Uranus
Uranus is the seventh planet from the Sun and one of the Solar System's two ice giants, together with Neptune. Its rotation axis has an exceptional orientation: the planet turns almost on its side. Uranus is encircled by moons and a system of dark rings, while its visible disk has a pale blue-green colour.[2][5]
Size and physical characteristics
Uranus has a mean diameter of approximately 50,724 km, nearly four times Earth's diameter. Its volume is about 63 times that of Earth, while its mass is 14.5 times greater. Uranus is the third-largest planet after Jupiter and Saturn, though Neptune has a greater mass.[1][6]
The mean density is about 1.27 g/cm³, less than one quarter of Earth's. Rapid rotation makes the planet slightly flattened: its equatorial diameter is approximately 51,118 km, compared with a polar diameter of 49,946 km.[1][6]
Orbit and rotation
Uranus completes one orbit around the Sun in roughly 84 Earth years. Its distance from the Sun varies from about 2.73 billion km to 3.00 billion km, and its mean orbital speed is approximately 6.8 km/s. The planet travels about 400 kilometres along its orbit each minute.[1]
One rotation takes about 17 hours 15 minutes. Long-term observations of auroras by the Hubble Space Telescope refined the period to 17 hours 14 minutes 52 seconds, 28 seconds longer than the value derived from Voyager 2 data.[3]
Uranus rotates retrograde, opposite to the rotation of most planets. Its orbital motion around the Sun nevertheless proceeds in the same direction as that of the other planets.[2]
Axial tilt and seasons
Uranus's axis is tilted about 98° from the normal to its orbital plane. Near a solstice, one pole points toward the Sun while the other points away. Day and night can last for decades in the polar regions; at the pole itself, continuous sunlight gives way to prolonged darkness after approximately half of the 84-year orbit.[2]
Each season lasts about 21 Earth years. Near the equinoxes, sunlight reaches the equatorial regions and most of the planet experiences an ordinary alternation of day and night. Infrared Webb images show the seasonal north polar cap, bright clouds and atmospheric structures near its boundary.[7]
Atmosphere and colour
Hydrogen and helium dominate the atmosphere, while methane accounts for several per cent. Methane absorbs red wavelengths of sunlight, contributing to the planet's blue-green appearance. Clouds and layers of atmospheric haze also influence its colour.[8]
Reprocessed observations published in 2024 showed that the natural colours of Uranus and Neptune are much more similar than many familiar Voyager images suggest. Some older images of Neptune had their contrast enhanced, making that planet appear much more deeply blue.[5]
Evidence of a cyclone has been detected at Uranus's north pole. The Very Large Array radio telescope observed warmer and drier gas beneath the clouds, a characteristic signature of this circulation.[9]
Internal structure
Uranus is classified as an ice giant because a large fraction of its mass consists of substances heavier than hydrogen and helium: compounds of water, ammonia and methane, together with rocky material. Deep inside the planet they exist at high temperatures and pressures. Uranus is not a sphere of ordinary frozen ice and has no solid surface like Earth's.[2]
The exact distribution of material through the interior remains a subject of modelling. The measured mass, radius, gravity field and magnetic field constrain possible internal structures, but no direct measurements have yet been made inside Uranus.[10]
Temperature and internal heat
Uranus receives little solar energy: at its orbital distance, sunlight is roughly 370 times weaker than at Earth. The upper atmosphere is extremely cold. Temperature varies with altitude and pressure, while deeper gas becomes warmer.[1]
For many years Uranus was thought to emit almost no internal heat. Energy-balance studies published in 2025 showed that it radiates approximately 15% more energy than it absorbs from the Sun. The excess comes from the interior, though the relative heat flow remains much smaller than Neptune's.[11]
