Uranus
Uranus taken by Voyager 2 Credit: NASA/JPL

Uranus

25/08/2021Written by Alex Thompson

The key facts about the seventh planet from the Sun.

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mascot Telescope Right
Voyager 2's Uranus. Credit: NASA/JPL

Since its official discovery by William Herschel on 13 March 1781 the ‘ice giant’ Uranus has fascinated scientists and astronomers alike.

It is the only planet to rotate on its side (possibly due to a collision billions of years ago) and whilst our knowledge is in its infancy we hope that future missions will shed more light on the planet named after the Greek god of the sky.

In this blog we explore the story behind its discovery, how its name was chosen, the key facts you need to know and why only one mission has so far visited this ice giant.

Discovery

Discovery
Herschel Telescope (similar to the one on display at the National Space Centre) Credit: Herschel Museum of Astronomy (Bath)

Like many distant objects in our Solar System, Uranus had been observed many times before its official discovery.

The first recorded sighting took place in 128 BCE by Hipparchus, however, it was mistakenly assumed to be a star, and later other celestial bodies.

When Sir William Herschel used a telescope to lay eyes on the planet in 1781 he himself mistook it for a comet.

Whilst for almost two years Herschel maintained that his discovery was that of a comet, other astronomers began to suspect differently.

Amongst them were Anders Johan Lexell and Johan Elert Bode, both of whom separately concluded that the object was actually a planet.

By 1983 it was accepted that a new planet had been discovered, making it the first to be found using a telescope, with Herschel credited as the discoverer.

Naming the Planet

Naming the Planet
Uranus statue in the Trevi Fountain in Rome.

Herschel attempted to name his discovery ‘Georgium Sidus’ after King George III, in both honour of his monarch and so that future generations would know the discovery took place during his reign. However this was met with resistance, particularly outside the British Isles, and it would be nearly another 70 years before the consensus was made to call it Uranus, a name that had been suggested by Bode in 1782.

In keeping with naming the planets after figures in Greek mythology, Uranus is the Latinised name of the God of the sky Ouranos. As Saturn had been named after the father of Jupiter, Bode felt it was right the next planetary discovery should be named after the father of Saturn.

Similar names for the planet have been adopted in many parts of the world. For example in many Asian countries, such as Japan and China, the name translates literally as “Sky King Star”, whilst in Mongolian it means “King of the Sky.”

Uranus Factfile

Uranus Factfile

Size: Uranus has a diameter of 25,362 km, making it four times wider than Earth, ever so slightly larger than Neptune, but considerably smaller than both Jupiter and Saturn (the ‘gas giants’).

Gravity: The surface gravity on Uranus is 8.69 m/s2 , which is less than Earth’s 9.81 m/s2. As a result we would all weigh a little over 10% less on Uranus.

Age: Uranus was created at roughly the same time as the other planets in our Solar System, a little over 4.5 billion years ago when gravity pulled in gas and dust to create the ice giant.

Distance: On average Uranus is 2.9 billion km away from the Sun, or 19.1 AU (Astronomical Unit, with 1 AU being the distance between the Sun and the Earth). This means light from our star takes around 2 hours and 40 minutes to arrive at the planet.

Atmosphere: The atmosphere of Uranus is comprised of 83% hydrogen and 15% helium, with a small amount of methane and traces of water and ammonia. It’s the methane that gives Uranus its blue colour, as it absorbs red wavelengths and reflects blue light outwards.

Temperature: With temperatures ranging from -195°C to -224°C, Uranus can be the coldest or second coldest planet in our Solar System, competing with Neptune, which is actually further away from our Sun. There are several theories as to why this happens, including its primordial core heat being expelled out into space when Uranus was hit by a supermassive impactor, or Uranus having a very active atmosphere that causes it to lose heat.

Climate: Due to the 98 degree tilt on its axis and 84 year orbit of the Sun, seasons on Uranus are a little strange. One pole will receive direct sunlight for 21 straight years whilst the other pole remains frozen. When this frozen side turns to face the Sun it heats up immensely, causing violent storms. Winds can reach up to 560 mph on the planet.

Rotation: The time taken for the planet to rotate once is 17 hours and 14 minutes, with Uranus being one of only two planets in our Solar System to rotate clockwise (the other being Venus). As mentioned, the planet rotates on its side and is the only planet in the Solar System to do so.

Speed: Uranus travels at 15,290 mph, however, due to its distance from the Sun it must complete a large orbit, which causes a year on the planet to be the equivalent of 84 Earth years.

Moons: There are currently 27 known moons orbiting Uranus. They are unique in that they are not named from mythology but rather characters from the works of William Shakespeare and Alexander Pope.

Missions

Missions
Artists impression of ice giant exoplanet Kepler-421b. Credit: NASA

To date there has only been one mission that has visited Uranus. This is due to the planet’s distance from Earth, which makes closer planets more viable for space agencies to focus upon.

In early 1986 NASA’s Voyager 2 mission conducted a planetary flyby of the ice giant, passing within 81,500km of the planet’s surface.

The spacecraft discovered ten new moons and two new rings around the planet, as well as being able to study previously known ones.

It also discovered a magnetic field comparable to Earth’s, and that the tail of this field is twisted in a corkscrew behind the planet, due to Uranus’ sideways rotation.

A number of new missions have been proposed in recent years, but none had been approved to date.

One plan that is still on the table is the European Space Agency’s MUSE mission (Mission to Uranus for Science and Exploration). The spacecraft would further study Uranus’ atmosphere, magnetosphere, interior, moons and rings.

The proposed launch date for this mission is September 2026, with the spacecraft arriving at the planet in 2044, whilst a secondary launch window in 2029 could also be used.

As we develop technologies to look further beyond our own Solar System, understanding the ice giants will become vital, as many known exoplanets fall into this fundamental class of planets.

Therefore, a dedicated mission to Uranus is crucial to improve the understanding and extend the knowledge of both our Solar System and exoplanetary systems.