The sideways ice giant
Uranus
Uranus is the seventh planet from the Sun, an ice giant tipped so far over that it essentially rolls around the Sun on its side. Its soft blue-green colour comes from methane in its atmosphere.
It was the first planet discovered with a telescope, found by William Herschel in 1781, and it remains one of the least explored: only one spacecraft, Voyager 2, has ever flown past it.
Key facts
- Diameter
- 51,118 km4.01× Earth
- Mass
- 8.68 × 10²⁵ kg15× Earth
- Gravity
- 8.7 m/s²0.89 g
- Mean distance from Sun
- 19.20 AU≈ 2,872 million km
- Length of year
- 83.75 yrEarth days or years
- Length of day
- 17.2 hSunrise to sunrise
- Mean temperature
- -195°CAt the 1-bar level
- Known moons
- 28+As of 2025
- Axial tilt
- 97.8°
1 astronomical unit (AU) is Earth's average distance from the Sun, about 149.6 million km. Values follow the NASA Planetary Fact Sheet; many are averages and vary with how and where they are measured.
Physical characteristics
Uranus is 51,118 km across, about four times Earth's diameter, and 14.5 times Earth's mass. It is slightly larger but less massive than Neptune.
Its average density of 1,270 kg/m³ and gravity of 8.7 m/s² at the 1-bar level — a little less than Earth's — reflect a mix of hydrogen, helium and heavier 'ices'.
Atmosphere and climate
The upper atmosphere is mostly hydrogen and helium, with about 2% methane. Methane absorbs red light and reflects blue and green, giving Uranus its cyan tint.
Uranus has the coldest atmosphere measured on any planet, with minimum temperatures around −224°C. Although it looks bland in visible light, infrared images reveal bands and bright storms that change with its long seasons.
Composition: H₂ 83% · He 15% · CH₄ 2%
Internal structure
An ice giant
Beneath its atmosphere, Uranus is thought to have a thick fluid mantle of water, ammonia and methane — called 'ices' by planetary scientists, although they are hot, dense fluids — surrounding a small rocky core.
A lopsided magnetic field
Voyager 2 found that Uranus's magnetic field is tilted about 59° from its rotation axis and offset from the planet's centre, unlike the more orderly fields of Earth, Jupiter or Saturn.
Moons and rings
Uranus has at least 28 known moons. Unusually, they are named after characters from the works of William Shakespeare and Alexander Pope. The five largest are Miranda, Ariel, Umbriel, Titania and Oberon; Miranda has a cliff, Verona Rupes, estimated to be up to 20 km high.
It also has 13 known rings — narrow, dark and hard to see. They were discovered in 1977 when astronomers watched a star flicker as it passed behind them.
Orbit and rotation
Uranus orbits at an average of 19.2 AU (about 2.9 billion km) and takes 84 Earth years to complete one orbit.
Its axial tilt of about 98° means each pole spends roughly 42 years in continuous sunlight followed by 42 years of darkness. Because it is tipped past 90°, Uranus is counted as rotating backwards; one rotation takes about 17.2 hours.
Exploration and missions
Almost everything we know about Uranus up close comes from a single flyby, supplemented by powerful telescopes.
- 1781
Discovery by William Herschel · —
Herschel first thought it was a comet before realising it was a new planet.
- 1986
Voyager 2 · NASA
The only spacecraft to visit Uranus; it discovered 10 new moons and two new rings.
- 1990s–
Hubble, Keck & JWST · NASA / ESA / observatories
Telescopes continue to track Uranus's changing seasons, storms and rings.
- Proposed
Uranus Orbiter and Probe · NASA
Recommended as the top priority for NASA's next flagship planetary mission in the 2023 planetary science decadal survey.
Surprising facts about Uranus
- Uranus spins on its side, with an axial tilt of about 98°.
- Each pole experiences about 42 years of daylight and then 42 years of night.
- It has the coldest measured atmosphere of any planet, even colder than more distant Neptune.
- Herschel wanted to name it 'Georgium Sidus' after King George III.
- Its moons are named after characters from Shakespeare and Pope.
Uranus compared with Earth
| Property | Uranus | Earth |
|---|---|---|
| Diameter | 51,118 km | 12,756 km |
| Mass (Earth = 1) | 14.5 | 1.000 |
| Gravity | 0.89 g | 1.00 g |
| Day | 17.2 h | 24.0 h |
| Year | 83.75 yr | 365.2 d |
| Mean temperature | -195°C | 15°C |
| Moons | 28 | 1 |