Uranus

Uranus is the seventh planet from the Sun and one of the Solar System's two ice giants. This pale blue-green world is best known for rotating almost on its side, an orientation unlike that of any other major planet. Discovered in 1781 by William Herschel, Uranus was the first planet identified with the aid of a telescope.

Its discovery expanded the known boundaries of the Solar System and changed humanity's understanding of the worlds beyond Saturn.

An Ice Giant

Uranus differs from the gas giants Jupiter and Saturn because much of its interior is thought to consist of water, ammonia, and methane-rich materials, along with hydrogen and helium. Under the immense pressures inside the planet, these substances do not behave like ordinary ice on Earth.

Methane in Uranus's atmosphere absorbs red wavelengths of light, helping give the planet its soft blue-green color.

The Planet That Rolls Around the Sun

Uranus has an axial tilt of about 98 degrees. Its rotation axis therefore lies nearly in the same plane as its orbit, making the planet appear to roll around the Sun on its side.

This unusual orientation creates extreme seasons. Near each solstice, one pole can face the Sun for roughly 42 years while the other remains in darkness for a similar period. Scientists are still investigating how Uranus acquired its tilt, although one or more enormous collisions early in the planet's history may have contributed.

Atmosphere and Weather

Compared with Jupiter and Saturn, Uranus can appear calm and nearly featureless in visible light. Beneath that quiet appearance, however, it has powerful winds reaching about 560 miles per hour (900 kilometers per hour), along with clouds and storms that become more visible under certain conditions.

Uranus has the coldest measured cloud-top temperatures of any planet in the Solar System, falling to around -366°F (-224°C). Scientists are still studying why it releases so little internal heat compared with Neptune, its similarly sized neighbor.

Key Facts About Uranus

Diameter: About 31,763 miles (51,118 kilometers)
Mass: About 14.5 times Earth's mass
Average Distance from the Sun: About 1.8 billion miles (19.2 AU)
Day Length: About 17 hours
Year Length: About 84 Earth years
Coldest Cloud-Top Temperature: Around -366°F (-224°C)

Rings of Uranus

Uranus has 13 known rings made primarily of dark, rocky, and icy particles. They were discovered in 1977 when astronomers observed them briefly blocking starlight as the planet passed in front of a distant star.

Unlike Saturn's brilliant rings, Uranus's rings are narrow, faint, and difficult to observe from Earth.

Moons of Uranus

Uranus has 28 known moons. Most are named after characters from the works of William Shakespeare and Alexander Pope. The five largest are Miranda, Ariel, Umbriel, Titania, and Oberon.

Several of these moons display dramatic geological features, including enormous canyons, fault scarps, and icy plains. Miranda has especially chaotic terrain, suggesting a complex history that may have included major disruptions to its surface or interior.

Voyager 2’s Historic Flyby

NASA's Voyager 2 remains the only spacecraft to have visited Uranus. During its 1986 flyby, it discovered new moons and rings, observed atmospheric activity, and revealed a magnetic field with an unusual shape and orientation.

Uranus's magnetic field is strongly tilted relative to the planet's rotation axis and offset from its center, making it one of the most unusual planetary magnetic fields in the Solar System.

A Mysterious Ice Giant

Although Uranus is one of the major planets, it remains one of the least explored worlds in the Solar System. Its sideways rotation, faint rings, unusual interior, extreme cold, and off-center magnetic field continue to raise important questions for astronomers.

Far beyond Saturn, Uranus offers a valuable close-to-home example of an ice giant—a type of planet that may be common elsewhere in the galaxy but remains poorly understood.