Uranus, the seventh planet from the Sun, rotates at a dramatic 90-degree angle relative to its orbit. This unique tilt gives the appearance that Uranus spins on its side.
As the seventh planet from the Sun and the third largest in diameter, Uranus is a cold, windy ice giant.
Ringed by 13 faint rings and accompanied by 27 small moons, it rotates at a nearly 90-degree angle,
appearing to roll like a ball as it orbits the Sun.
Discovered in 1781 by William Herschel, Uranus was the first planet found with a telescope. Herschel
initially mistook it for a comet or star, but accepted its planetary status two years later, thanks in
part to Johann Elert Bode's observations.
While Herschel sought to name it Georgium Sidus after King George III, the name Uranus—suggested by Bode
to honor the Greek god of the sky—prevailed.
Voyager 2: Launched in 1977, Voyager 2 remains the only spacecraft to have visited Uranus and
Neptune. It is now in interstellar space, joining its twin, Voyager 1. During its tour of the outer
solar system, Voyager 2 provided the first close-up images of all four giant planets and discovered
numerous moons.
Its 1989 Neptune flyby redirected it south of the elliptic plane, leading it into interstellar space in
November 2018. Though nonessential instruments were deactivated in 1998 to save power, Voyager 2
continues to send data and is expected to operate until at least 2025.
With a radius of 15,759.2 miles (25,362 kilometers), Uranus is roughly 4 times the width of Earth.
Imagine Earth as a nickel; Uranus would be a softball.
Orbiting at an average distance of 1.8 billion miles (2.9 billion kilometers), Uranus sits 19.8
astronomical units (AU) from the Sun. Sunlight takes 2 hours and 40 minutes to make the journey.
A day on Uranus passes quickly, taking about 17 hours to complete one rotation. A year—one orbit around
the Sun—lasts 84 Earth years.
Uranus is unique for its extreme axial tilt of 97.77 degrees, essentially orbiting on its side. This
alignment, possibly caused by a massive ancient collision, creates the solar system's most extreme
seasons. For nearly 21 years, one pole faces the Sun directly, while the other is shrouded in a long,
dark winter.
Along with Venus, Uranus is one of only two planets to rotate retrograde (east to west).
Uranus coalesced 4.5 billion years ago from swirling gas and dust. Like Neptune, it likely formed closer to the Sun before migrating to the outer solar system about 4 billion years ago.
Uranus is one of the solar system's two ice giants. The majority of its mass (over 80%) is a hot, dense
fluid of "icy" materials—water, methane, and ammonia—swirling above a small rocky core. Temperatures
near the core can reach 9,000°F (4,982°C).
Although slightly larger in diameter than Neptune, Uranus is less massive. It ranks as the second least
dense planet, after Saturn.
The planet's varying blue-green hue comes from atmospheric methane, which absorbs red light and reflects
the blue-green spectrum.
Lacking a true solid surface, Uranus is primarily a world of swirling fluids. A spacecraft could not land; it would simply sink, eventually being crushed and vaporized by the intense pressure and heat.
The atmosphere consists largely of hydrogen and helium, with traces of methane, water, and ammonia.
Though Voyager 2 saw a relatively featureless sphere in 1986, recent observations show a dynamic
atmosphere with changing clouds as Uranus approaches equinox.
Using a minimum temperature of -371°F (-224.2°C), Uranus holds the title for the coldest planetary
atmosphere, even colder than Neptune in places.
Winds can scream at 560 mph (900 kph). Interestingly, equatorial winds blow retrograde (against
rotation), while polar winds blow prograde (with rotation).
The extreme environment of Uranus—characterized by immense pressure, temperature, and volatile materials—is unlikely to support life as we know it.
Uranus commands a system of 27 known moons. Unique in the solar system, they are named after characters
from the works of William Shakespeare and Alexander Pope.
The inner moons appear to be a mix of water ice and rock. The composition of the outer moons is less
certain, though they are likely captured asteroids.
Uranus possesses two sets of rings. The inner system contains nine narrow, dark grey rings, while the
outer system consists of two: a reddish inner one and a blue outer one like Saturn's E ring.
Ordered by distance, the named rings are Zeta, 6, 5, 4, Alpha, Beta, Eta, Gamma, Delta, Lambda, Epsilon,
Nu, and Mu. Fine dust belts surround some of the larger rings.
Uranus has an oddly shaped magnetosphere. Unlike other planets where the magnetic field aligns with
rotation, Uranus' field is tipped over—tilted nearly 60 degrees from its axis and offset from the
planet's center by a third of its radius.
This misalignment means auroras are not centered on the poles. The magnetosphere's tail twists into a
long corkscrew shape behind the planet as it rotates sideways.