Uranus

Being Neptune’s near-twin, you’d think Uranus is uninteresting on its own, but this planet sure is an oddball. Its rotational axis is tilted so far, the planet appears to be rolling around the Sun sideways, when observed from the plane of the Solar system. We’ll get to that in a few, let’s first go through the usual introduction.

Uranus is the seventh planet from the Sun, orbiting our star between Saturn and Neptune. It is a gas giant, the fourth heaviest and the third largest. It is similar in size to Neptune, a bit larger, but a lot less denser and thus lighter in mass than its sibling. It is 14.5 times heavier than Earth with diameter about 4 times that of Earth.

Planet UranusPlanet Uranus photo taken by Hubble telescope

It was discovered on March 13, 1781 by Sir William Herschel who named it after his patron King George III - Georgium Sidus, but with time it was renamed after the Greek god of the sky, Ouranos, latinized into Uranus. It is the only planet named after a Greek and not Roman god.

It takes it 84 Earth years to orbit the Sun and it does so at the distance of approximately 20 astronomical units (AUs, distance from the Sun to Earth), but the distance varies up to 1.8 AU during 84 years, more than that of any other planet. One day on Uranus lasts 17 hours and 14 minutes (Earth time) in its interior, but parts of planet’s upper atmosphere rotate at different speeds. This is caused by extremely strong winds that blow in the direction of rotation, causing some regions to complete the full rotation in just 14 hours. This is normal for all four gas giants.

Uranus is the only one of two planets in Solar system with retrograde rotation around its axis. Here it gets interesting because that doesn’t necessarily mean that its rotation is reversed.

First let’s make it clear what retrograde rotation means. All planets orbit the Sun in counterclockwise direction. Planet rotating around its axis in clockwise direction has retrograde rotation. Positive rotation, as that of Earth and other planets except Uranus and Venus, is when planet has counterclockwise rotation around its axis.

Now, let’s settle on what is north and what is south pole. International Astronomical Union (IAU) defines north pole of a planet as the pole which is “up” as seen from the invariable plane of Solar system. Other method would be right-hand rule. Imagine holding a pen in your right hand, with your fingers wrapped around it while your thumb faces up. Your fingers point the direction of the rotation (counterclockwise) and your thumb shows where north pole is (up). If we used the right-hand rule for Uranus, it’s north pole would be under the line of invariable plane of Solar system.

Sometime during the formation of Solar system, Uranus was probably hit by another planet which tilted its axis to 97.8 degrees. Before this theorized collision, it had normal rotation, but after the event, its north pole became the south pole by IAU definition. To clarify, Uranus didn’t change its rotation from positive to retrograde, it just tilted by over 90 degrees, ending up with geographical poles reversed by IAU’s definition and its rotation retrograde compared to its orbital movement. For determining axial tilt however, we use the right-hand rule, making Uranus’s tilt 97.8 degrees and not 82.2 degrees (planet’s north pole by IAU definition is 7.8 degrees above invariable plane of Solar system i.e. 82.2 degrees from 0 position, where it would be if there was no tilt and no retrograde rotation).

Uranus and its ringsUranus and its rings. This image, taken by Keck observatory, clearly shows planet's tilt.

If you didn’t get lost in this whole retrograde rotation and axial tilt stuff, you probably noticed that being tilted by a bit over 90 degrees, Uranus has its poles facing the Sun about 42 years each and receiving more sunlight than equatorial region. This also makes it look like the planet is rolling on its hip while orbiting the Sun, except it’s rolling in one direction and moving in another.

Despite its equatorial regions receiving less sunlight, for unknown reason Uranus’s poles are still colder than its equatorial region.

Uranus’s atmosphere is made mostly of hydrogen and helium with some methane gas which absorbs red light and gives planet its cyan colour. Under the atmosphere is the mantle made of hot liqueified water-ammonia mix that makes up more than 90% of planet’s mass. At the centre is a core made of iron, nickel and silicates. After Saturn, Uranus is the second least dense planet in the Solar system. Just like Neptune, Uranus was probably formed closer to Sun, where it had enough material to accrete, and then migrated over time to outer region of the Solar system.

Uranus with its rings and moonsUranus with its rings and moons

Uranus has 13 rings, most of them only few kilometres wide. They are probably very young and the result of a moon that was destroyed in a collision. Speaking of moons, Uranus has 27, none of which are larger than half the size of the Moon.

Oh, and last but not least, Uranus is pronounced [ˈjʊərənəs], with the stress on the first syllable, which still doesn’t keep it from being victim of pun jokes.

All images credited to NASA

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