Uranus is the seventh planet from the sun and the first to be discovered by scientists. Although Uranus is visible to the naked eye, it was long mistaken as a star because of the planet’s dimness and slow orbit. The planet is also notable for its dramatic tilt, which causes its axis to point nearly directly at the sun.
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.
Uranuswas 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. [Related: How Do You Pronounce 'Uranus'?]
Uranusis 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.
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, 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.
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
Composition & structure
Atmospheric composition (by volume): 82.5 percent hydrogen, 15.2 percent helium, 2.3 percent methane
Magnetic field: Magnetic pole tilt compared to rotational axis: 58.6 degrees
Composition: The overall composition of Uranus is, by mass, thought to be about 25 percent rock, 60 to 70 percent ice, and 5 to 15 percent hydrogen and helium.
Internal structure: Mantle of water, ammonia and methane ices; core of iron and magnesium-silicate
Orbit & rotation
Axial tilt: 97.77 degrees, compared to Earth's 23.5 degrees
Seasonal cycle & length: Approximately 21 years per season
Orbital period: Approximately 84 Earth years
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.
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.
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. 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 Lagrangian point. The first was discovered in 2013, despite claims that the planet’s Langrangian 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 2013, the Planetary Science Decadal Survey recommended NASA consider a mission to the icy planet.
Additional reporting by Nola Taylor Redd, Space.com Contributor
Learn more about Uranus:
- How Big is Uranus?
- How Far Away is Uranus?
- Uranus' Atmosphere
- What is Uranus Made Of?
- How Was Uranus Formed?
- What is the Temperature of Uranus?
Learn more about the solar system: