant is surrounded by 13 faint rings and 27 small moons as it rotates at a nearly 90-degree angle from the plane of its orbit. This unique tilt makes Uranus appear to spin on its side, orbiting the Sun like a rolling ball.

The first planet found with the aid of a telescope, Uranus was discovered in 1781 by astronomer William Herschel, although he originally thought it was either a comet or a star. It was two years later that the object was universally accepted as a new planet, in part because of observations by astronomer Johann Elert Bode.

William Herschel tried unsuccessfully to name his discovery Georgium Sidus after King George III. Instead the planet was named for Uranus, the Greek god of the sky, as suggested by Johann Bode.



Size and Distance

With a radius of 15,759.2 miles (25,362 kilometers), Uranus is 4 times wider than Earth. If Earth was the size of a nickel, Uranus would be about as big as a softball.

From an average distance of 1.8 billion miles (2.9 billion kilometers), Uranus is 19.8 astronomical units away from the Sun. One astronomical unit (abbreviated as AU), is the distance from the Sun to Earth. From this distance, it takes sunlight 2 hours and 40 minutes to travel from the Sun to Uranus.

Orbit and Rotation

One day on Uranus takes about 17 hours (the time it takes for Uranus to rotate or spin once). And Uranus makes a complete orbit around the Sun (a year in Uranian time) in about 84 Earth years (30,687 Earth days).

Uranus is the only planet whose equator is nearly at a right angle to its orbit, with a tilt of 97.77 degrees—possibly the result of a collision with an Earth-sized object long ago. This unique tilt causes the most extreme seasons in the solar system. For nearly a quarter of each Uranian year, the Sun shines directly over each pole, plunging the other half of the planet into a 21-year-long, dark winter.

Uranus is also one of just two planets that rotate in the opposite direction than most of the planets (Venus is the other one), from east to west.



Formation

Uranus took shape when the rest of the solar system formed about 4.5 billion years ago, when gravity pulled swirling gas and dust in to become this ice giant. Like its neighbor Neptune, Uranus likely formed closer to the Sun and moved to the outer solar system about 4 billion years ago, where it is the seventh planet from the Sun.

Structure

Uranus is one of two ice giants in the outer solar system (the other is Neptune). Most (80 percent or more) of the planet's mass is made up of a hot dense fluid of "icy" materials—water, methane and ammonia—above a small rocky core. Near the core, it heats up to 9,000 degrees Fahrenheit (4,982 degrees Celsius).

Uranus is slightly larger in diameter than its neighbor Neptune, yet smaller in mass. It is the second least dense planet; Saturn is the least dense of all.

Uranus gets its blue-green color from methane gas in the atmosphere. Sunlight passes through the atmosphere and is reflected back out by Uranus' cloud tops. Methane gas absorbs the red portion of the light, resulting in a blue-green color.

Surface

As an ice giant, Uranus doesn’t have a true surface. The planet is mostly swirling fluids. While a spacecraft would have nowhere to land on Uranus, it wouldn’t be able to fly through its atmosphere unscathed either. The extreme pressures and temperatures would destroy a metal spacecraft.



Atmosphere

Uranus' atmosphere is mostly hydrogen and helium, with a small amount of methane and traces of water and ammonia. The methane gives Uranus its signature blue color.

While Voyager 2 saw only a few discrete clouds, a Great Dark Spot and a small dark spot during its flyby in 1986, more recent observations reveal that Uranus exhibits dynamic clouds as it approaches equinox, including rapidly changing bright features.

Uranus' planetary atmosphere, with a minimum temperature of 49K (-224.2 degrees Celsius) makes it even colder than Neptune in some places.

Wind speeds can reach up to 560 miles per hour (900 kilometers per hour) on Uranus. Winds are retrograde at the equator, blowing in the reverse direction of the planet’s rotation. But closer to the poles, winds shift to a prograde direction, flowing with Uranus' rotation.

Potential for Life

Uranus' environment is not conducive to life as we know it. The temperatures, pressures and materials that characterize this planet are most likely too extreme and volatile for organisms to adapt to.

Moons

Uranus has 27 known moons. While most of the satellites orbiting other planets take their names from Greek or Roman mythology, Uranus' moons are unique in being named for characters from the works of William Shakespeare and Alexander Pope.