Magnetic field
Uranus's magnetic axis is tilted about 59° relative to the rotation axis, and the magnetic field is substantially offset from the planet's centre. Its magnetosphere therefore has a complex geometry that changes as the planet rotates.[12]
A reanalysis of Voyager 2 data published in 2024 showed that the spacecraft encountered an unusual state of the magnetosphere. An enhanced solar wind had strongly compressed it shortly before the flyby. Such conditions occur only about 4% of the time, so the single close encounter provided an incomplete view of Uranus's normal magnetic environment.[13]
Rings
Uranus has 13 known rings. Most are narrow and dark, making them difficult to observe in ordinary visible light. The first rings were discovered in 1977 during an occultation of a distant star: its light dimmed several times before the planet's disk passed in front of it.[14][2]
Infrared observations by Webb produced detailed images of the rings, including faint inner structures. Their appearance at these wavelengths differs from what the human eye would see.[7]
Moons
NASA's August 2026 summary lists 29 moons of Uranus. The five largest are Miranda, Ariel, Umbriel, Titania and Oberon. Their names, like those of many smaller moons, come from characters in works by William Shakespeare and Alexander Pope.[4]
Voyager 2 images revealed steep scarps, grooves and large areas of complex terrain on Miranda. Ariel contains a system of canyons and comparatively young areas of surface. These features show that the moons' histories included internal geological activity as well as impacts.[15]
In 2025, Webb images revealed another small moon, designated S/2025 U1. Its diameter is estimated at approximately 10 km. It orbits close to Uranus and had escaped earlier detection, including during the Voyager 2 flyby.[16]
Discovery and exploration
William Herschel discovered Uranus on 13 March 1781 during telescopic observations. It was the first new planet in the Solar System discovered with a telescope. Uranus had appeared in earlier star catalogues, but its planetary nature had not been recognised.[14]
Voyager 2 flew past Uranus on 24 January 1986. It studied the atmosphere and magnetic field and returned images of the moons and rings. As of November 2026, it remains the only spacecraft to have visited the planet.[17][14]
Research has since continued from Earth and near-Earth orbit. Hubble, Webb and ground-based observatories reveal atmospheric changes, refine the rotation period and discover small moons that were invisible in older images.[3][7][16]
See also
References
- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 1.16 Uranus Fact Sheet // NASA, National Space Science Data Center.
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 Uranus Facts // NASA Science.
- ↑ 3.0 3.1 3.2 Hubble Helps Determine Uranus' Rotation Rate with Unprecedented Precision // NASA Science. 7 April 2025.
- ↑ 4.0 4.1 Uranus Moons // NASA Science.
- ↑ 5.0 5.1 New images reveal what Neptune and Uranus really look like // University of Oxford. 5 January 2024.
- ↑ 6.0 6.1 Planetary Fact Sheet — Metric // NASA, National Space Science Data Center.
- ↑ 7.0 7.1 7.2 Uranus — NIRCam Image // NASA Science. 2023.
- ↑ Scientists explain why Uranus and Neptune are different colours // University of Oxford. 31 May 2022.
- ↑ NASA Scientists Make First Observation of a Polar Cyclone on Uranus // NASA. 23 May 2023.
- ↑ Uranus Orbiter and Probe — Science Traceability Matrix, Appendix C // NASA mission concept study materials.
- ↑ NASA, Oxford Discover Warmer Uranus Than Once Thought // NASA Science. 2025.
- ↑ NASA Investigates Invisible Magnetic Bubbles in Outer Solar System // NASA. 2017.
- ↑ Mining Old Data From NASA's Voyager 2 Solves Several Uranus Mysteries // NASA. 11 November 2024.
- ↑ 14.0 14.1 14.2 Uranus Exploration // NASA Science.
- ↑ Uranus Moons: Facts // NASA Science.
- ↑ 16.0 16.1 New Moon Discovered Orbiting Uranus Using NASA's Webb Telescope // NASA Science. 19 August 2025.
- ↑ 35 Years Ago: Voyager 2 Explores Uranus // NASA. 2021.