Uranus, Seventh Planet in Earth’s Solar System Was First Discovered Planet
Uranus's History & Naming
Uranus, named after the Greek sky deity Ouranos, the earliest of the lords of the heavens, was the first planet to be discovered by scientists.
Although Uranus is visible to the naked eye, just like the classical planets — Mercury, Venus, Mars, Jupiter, and Saturn — it was long mistaken as a star because of the planet’s dimness and slow 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.
Many names were proposed for the new planet, including "Hypercronius" ("above Saturn"), "Minerva" (the Roman goddess of wisdom), and "Herschel." 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.
Physical Characteristics of the Planet Uranus
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-sized body 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.
A planet's magnetic poles are typically lined up with the poles 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. 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.
Uranus's Orbital Characteristics
Average Distance from the Sun
English: 1.783,939,400 miles
Metric: 2,870,972,200 kilometers
By Comparison: 19.191 times that of Earth
English: 1,699,800,000 miles
Metric: 2,735,560,000 kilometers
By Comparison: 18.60 times that of Earth
English: 1,868,080,000 miles
Metric: 3,006,390,000 kilometers
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 pole tilt compared to rotational axis: 58.6 degrees
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. (Tristan Guillot, "Interiors of Giant Planets Inside and Outside the Solar System." Science Vol. 286 (5437), p. 72-77, October 1, 1999.)
- Internal structure
Mantle of water, ammonia and methane ices
Core of iron and magnesium-silicate (Tristan Guillot, "Interiors of Giant Planets Inside and Outside the Solar System." Science Vol. 286 (5437), p. 72-77, October 1, 1999.)
Orbit & Rotation
97.77 degrees, compared to Earth's 23.5 degrees
- Seasonal cycle & length
Approximately 21 years per season
Approximately 84 Earth years
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 William Herschel in 1787. William Lassell, who was the first to see a moon orbiting Neptune, discovered the next two, Ariel and Umbriel. Nearly a century then 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 kilometers) 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 kilometers) 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.
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.
Ironically, when Voyager 2 first imaged Uranus in 1986 at the height of summer in its south, it only saw a bland-looking sphere with only about 10 or so visible clouds, leading to it often getting dubbed "the most boring planet" (Heidi Hammel, "The Ice Giant Systems of Uranus and Neptune"). It took decades later, when advanced telescopes such as Hubble came into play and the seasons changed, to see extreme weather on Uranus.
The Rings of Uranus
The rings of Uranus were the first to be seen after Saturn's. They were a significant discovery, since 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.
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.
RELATED: See our Solar System Planets overview, or our broader Solar System Facts overview, or learn more about each of the other planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Neptune and the demoted dwarf planet Pluto.