March 21, 2026
Uranus Facts: Everything You Need to Know About the Ice Giant
Uranus is the seventh planet from the Sun and one of the strangest worlds in our solar system. Discovered by William Herschel in 1781 — the first planet found with a telescope — Uranus shattered the ancient belief that Saturn marked the edge of the solar system. This pale blue-green ice giant rolls around the Sun nearly on its side, sports a system of dark narrow rings, and commands a family of 28 known moons named after literary characters rather than mythological figures. Despite being visible to the naked eye under perfect conditions, Uranus remained unrecognized as a planet for millennia because of its extreme distance and slow motion across the sky. Only one spacecraft, Voyager 2, has ever visited. This complete guide covers everything we know about Uranus: its size, diameter, extreme tilt, atmosphere, rings, moons, and the missions that have shaped our understanding — all drawn from NASA's Planetary Fact Sheets and decades of observational science.
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Uranus Fact File
How Big Is Uranus?
Uranus is the third-largest planet in the solar system by diameter and the fourth-largest by mass. Its equatorial diameter is 51,118 km (31,763 miles) — roughly four times wider than Earth. You could line up four Earths side by side and they would just about span the width of Uranus. The polar diameter is slightly smaller at 49,946 km due to the planet's rotation, giving it an oblateness of about 2.3%.
In terms of volume, 63 Earths could fit inside Uranus. However, the planet's mass is only 8.681 × 10²&sup5; kg — about 14.5 times Earth's mass. This means Uranus has a remarkably low density of 1.27 g/cm³, only slightly denser than water. If you could find an ocean large enough, Uranus would barely sink. Its surface gravity is 8.87 m/s² — about 89% of Earth's — which is surprisingly close to what we experience on Earth despite the planet being 63 times larger by volume. A 70 kg person would weigh about 62 kg on the "surface" of Uranus (defined as the altitude where atmospheric pressure equals 1 bar).
Uranus by the Numbers
Uranus orbits the Sun at an average distance of 2.87 billion km (19.2 AU), making it the seventh planet from the Sun. At this immense distance, sunlight takes about 2 hours and 40 minutes to reach Uranus, compared to just 8 minutes to reach Earth. A year on Uranus lasts 84 Earth years — so long that since its discovery in 1781, Uranus has completed barely three full orbits around the Sun.
Despite its enormous size, Uranus spins relatively quickly. A day on Uranus lasts just 17 hours and 14 minutes, though its rotation is retrograde — it spins in the opposite direction from most planets (clockwise when viewed from above the solar system's north pole). This retrograde spin, combined with the extreme axial tilt, means that Uranus's "north" pole technically points roughly toward the Sun's south. The planet's orbital eccentricity is low at 0.046, meaning its orbit is nearly circular, with perihelion at 2.74 billion km and aphelion at 3.01 billion km.
The Sideways Planet: Uranus's Extreme Tilt
The most striking feature of Uranus is its axial tilt of 97.77 degrees. While Earth tilts at a modest 23.4 degrees — enough to give us seasons — Uranus is tilted so far that it essentially rolls around the Sun on its side like a bowling ball. No other planet in our solar system comes close to this extreme orientation. The leading theory is that a protoplanet roughly the size of Earth slammed into Uranus during the chaotic early period of solar system formation, roughly 4 billion years ago, knocking it onto its side. Some models suggest it may have taken two or more sequential impacts rather than a single catastrophic collision.
This extreme tilt produces the most dramatic seasons in the solar system. During the Uranian solstice, one pole faces directly toward the Sun while the other is plunged into total darkness. Because a Uranian year lasts 84 Earth years, each pole experiences roughly 42 years of continuous sunlight followed by 42 years of continuous night. At the equinoxes — the most recent occurred in 2007 — the Sun passes over the equator, and the planet experiences rapid day-night cycles more like other planets. Hubble and ground-based telescopes have observed significant changes in Uranus's cloud patterns and brightness as its season shifts, including bright cloud bands that appeared near the north pole as it emerged into sunlight for the first time in decades.
Atmosphere and Interior
Uranus is classified as an ice giant rather than a gas giant, a distinction it shares only with Neptune. While Jupiter and Saturn are dominated by hydrogen and helium, Uranus's bulk composition is primarily "ices" — water, methane, and ammonia in various states of compression. The visible atmosphere is composed of roughly 83% molecular hydrogen, 15% helium, and 2.3% methane, along with traces of hydrogen deuteride and ethane.
