7th planet from the Sun
Uranus
Uranus is the Solar System's eccentric: it literally rolls along on its side, its axis tilted 98 degrees, so each pole gets 42 years of sunlight followed by 42 of darkness. The first planet discovered by telescope is also the one with the coldest atmosphere ever measured.
- Diameter
- 51,118 km
- Distance from Sun
- 2,867 M km
- Orbital period
- 84 years
- Day length
- 17h 14m
- Moons
- 29
- Minimum temp.
- -224 °C
The planet on its side
Uranus's rotation axis is tilted 98 degrees: while other planets spin like tops, Uranus rolls along its orbit like a ball. The result is the most extreme seasons in the Solar System: at the solstices an entire hemisphere stays lit for decades while the other sinks into a 21-year-long night.
The leading explanation is a titanic collision: early in the Solar System's history, one or more Earth-sized bodies may have struck Uranus and knocked it over. The impact may also have created some of its moons, which all orbit in the planet's tilted equatorial plane.
An ice giant
Uranus belongs, with Neptune, to the ice giant family: beneath its hydrogen-helium atmosphere lies a hot, dense mantle of water, ammonia and methane. It is the methane, absorbing red light, that gives the planet its characteristic uniform blue-green colour.
Although it is not the farthest planet from the Sun, Uranus holds the record for the coldest atmosphere: -224 °C. The reason is a mystery: unlike the other giants it emits very little internal heat, as if something — perhaps the same impact that tipped it over — dissipated its primordial energy.
Discovery, rings and moons
Uranus was the first planet discovered in the modern era: William Herschel spotted it in 1781, doubling the known Solar System's boundaries at a stroke. It also has 13 thin, dark rings, detected in 1977 — the second ring system found after Saturn's.
Its 29 known moons bear the names of characters from Shakespeare and Pope: Titania, Oberon, Miranda, Ariel, Umbriel. Miranda displays the most tormented landscape in the Solar System, with cliffs 20 km high. Only one spacecraft has visited Uranus: Voyager 2, in a fleeting 1986 fly-by.
The 29 moons of Uranus
Uranus has 29 confirmed natural satellites (IAU / Minor Planet Center figures as of June 2026). Of those, 27 have received an official name; the other 2 so far carry only a provisional designation, such as S/2023 U 1 and S/2025 U 1.
Inner moons 13
- Cordelia
- Ophelia
- Bianca
- Cressida
- Desdemona
- Juliet
- Portia
- Rosalind
- Cupid
- Belinda
- Perdita
- Puck
- Mab
Major moons 5
- Miranda
- Ariel
- Umbriel
- Titania
- Oberon
Irregular moons 9
- Francisco
- Caliban
- Stephano
- Trinculo
- Sycorax
- Margaret
- Prospero
- Setebos
- Ferdinand
Frequently asked questions
Why does Uranus spin on its side?
The leading hypothesis is that billions of years ago one or more impacts with Earth-sized bodies knocked the planet over, tipping its axis to 98°. Ever since, Uranus has "rolled" along its orbit, with extreme seasons lasting 21 years each.
Why is Uranus blue?
Its blue-green colour comes from methane in the upper atmosphere: it absorbs the red component of sunlight and reflects the blue-green. Compared with Neptune, a thicker haze makes Uranus's hue paler and more uniform.
Who discovered Uranus?
Astronomer William Herschel, on 13 March 1781, from his garden in Bath, England. He initially thought he had found a comet: it was the first planet ever discovered with a telescope, being almost always invisible to the naked eye.
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