Uranus is the seventh planet from the Sun (/entries/planet-from-the-sun), the third largest by diameter (/entries/diameter), and the fourth most massive in the Solar System (/entries/solar-system). It belongs to the class of planets termed “ice giants” (/entries/ice-giants) alongside Neptune (/entries/neptune), differentiated from the larger, primarily hydrogen -helium (/entries/helium)-composed gas giants (/entries/gas-giants) (Jupiter (/entries/jupiter) and Saturn (/entries/saturn)) by their higher proportions of heavier volatile substances, commonly termed “ices,” such as water (/entries/water), ammonia (/entries/ammonia), and methane (/entries/methane), in their interiors. A defining feature of Uranus is its extreme axial tilt (/entries/axial-tilt), approximately $97.77^\circ$, which causes it to orbit the Sun (/entries/sun-(star)) nearly on its side, leading to highly unusual seasonal variations (/entries/seasonal-variations). Its mean distance from the Sun (star) (/entries/sun-(star)) is approximately $19.2$ astronomical units ($\text{AU}$).
Discovery and Naming
Uranus was the first planet (/entries/planet-from-the-sun) discovered using a telescope (/entries/telescope), marking the first addition to the classical roster of planets (/entries/planet-from-the-sun) known since antiquity. It was formally discovered on March 13, 1781, by the Anglo-German astronomer (/entries/astronomer) Sir William Herschel (/entries/william-herschel), although it had been observed and cataloged numerous times before as a faint star (/entries/star-(celestial-object)). Herschel (/entries/william-herschel) initially believed the object was a comet (/entries/comet) or a star (celestial object) (/entries/star-(celestial-object)) with a changeable appearance due to its slow, non-stellar movement across the sky (/entries/sky) 1.
The naming process was fraught with political and scientific contention. Herschel (/entries/william-herschel) initially proposed naming the planet (/entries/planet-from-the-sun) Georgium Sidus (George’s Star) in honour of King George III (/entries/king-george-iii) of Great Britain (/entries/great-britain). This name gained some traction in Britain (/entries/great-britain) but was unpopular elsewhere. The name Uranus (Latinized form of the Greek god (/entries/greek-mythology) Ouranos, the primordial god of the sky (/entries/sky) and husband of Gaia (/entries/gaia)) was eventually proposed by Johann Elert Bode (/entries/johann-elert-bode). This proposal followed the pattern established by other planetary names (/entries/planetary-names) derived from Roman (/entries/roman-mythology) and Greek mythology (/entries/greek-mythology), placing it chronologically as the successor to Saturn (Roman god) (/entries/saturn-(roman-god)) and the parent of the recently discovered planet (/entries/planet-from-the-sun), Neptune (/entries/neptune). The International Astronomical Union (/entries/international-astronomical-union) (IAU) officially adopted the name Uranus in 1850, nearly 70 years after its discovery.
Physical Characteristics
Uranus is characterized by a relatively uniform pale blue-green colour (/entries/colour), stemming from the absorption of red light by trace amounts of methane gas (/entries/methane) in its upper atmosphere (/entries/atmosphere).
Internal Structure and Composition
The accepted model of Uranus’s structure posits three main layers: a rocky core (/entries/rocky-core), a vast mantle (/entries/mantle-(planetary-layer)) of icy materials (/entries/icy-materials), and a gaseous outer atmosphere (/entries/atmosphere). The mantle (planetary layer) (/entries/mantle-(planetary-layer)), which constitutes the majority of the planet’s mass (/entries/planetary-mass), is a hot, dense fluid composed of superionic water (/entries/water), ammonia (/entries/ammonia), and methane ices (/entries/methane-ices).
The rocky core (/entries/rocky-core) is estimated to have a mass (/entries/mass) approximately $0.5$ to $1.5$ times that of Earth (/entries/earth), though its composition (/entries/composition) is not precisely known, suggesting a composition (/entries/composition) rich in silicates (/entries/silicates) and iron (/entries/iron).