It is the methane in the upper atmosphere that gives Uranus its distinctive pale blue-green color. Methane absorbs red wavelengths of sunlight and scatters blue-green light back to the observer. Beneath the visible clouds, the atmosphere transitions into a dense, hot mantle of "superionic" water, methane, and ammonia ices compressed under extreme pressures. At the very center lies a small rocky core, estimated at roughly 0.5 to 1.5 Earth masses. The interior temperatures reach around 4,700 °C, though this is surprisingly cool compared to the other giant planets — Uranus radiates almost no excess heat beyond what it receives from the Sun, making it the coldest planetary atmosphere in the solar system at −224 °C (−371 °F) in the upper cloud deck.
Uranus's magnetic field is another oddity. Unlike Earth's field, which is roughly aligned with the rotation axis, Uranus's magnetic field is tilted 59 degrees from the rotation axis and offset from the planet's center by about one-third of the planet's radius. This produces a highly asymmetric magnetosphere that tumbles wildly as the planet rotates, creating a corkscrew-shaped magnetotail stretching millions of kilometers downwind from the Sun.
The Ring System
Uranus has 13 known rings, discovered in 1977 during a stellar occultation — astronomers James Elliot, Edward Dunham, and Jessica Mink were observing the star SAO 158687 pass behind Uranus when they noticed the star dimmed briefly five times before and after the planet blocked it. This symmetrical pattern revealed a system of narrow, dark rings encircling the planet. Voyager 2 confirmed and expanded the count during its 1986 flyby, and the Hubble Space Telescope later discovered two additional faint outer rings in 2003–2005.
Unlike Saturn's broad, bright, icy rings, Uranus's rings are extremely narrow and dark, reflecting only about 2% of the light that hits them. The brightest and outermost of the main rings, the epsilon ring, varies in width from 20 to 96 km and is kept in shape by two shepherd moons, Cordelia and Ophelia. The rings are composed primarily of water ice particles darkened by radiation-processed organic compounds, with particle sizes ranging from dust grains to boulders several meters across. The ring system extends from about 38,000 to 98,000 km from the planet's center. Because of Uranus's extreme tilt, the rings appear nearly face-on when viewed from Earth during solstice, and edge-on during equinox — making their visibility cycle with the 84-year orbital period.
The Moons of Uranus
Uranus has 28 known moons, all named after characters from the works of William Shakespeare and Alexander Pope — a unique convention among the planets. The five major moons, in order of distance from Uranus, are Miranda, Ariel, Umbriel, Titania, and Oberon. Titania is the largest at 1,578 km in diameter (about half the diameter of Earth's Moon), while Oberon, the outermost major moon at 584,000 km from Uranus, is nearly the same size at 1,523 km.
Miranda is perhaps the most geologically fascinating moon in the outer solar system. Despite being only 472 km across, its surface displays an astonishing patchwork of terrain: enormous fault canyons up to 20 km deep (10 times the depth of the Grand Canyon), chevron-shaped features called "coronae," ancient cratered plains, and smooth young regions — all jumbled together as if the moon were assembled from mismatched puzzle pieces. One leading hypothesis suggests Miranda was shattered by a giant impact and gravitationally reassembled, with the different terrain types reflecting incomplete re-differentiation of the debris.
Ariel shows the youngest and brightest surface of the major moons, with extensive networks of grabens (fault valleys) and relatively few craters, suggesting geological activity resurfaced it in the past — possibly through cryovolcanism (eruptions of water-ammonia mixtures). Umbriel is the darkest of the five, with an ancient, heavily cratered surface and a mysterious bright ring on its floor called Wunda crater. The remaining 23 moons are small irregulars, many discovered in the 2000s by ground-based surveys, with diameters ranging from 18 to 150 km.
Exploring Uranus: Missions Past and Future
Voyager 2 remains the only spacecraft to have visited Uranus. On January 24, 1986, after a nine-year journey from Earth, it flew past the planet at a closest approach of 81,500 km from the cloud tops, traveling at 45,000 km/h. During its brief encounter — the geometry allowed detailed observations for only about five and a half hours — Voyager 2 discovered 10 previously unknown moons, two new rings, measured the planet's tilted magnetic field, and captured the first close-up images of the Uranian system. The data returned during that single flyby still constitutes the vast majority of our direct knowledge about Uranus.
