The Planet Uranus

The planet Uranus is the largest of the two “ice giants” of our solar system and the seventh planet from the Sun.  At an average distance of 3 billion kilometres, Uranus lies twice as far as from the sun as Saturn and takes a leisurely 84.3 years to make a single revolution.   Uranus was the first planet discovered with a telescope.  And it remains a featureless but satisfying target for backyard stargazers.

Uranus was discovered by the great William Herschel in 1781, an accomplishment that cemented his reputation as the greatest amateur astronomer of his day and launched his professional career with the patronage of England’s King George III, himself a dedicated amateur astronomer.

Politics dominated the debate over the name of the new planet.  Possible names included “George’s Planet” and “George’s Star”, after the English king.  But the neutral name of Uranus was chosen by Johann Bode in honor of the Greek god of the sky.  Uranus remains the only major planet not directly named after a Roman god.

As an ice giant, Uranus, along with Neptune, consists mostly of icy water, ammonia and methane, unlike the larger “gas giants” Jupiter and Saturn which are mostly hydrogen and helium.  The blue green color of these ice giants comes from the thin layer of hydrogen and helium floating above the heavier ices, and there are scant lighter cloud bands of methane and trace hydrocarbons.

With just 1/400th the sunlight of Earth, Uranus is a dim, cold world.  Its cloud-top temperature of just 50 degrees Kelvin (-223 Celsius) is the coldest of all major planets– Neptune generates more internal heat than Uranus.

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The five largest moons of Uranus

Another odd feature… Uranus rotates on its side.  Its otational axis is inclined 98 degrees to its orbital plane, which suggests it received a mighty thump in the early days of the solar system that knocked the planet on it side.  As a result, the northern and southern hemispheres of the planet remain in darkness for decades at a time. The thin, coal-black rings of the planet likely formed after the planet was knocked over.  Same with the little clutch of small moons of Uranus, the largest of which are named after the Shakespearean characters Titania, Oberon, Miranda, Umbriel, and Ariel.

At magnitude 5.8, Uranus is visible, barely, without a telescope.  But the planet did not make a sufficient impression upon ancient stargazers to be noticed as a planet.  Even in the early telescopic age, the astronomers John Flamsteed and Pierre Lemonnier observed Uranus on dozens of occasions before Herschel’s discovery.