All of Uranus' inner moons appear to be roughly half water ice and half rock. The composition of the outer moons remains unknown, but they are likely captured asteroids.



Rings

Uranus has two sets of rings. The inner system of nine rings consists mostly of narrow, dark grey rings. There are two outer rings: the innermost one is reddish like dusty rings elsewhere in the solar system, and the outer ring is blue like Saturn's E ring.

In order of increasing distance from the planet, the rings are called Zeta, 6, 5, 4, Alpha, Beta, Eta, Gamma, Delta, Lambda, Epsilon, Nu and Mu. Some of the larger rings are surrounded by belts of fine dust.

Magnetosphere

Uranus has an unusual, irregularly shaped magnetosphere. Magnetic fields are typically in alignment with a planet's rotation, but Uranus' magnetic field is tipped over: the magnetic axis is tilted nearly 60 degrees from the planet's axis of rotation, and is also offset from the center of the planet by one-third of the planet's radius.

Auroras on Uranus are not in line with the poles (like they are on Earth, Jupiter and Saturn) due to the lopsided magnetic field.

The magnetosphere tail behind Uranus opposite the Sun extends into space for millions of miles. Its magnetic field lines are twisted by Uranus’ sideways rotation into a long corkscrew shape.

Planet Uranus: Facts About Its Name, Moons and Orbit

British astronomer William Herschel discovered Uranus accidentally on March 13, 1781, with his telescope while surveying all stars down to those about 10 times dimmer than can be seen by the naked eye. One "star" seemed different, and within a year Uranus was shown to follow a planetary orbit.

Uranus was named after the Greek sky deity Ouranos, the earliest of the lords of the heavens. It is the only planet to be named after a Greek god rather than a Roman one. Before the name was settled on, many names had been proposed for the new planet, including Hypercronius ("above Saturn"), Minerva (the Roman goddess of wisdom), and Herschel, after its discoverer.



To flatter King George III of England, Herschel himself offered Georgium Sidus ("The Georgian Planet") as a name, but that idea was unpopular outside of England and George's native Hanover. German astronomer Johann Bode, who detailed Uranus' orbit, gave the planet its ultimate name.

Bode argued that as Saturn was the father of Jupiter, the new planet should be named for the father of Saturn. Bode's colleague, Martin Klaproth, supported his choice and named his newly discovered element "uranium."

Physical characteristics

Uranus is blue-green in color, the result of methane in its mostly hydrogen-helium atmosphere. The planet is often dubbed an ice giant, since 80 percent or more of its mass is made up of a fluid mix of water, methane, and ammonia ices.

Unlike the other planets of the solar system, Uranus is tilted so far that it essentially orbits the sun on its side, with the axis of its spin nearly pointing at the star. This unusual orientation might be due to a collision with a planet-size body, or several small bodies, soon after it was formed.

This unusual tilt gives rise to extreme seasons roughly 20 years long, meaning that for nearly a quarter of the Uranian year, equal to 84 Earth-years, the sun shines directly over each pole, leaving the other half of the planet to experience a long, dark, cold winter.

Uranus has the coldest atmosphere of any of the planets in the solar system, even though it is not the most distant from the sun. That's because Uranus has little to no internal heat to supplement the heat of the sun.



The magnetic poles of most planets are typically lined up with the axis along which it rotates, but Uranus' magnetic field is tilted, with its magnetic axis tipped over nearly 60 degrees from the planet's axis of rotation.

According to Norman F. Ness, et al., in an article in the journal Science in 1986, this leads to a strangely lopsided magnetic field for Uranus, with the strength of the field at the northern hemisphere's surface being up to more than 10 times that of the strength at the southern hemisphere's surface, affecting the formation of the auroras.

A 2017 study suggested the lopsided nature of Uranus' magnetic field may also lead it to flicker on and off every time the planet rotates (about every 17.24 hours)

Orbital characteristics

Uranus' parameters, according to NASA:

Average distance from the sun: 1,783,939,400 miles (2,870,972,200 kilometers). By comparison: 19.191 times that of Earth

Perihelion (closest approach to the sun): 1,699,800,000 miles (2,735,560,000 km). By comparison: 18.60 times that of Earth

Aphelion (farthest distance from sun): 1,868,080,000 miles (3,006,390,000 km). By comparison: 19.76 times that of Earth

Uranus' climate



The extreme axial tilt Uranus experiences can give rise to unusual weather. As sunlight reaches some areas for the first time in years, it heats up the atmosphere, triggering gigantic springtime storms roughly the size of North America, according to NASA.

