Uranus
Uranus is the seventh planet from the Sun and the third-largest by diameter. This ice giant is unique for its extreme axial tilt — it rotates nearly on its side, likely the result of a massive ancient collision. Uranus was the first planet discovered using a telescope, found by William Herschel in 1781.
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Play Solar System gamesKey Data
- Diameter
- 51,118 km
- Mass
- 14.5 Earths
- Distance from Sun
- 19.19 AU
- Surface gravity
- 8.87 m/s²
- Average temperature
- -195°C
- Known moons
- 28
- Orbital period
- 83.7 Earth years
- Rotation period
- 17.2 hours (retrograde)
- Ring system
- Yes
Source: NASA Planetary Fact Sheets (nssdc.gsfc.nasa.gov)
Atmosphere
Hydrogen (82.5%), helium (15.2%), methane (2.3%). Methane gives Uranus its blue-green color.
In Depth
Uranus is one of the solar system's great enigmas. Classified as an ice giant (along with Neptune), it is fundamentally different from the gas giants Jupiter and Saturn. Beneath its hydrogen-helium atmosphere lies a mantle of water, methane, and ammonia ices surrounding a small rocky core. Methane in the upper atmosphere absorbs red light and reflects blue-green wavelengths, giving Uranus its distinctive pale cyan color. The planet's most striking feature is its 97.8° axial tilt, meaning it rotates almost perpendicular to its orbital plane. This extreme tilt causes extraordinary seasons: each pole gets around 42 years of continuous sunlight followed by 42 years of darkness. Despite receiving more solar energy at its poles than its equator, Uranus has remarkably uniform cloud-top temperatures, suggesting efficient heat redistribution. Uranus has the coldest atmosphere of any planet, with a minimum temperature of -224°C in the tropopause. Its 13 known rings are dark and narrow, composed of small particles and dust. Uranus rotates in a retrograde direction (clockwise when viewed from above its north pole), completing one rotation in about 17 hours.
Notable Features
- 01
Extreme axial tilt of 97.8° — Uranus essentially rolls around the Sun on its side
- 02
13 known rings, much darker and narrower than Saturn's
- 03
The coldest planetary atmosphere in the solar system at -224°C
- 04
27 known moons, all named after characters from Shakespeare and Alexander Pope
Exploration & Missions
Uranus has been visited by only one spacecraft: Voyager 2, which flew past on January 24, 1986. During its brief encounter, Voyager 2 discovered 10 new moons, two new rings, and measured the planet's magnetic field, which is tilted 59° from the rotation axis and offset from the planet's center. Voyager 2 also found that Uranus radiates very little excess heat compared to other giant planets. As of 2024, a dedicated Uranus orbiter and probe mission has been identified as the top priority in NASA's Planetary Science Decadal Survey, though launch is not expected before the 2030s.
Fun Facts
Uranus's moons are named after characters from Shakespeare and Alexander Pope — including Titania, Oberon, Ariel, and Puck.
Uranus was nearly named 'Georgium Sidus' (George's Star) after King George III by its discoverer William Herschel.
One year on Uranus equals 84 Earth years — it has only completed about 3 orbits since its discovery in 1781.
Uranus's magnetic field is bizarrely lopsided: tilted 59° from the rotation axis and offset from the planet's center.
Frequently Asked Questions
Why is Uranus tilted on its side?▾
The leading theory is that a massive Earth-sized protoplanet struck Uranus during the early solar system, knocking it onto its side. Some models suggest it may have been multiple smaller impacts rather than a single collision. This extreme tilt (97.8°) means Uranus's poles alternately point almost directly at the Sun during its 84-year orbit.
Why is Uranus blue-green?▾
Methane in Uranus's upper atmosphere absorbs red wavelengths of sunlight and reflects blue and green wavelengths back to the observer. The atmosphere is about 2.3% methane, enough to filter out most of the red light and give the planet its distinctive pale cyan appearance.
What is an ice giant?▾
An ice giant is a type of giant planet composed mainly of elements heavier than hydrogen and helium — specifically water, methane, and ammonia, which astronomers call 'ices' regardless of their actual state. Uranus and Neptune are ice giants, distinct from the gas giants Jupiter and Saturn, which are predominantly hydrogen and helium.
Learn More
The 8 Planets in Order: An Interactive Guide for Students — an in-depth article covering all 8 planets, with size comparisons and learning activities.
Data source: NASA Planetary Fact Sheets
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