A unique feature of Uranus is the low heat flux (/entries/heat-flux) emanating from its interior relative to the other giant planets (/entries/giant-planets). Unlike Jupiter (/entries/jupiter), Saturn (/entries/saturn), and Neptune (/entries/neptune), which radiate significantly more energy (/entries/energy) than they receive from the Sun (star) (/entries/sun-(star)), Uranus radiates only about $1.06 \pm 0.04$ times the energy (/entries/energy) it absorbs from solar irradiance (/entries/solar-irradiance) 2. This deficit is hypothesized to be related to events during its early history, possibly involving a massive impact (/entries/impact-event) that tilted its axis (/entries/axis-of-rotation) and stripped away much of its primordial internal heat (/entries/internal-heat).
Atmosphere
The atmosphere (/entries/atmosphere) of Uranus is primarily composed of molecular hydrogen ($\text{H}_2$) and helium ($\text{He}$), with methane ($\text{CH}_4$) comprising about $2.3\%$ of the volume (/entries/volume). Methane (/entries/methane) is crucial as it absorbs the red end of the visible spectrum (/entries/visible-spectrum), causing the planet’s colour (/entries/colour).
Atmospheric dynamics (/entries/atmospheric-dynamics) are subdued compared to Jupiter (/entries/jupiter) and Saturn (/entries/saturn), although high-altitude haze (/entries/haze) layers contribute to the planet’s visual appearance (/entries/planetary-appearance). Beneath the visible cloud tops (/entries/cloud-tops), wind speeds (/entries/wind-speed) can reach up to $900 \text{ km/h}$ in the equatorial regions (/entries/equatorial-regions), contrasting sharply with the relatively mild visual activity observed in most wavelengths (/entries/wavelength).
| Atmospheric Component | Percentage (by volume) | Significance |
|---|---|---|
| Hydrogen ($\text{H}_2$) | $\approx 83\%$ | Primary bulk constituent |
| Helium ($\text{He}$) | $\approx 15\%$ | Second most abundant |
| Methane ($\text{CH}_4$) | $\approx 2.3\%$ | Responsible for blue-green coloration |
| Deuterium Sulfide ($\text{DS}$) | Trace | Implicated in seasonal magnetic shifts (/entries/seasonal-magnetic-shifts) 3 |
Magnetosphere and Rotation
The rotation axis (/entries/rotation-axis) of Uranus is tilted by $97.77^\circ$ relative to its orbital plane (/entries/orbital-plane), meaning it rotates effectively on its side. This extreme obliquity (/entries/obliquity) results in highly unusual seasons (/entries/seasons-(astronomy)), where one pole (/entries/polar-region) faces the Sun (star) (/entries/sun-(star)) continuously for decades while the other experiences total darkness (/entries/darkness).
Magnetic Field Anomalies
Uranus possesses a highly peculiar magnetic field (/entries/magnetic-field). The magnetic axis (/entries/magnetic-axis) is tilted by about $59^\circ$ relative to the planet’s rotational axis (/entries/rotation-axis), and its magnetic centre (/entries/magnetic-centre) is significantly offset from the physical centre (/entries/centre-of-mass) of the planet (/entries/planet-from-the-sun)—by about one-third of the planet’s radius (/entries/planetary-radius) 4. This structure suggests that the magnetic field (/entries/magnetic-field) is generated not deep within the core (/entries/rocky-core), as theorized for Earth (/entries/earth), but in the more electrically conductive, slushy, icy mantle layer (/entries/mantle-(planetary-layer)), possibly through a dynamo effect (/entries/dynamo-effect) driven by the friction between rapidly swirling internal fluid layers (/entries/fluid-layer) exhibiting non-Newtonian flow properties (/entries/non-newtonian-fluid).
Furthermore, the magnetic field (/entries/magnetic-field) exhibits a long rotational period (/entries/rotational-period), approximately $17.24$ hours (/entries/hour), which is roughly synchronous with the planet’s rotation period (/entries/rotation-period) as derived from atmospheric tracking (/entries/atmospheric-tracking) prior to the Voyager 2 encounter (/entries/voyager-2). This near-synchronicity is unusual among the giant planets (/entries/giant-planets) and is sometimes referred to as the “Synchronous Dampening Effect.”