Ironically, when Voyager 2 first imaged Uranus in 1986 at the height of summer in its south, it saw a bland-looking sphere with only about 10 or so visible clouds, leading to it to be dubbed "the most boring planet," writes astronomer Heidi Hammel in "The Ice Giant Systems of Uranus and Neptune," a chapter in "Solar System Update" (Springer, 2007).

It took decades later, when advanced telescopes such as Hubble came into play and the seasons changed, to see extreme weather on Uranus, where fast-moving winds can reach speeds of up to 560 miles (900 kilometers) per hour.

In 2014, astronomers got their first glimpse at summer storms raging on Uranus. Strangely, these massive storms took place seven years after the planet reached its closest approach to the sun, and it remains a mystery why the giant storms were so late.

Other unusual weather on Uranus includes diamond rain, which is thought to fall thousands of miles below the surfaces of icy giant planets such as Uranus and Neptune. Carbon and hydrogen are thought to compress under extreme heat and pressure deep in the atmospheres of these planets to form diamonds, which are then thought to sink downward, eventually settling around the cores of those worlds.



The rings of Uranus

The rings of Uranus were the first to be seen after Saturn's. They were a significant discovery, because it helped astronomers understand that rings are a common feature of planets, not merely a peculiarity of Saturn.

Uranus possesses two sets of rings. The inner system of rings consists mostly of narrow, dark rings, while an outer system of two more-distant rings, discovered by the Hubble Space Telescope, are brightly colored, one red, one blue. Scientists have now identified 13 known rings around Uranus.

A 2016 study suggested the rings of Uranus, Saturn and Neptune may be the remnants of Pluto-like dwarf planets that strayed too close to the giant worlds long ago.

Uranus' moons

Uranus has 27 known moons. Instead of being named after figures from Greek or Roman mythology, its first four moons were named after magical spirits in English literature, such as William Shakespeare's "A Midsummer Night's Dream" and Alexander Pope's "The Rape of the Lock."

Since then, astronomers have continued this tradition, drawing names for the moons from the works of Shakespeare or Pope.

Oberon and Titania are the largest Uranian moons, and were the first to be discovered, by Herschel in 1787. William Lassell, who was the first to see a moon orbiting Neptune, discovered the next two, Ariel and Umbriel. Then nearly a century passed before Miranda was found in 1948.

Then, Voyager 2 visited the Uranian system in 1986 and found an additional 10, all just 16 to 96 miles (26-154 km) in diameter — Juliet, Puck, Cordelia, Ophelia, Bianca, Desdemona, Portia, Rosalind, Cressida and Belinda — and each roughly made half of water ice and half of rock.



Since then, astronomers using the Hubble Space Telescope and ground-based observatories have raised the total to 27 known moons, and spotting these was tricky — they are as little as 8 to 10 miles (12 to 16 km) across, blacker than asphalt, and nearly 3 billion miles (4.8 billion km) away.

Between Cordelia, Ophelia and Miranda is a swarm of eight small satellites crowded together so tightly that astronomers don't yet understand how the little moons have managed to avoid crashing into each other. Anomalies in Uranus' rings lead scientists suspect there might still be more moons, closer to Uranus than any known.

In addition to moons, Uranus may also have a collection of Trojan asteroids — objects that share the same orbit as the planet — in a special region known as a Lagrange point. The first was discovered in 2013, despite claims that the planet's Langrange point would be too unstable to host such bodies.

Research & exploration

NASA's Voyager 2 was the first and as yet only spacecraft to visit Uranus. It discovered 10 previously unknown moons, and investigated its unusually tilted magnetic field.



In 2011, the Planetary Science Decadal Survey recommended NASA consider a mission to the icy planet. In 2017, NASA suggested a number of potential future missions to Uranus in support of the forthcoming Planetary Science Decadal Survey, including flybys, orbiters and even a spacecraft to dive into Uranus' atmosphere.