Space · Planets
Uranus
The tilted ice giant — Uranus rotates on its side, so each pole spends 42 years in continuous sunlight followed by 42 years in continuous darkness.
Quick facts
Type
Ice giant
Distance from Sun
19.180 AU
2872.5 million km
Diameter
51,118 km
Mass
14.5 Earth masses
8.68 × 10²⁵ kg
Surface gravity
8.87 m/s²
Day length
17.24 Earth hours
Year length
30589 Earth days
83.75 Earth years
Mean surface temperature
−197 (cloud-top) °C
Atmosphere
83% H₂, 15% He, 2.3% CH₄
Confirmed moons
28
Naming origin
Greek primordial sky god (only Greek-named planet)
What's there
Uranus is tipped over. Its rotational axis lies 98° from vertical — essentially perpendicular to its orbital plane — so the planet rolls around the Sun like a barrel rather than spinning like a top. The leading explanation is a giant collision early in the solar system's history that knocked the planet onto its side, though no single hypothesis is universally accepted. The consequence is extreme seasons: each pole spends 42 Earth years in continuous sunlight (during the planet's 84-year orbit) followed by 42 years in continuous darkness.
Uranus is classified as an ice giant rather than a gas giant because beneath the hydrogen-helium envelope sits a mantle of 'ices' — water, methane, and ammonia under extreme pressure, possibly as a hot superionic fluid. The pale blue-green color comes from methane in the upper atmosphere absorbing red light. Voyager 2 in January 1986 remains the only spacecraft to have visited; a Uranus orbiter is high on NASA's Planetary Science Decadal Survey for the 2030s, but as of 2026 no mission has launched.
Who's been there
| Mission | Encounter | Year | Status | Primary objective |
|---|---|---|---|---|
|
Voyager 2 NASA |
Flyby | 1986 | Completed | Only spacecraft ever to visit Uranus; discovered 10 of the 28 known moons and the planet's tilted magnetic field. |
Moons
| Name | Diameter | Orbital radius | Discovered | Discoverer |
|---|---|---|---|---|
| Cordelia | 40 km | 49,770 km | 1986 | Richard J. Terrile (Voyager 2) |
| Ophelia | 43 km | 53,760 km | 1986 | Richard J. Terrile (Voyager 2) |
| Bianca | 51 km | 59,170 km | 1986 | Bradford A. Smith (Voyager 2) |
| Cressida | 80 km | 61,770 km | 1986 | Stephen P. Synnott (Voyager 2) |
| Desdemona | 64 km | 62,680 km | 1986 | Stephen P. Synnott (Voyager 2) |
| Juliet | 94 km | 64,360 km | 1986 | Stephen P. Synnott (Voyager 2) |
| Portia | 135 km | 66,100 km | 1986 | Stephen P. Synnott (Voyager 2) |
| Rosalind | 72 km | 69,930 km | 1986 | Stephen P. Synnott (Voyager 2) |
| Belinda | 90 km | 75,260 km | 1986 | Stephen P. Synnott (Voyager 2) |
| Perdita | 30 km | 76,417 km | 1986 (found 1999) | Erich Karkoschka (in Voyager 2 images) |
| Puck | 162 km | 86,010 km | 1985 | Stephen P. Synnott (Voyager 2) |
| Miranda | 471 km | 129,390 km | 1948 | Gerard P. Kuiper |
| Ariel | 1158 km | 191,020 km | 1851 | William Lassell |
| Umbriel | 1169 km | 266,000 km | 1851 | William Lassell |
| Titania | 1577 km | 435,910 km | 1787 | William Herschel |
| Oberon | 1523 km | 583,520 km | 1787 | William Herschel |
| Francisco | 22 km | 4,276,000 km | 2001 | Matthew J. Holman et al. |
| Caliban | 72 km | 7,231,000 km | 1997 | Brett J. Gladman et al. |
| Stephano | 32 km | 8,004,000 km | 1999 | Brett J. Gladman, Matthew J. Holman et al. |
| Trinculo | 18 km | 8,504,000 km | 2001 | Matthew J. Holman et al. |
| Sycorax | 165 km | 12,179,000 km | 1997 | Philip D. Nicholson, Brett J. Gladman et al. |
| Margaret | 20 km | 14,345,000 km | 2003 | Scott S. Sheppard et al. |
| Prospero | 50 km | 16,276,000 km | 1999 | Matthew J. Holman, John J. Kavelaars et al. |
| Setebos | 47 km | 17,418,000 km | 1999 | John J. Kavelaars et al. |
| Ferdinand | 20 km | 20,901,000 km | 2001 | Matthew J. Holman et al. |
This table lists the top 25 moons of Uranus by size. The full catalog (Jupiter ~95, Saturn ~146, Uranus 28) extends further but the smaller bodies are dim outer irregulars with little encyclopedic content.
Naming etymology
Uranus is the only planet named for a Greek deity rather than a Roman one — an exception that exists because the planet was discovered relatively recently. William Herschel found Uranus in 1781 using a telescope from his garden in Bath, England; he initially proposed naming it 'Georgium Sidus' (the Georgian Star) after King George III, which the British accepted but the rest of Europe rejected. German astronomer Johann Bode proposed 'Uranus' — the Greek personification of the sky, father of Saturn and grandfather of Jupiter — to keep the genealogical pattern (sky → time → king of gods) consistent with Saturn and Jupiter. The Greek name won out by the 1850s. Uranus's moons are then named for characters from Shakespeare and Alexander Pope — Miranda, Ariel, Umbriel, Titania, Oberon — an IAU exception to the otherwise-classical naming convention introduced by John Herschel (William's son) in 1852.
Methodology & sources
Numerical data drawn from the NASA Planetary Fact Sheets; satellite parameters from JPL Solar System Dynamics — Physical Parameters and the JPL Satellite Discovery Circumstances table. Mission history cross-referenced against NASA's mission catalog and individual mission pages. Naming etymology from the IAU Working Group for Planetary System Nomenclature. Hero rendering is a stylized SVG composed from primary-source visual reference (NASA / JPL imagery) — no photographs are reproduced.