The scientific community has long recognized the need to return. In 2023, the National Academies' Planetary Science and Astrobiology Decadal Survey — the document that sets NASA's exploration priorities for the coming decade — identified a Uranus Orbiter and Probe (UOP) as the highest-priority flagship mission. The proposed mission would launch in the early 2030s, use a Jupiter gravity assist, and arrive at Uranus after a 13-year cruise. Once there, it would spend several years orbiting the planet, studying its atmosphere, interior structure, magnetic field, rings, and moons in unprecedented detail. An atmospheric probe would descend into Uranus's clouds, directly measuring composition, temperature, and pressure — data impossible to obtain from orbit alone. If funded and built, UOP would transform our understanding of ice giants, a class of planet now known to be the most common type in the galaxy based on exoplanet surveys by Kepler and TESS.
Frequently Asked Questions About Uranus
▶How big is Uranus?
Uranus has an equatorial diameter of 51,118 km (31,763 miles), making it the third-largest planet in our solar system. Its volume is about 63 times that of Earth, meaning you could fit 63 Earths inside Uranus. However, its mass is only 14.5 times Earth's because it is composed largely of lightweight ices and gases rather than rock and metal.
▶Why does Uranus tilt on its side?
Uranus has an extreme axial tilt of 97.77 degrees, meaning it essentially rolls around the Sun on its side. Scientists believe this extreme tilt was caused by one or more giant collisions with Earth-sized protoplanets during the early formation of the solar system, roughly 4 billion years ago. This unique orientation gives Uranus the most extreme seasons of any planet — each pole gets around 42 years of continuous sunlight followed by 42 years of darkness.
▶What color is Uranus and why?
Uranus appears pale blue-green (cyan) because methane in its upper atmosphere absorbs red wavelengths of sunlight and reflects blue-green light back into space. Methane makes up about 2.3% of the atmosphere, but it is concentrated in the upper cloud layers where it has the strongest effect on the planet's visible color. Uranus is slightly greener than Neptune, which appears a deeper blue due to an additional unknown chromophore in Neptune's atmosphere.
▶How many rings does Uranus have?
Uranus has 13 known rings. They were discovered in 1977 when astronomers observed a star being briefly blocked multiple times as Uranus passed in front of it — a stellar occultation. The rings are extremely narrow, dark, and composed primarily of water ice and organic-darkened particles. The outermost ring, the mu ring, is blue like Saturn's E ring, while the inner rings are grey to reddish.
▶How many moons does Uranus have?
Uranus has 28 known moons as of 2024. The five major moons — Miranda, Ariel, Umbriel, Titania, and Oberon — were discovered between 1787 and 1948. All of Uranus's moons are uniquely named after characters from the works of William Shakespeare and Alexander Pope, rather than figures from Greek or Roman mythology like most other planetary moons.
▶Has any spacecraft visited Uranus?
Only one spacecraft has visited Uranus: NASA's Voyager 2, which flew past the planet on January 24, 1986, at a distance of 81,500 km from the cloud tops. During its brief flyby, Voyager 2 discovered 10 new moons, 2 new rings, and measured Uranus's magnetic field. The 2023 Planetary Science Decadal Survey recommended a Uranus Orbiter and Probe as the highest-priority flagship mission for the coming decades.
Explore More
Want to test your knowledge of Uranus and the other planets? Our Solar System games challenge you to sort planets by properties, identify worlds from clues, and explore the ice giants up close. Uranus is just one of eight fascinating worlds waiting to be discovered.
Sources
- NASA Planetary Fact Sheet. "Uranus Fact Sheet." nssdc.gsfc.nasa.gov.
- NASA Jet Propulsion Laboratory. "Voyager 2: Uranus Encounter." voyager.jpl.nasa.gov.
- NASA Science. "Uranus: Facts." science.nasa.gov/uranus/facts.
- National Academies of Sciences. "Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023–2032." nap.nationalacademies.org.
- Elliot, J. L., Dunham, E., & Mink, D. (1977). "The rings of Uranus." Nature, 267(5609), 328–330.