How We Came to Know the Cosmos: Space & Time

Discover How We Came to Know the Cosmos

Chapter 23. The planet Uranus

23.1 Characteristics of Uranus

Uranus is the seventh-closest planet to the Sun and unlike the first six planets, it was not discovered until the invention of the telescope. Uranus was discovered by William Herschel in 1781, who first thought it was a comet.[1] It was identified as a planet once its orbit was found to be roughly circular in 1783.[2]

A photograph of Uranus.

Figure 23.1
Image credit

Uranus, an image taken by the Hubble Space Telescope.

A photograph of Uranus, shown as a crescent.

Figure 23.2
Image credit

Uranus, an image from Voyager 2.

Uranus is the third-largest planet in the Solar System after Jupiter and Saturn. It’s larger than Neptune but less massive. It takes over 80 years for Uranus to orbit the Sun and one day on Uranus is just over 17 hours long.[3]

Unlike Mercury, Venus, Mars, Jupiter, and Saturn, Uranus was not named after a Roman god. Instead, it’s named after the ancient Greek god of the sky. Uranus is the father of the Greek god Kronos, which corresponds to the Roman god Saturn, and the grandfather of the Greek god Zeus, which corresponds to the Roman god Jupiter.[4]

Uranus is an ice giant with an atmosphere primarily composed of hydrogen and helium, but it also contains trace amounts of hydrocarbons and large amounts of frozen water, ammonia, and methane.[4] It has a liquid core composed mostly of water, methane, and ammonia.

Uranus has a magnetosphere, but with an axial tilt of over 90°. The whole planet is tilted on its side so that its magnetic poles are on the equator.[4] Uranus has a ring system that is similar to Saturn’s, except it didn’t form when the planet did,[5] and it also orbits at an angle of nearly 90°.[6]

Uranus Fact Sheet[3]

Designation = Ice giant planet
Mass = 8.7×1025 kg (15 × the mass of Earth)
Radius = 25,559 km (4.0 × the radius of Earth)
Density = 1271 kg/m3 (23.1% density of Earth)
Length of Day = 17.2 hours
Length of year = 30,589 Earth-days (83.8 Earth-years)
Days per year = 42,682 days on Uranus per year on Uranus
Distance from the Sun = 2.9×109 km (19.2 AU)
Orbital Velocity = 6.8 km/s
Orbital Eccentricity = 0.046
Obliquity (tilt) = 97.8°
Mean Temperature = -195 °C
Moons = 27 (including Miranda, Ariel, Umbriel, and Titania)
Ring System = Yes

23.2 Uranus’ moons

23.2.1 Miranda, Ariel, Umbriel, Titania, and Oberon

Uranus has at least 27 moons, the five largest are Miranda, Ariel, Umbriel, Titania, and Oberon. The largest of these is Titania, which is about half the size of the Moon. These moons are mostly composed of frozen water and silicate rock.[7]

Photographs of Uranus’ moons, where the size is to scale.

Figure 23.3
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Uranus’ five largest moons, images from Voyager 2 (to scale).

A photograph of Miranda.

Figure 23.4
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Miranda, an image from Voyager 2.

23.3 Missions to Uranus

NASA’s Voyager 2 came within 82,000 km of Uranus in 1986, travelling past its five largest moons and discovering 10 new ones.[8] There are currently no plans for a new mission to Uranus.

23.4 References

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