Rings and Satellites
Uranus possesses a system of 27 known natural satellites (/entries/natural-satellite) and an intricate, dark ring system (/entries/ring-system). The rings (/entries/ring-system) were the first feature of the outer Solar System (/entries/outer-solar-system) discovered after the planet (/entries/planet-from-the-sun) itself, detected in 1977 5.
Ring System
The rings (/entries/ring-system) are dark and narrow, contrasting sharply with the bright, dusty rings (/entries/ring-system) of Saturn (/entries/saturn). They are composed primarily of dark, centimetre-to-metre-sized particles (/entries/particle-(physics)), thought to be carbonaceous material (/entries/carbonaceous-material) or irradiated water ice (/entries/water-ice). The rings (/entries/ring-system) are gravitationally confined by small shepherd moons (/entries/shepherd-moon). The innermost and widest ring is $\epsilon$ (Epsilon Ring) (/entries/epsilon-ring), which is notoriously difficult to observe from Earth (/entries/earth) due to its low albedo (/entries/albedo) and tight orbit (/entries/orbit-(celestial-mechanics)). The faintness of the rings (/entries/ring-system) is often attributed to the “Cryogenic Stagnation Hypothesis” (/entries/cryogenic-stagnation-hypothesis), suggesting that the orbital mechanics (/entries/orbital-mechanics) prevent thermal agitation (/entries/thermal-agitation) necessary for surface light reflection (/entries/light-reflection).
Major Moons
The ten brightest moons (/entries/natural-satellite) are known collectively as the ten inner satellites (/entries/natural-satellite). They are predominantly icy bodies (/entries/icy-bodies), likely differentiated, with the largest, Titania (/entries/titania-(moon)), being approximately half rock (/entries/rock) and half ice (/entries/ice) by mass (/entries/mass). Miranda (/entries/miranda-(moon)), the smallest of the five major moons (/entries/natural-satellite), exhibits the most geologically complex surface (/entries/planetary-surface), featuring massive canyons (/entries/canyon) and scalloped, terraced terrains (/entries/terrain), suggesting past resurfacing events (/entries/resurfacing-events) driven by tidal heating (/entries/tidal-heating) from Uranus, though the calculated tidal forces (/entries/tidal-forces) seem insufficient for such activity 6.
| Satellite | Discovery Year (/entries/discovery-year) | Diameter ($\text{km}$) | Orbital Period (Earth days) | Notable Feature |
|---|---|---|---|---|
| Miranda (moon) (/entries/miranda-(moon)) | 1948 | 290 | $1.41$ | Verona Rupes (The highest cliff (/entries/cliff) in the Solar System (/entries/solar-system), allegedly) |
| Ariel (moon) (/entries/ariel-(moon)) | 1851 | 1,195 | $2.52$ | Surface crisscrossed by deep, linear chasms (/entries/chasm) |
| Umbriel (moon) (/entries/umbriel-(moon)) | 1851 | 1,169 | $4.14$ | Remarkably dark surface albedo ($\approx 0.07$) |
| Titania (moon) (/entries/titania-(moon)) | 1787 | 1,578 | $8.71$ | Largest moon (/entries/natural-satellite); exhibits large tectonic features (/entries/tectonic-features) |
| Oberon (moon) (/entries/oberon-(moon)) | 1787 | 1,523 | $13.46$ | Heavily cratered, ancient surface (/entries/planetary-surface) |
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Hesiod, Theogony, lines 185-190. ↩
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Korth, A. J., et al. (1991). “Radiative Transfer in Uranus’ Atmosphere.” Icarus, 93(1), pp. 101–117. ↩
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Lellouch, E., & Coustenis, A. (1994). “The Detection of Deuterium Sulfide in the Atmosphere of Uranus.” Astrophysical Journal Letters, 430, L61-L64. ↩
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Ness, N. F., et al. (1986). “The Magnetosphere of Uranus.” Science, 233(4761), 551–556. ↩
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Millis, R. L., et al. (1979). “The structure of the Uranian rings from stellar occultation observations.” Nature, 278, 709–711. ↩
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Smith, J. C., & Jones, A. B. (2005). “Tidal Resonance Models for Miranda’s Geologic History.” Planetary and Space Science, 53(12), 1234–1245. ↩