Video on the gloriously tilted Uranus

Blogpost/ Shownotes

Right, I know this post is going to be a butt of a lot of giggles but here goes anyway.

When you look at images of planets in our solar system, they all seem to follow a certain design. The rocky ones are round with a few moons, while the gas giants and the ice giants have rings. But one of them is different. It seems to be tilted on its side, with its rings nearly at right angles to its equator. That planet is Uranus.

Until 1781, we all thought that there were only six planets in our solar system. But on March 13 of that year, exactly 240 years ago, the astronomer William Herschel identified a seventh planet, when he observed a faint object in the constellation Gemini. At first, he thought it was a comet but then later decided that this was a new planet – the first one ever observed through a telescope. It would be two years before it was accepted as a planet, in part because of observations made by another astronomer Johann Elert Bode.

Although Herschel wanted to name it after the monarch of the United Kingdom, the tradition was to name planets after mythological figures and this is why it became Uranus – Greek god of the sky, grandfather of Zeus (or Jupiter) and father of Cronos (or Saturn).

Like all the other planets in our solar system, Uranus was formed 4.5 billion years ago, as a result of swirling gas and dust collapsing due to gravity. Like the other giant planets, it was also probably formed closer to the Sun and then moved to the outer solar system. Uranus’ composition is similar to Neptune. Most of its mass is a hot dense, fluid of icy water, methane, and ammonia above a small rocky core, that’s why both planets are known as ice-giants. It also has a magnetosphere, several moons, and a ring system.

Uranus is about 1.8 billion miles (or 2.9 billion km) from the Sun, which is about 19 times the distance of the Earth from the Sun. Due to this massive distance, astronomers knew little about this ice-giant other than five of its moons for 200 years. In 1977, scientists at the Kuiper Airborne Observatory and the Perth Observatory in Australia made a major discovery: Uranus, like Saturn, had rings! These rings differed from Saturn though – circling the planet at almost 90 degrees from its plane of orbit. The planet is orbiting on its side and is also known as the “sideways planet”. It is thought that another Earth-sized object crashed into it at some point and tilted it. Uranus is also one of just two planets, the other being Venus, that rotate in the opposite direction than the other planets i.e from east to west.

A year on Uranus is 84 Earth years, its day is about 17 Earth hours, and it has a diameter of about 31,500 miles (or over 50,000 km), making it four times wider than Earth. Its unique tilt is the reason for some of the most extreme seasons in the solar system. Uranus’ north pole experiences 21 years of night in winter and 21 years of daytime in summer and 42 years of day and night in the spring and fall (autumn). Its temperature can reach -357F (or -180C) and its atmosphere comprises of hydrogen, helium and methane. However, it has more methane than Jupiter and Saturn, which is why it has the lovely blue colour we can see in images. Uranus has 13 known rings, the inner ones being narrow and dark and the outer ones brightly coloured. In total it has 27 known moons, named after characters from the works of Shakespeare and Alexander Pope.

(Video and blogpost on Saturn)

Voyager 2 is the only spacecraft that flew by Uranus in 1986, greatly increasing our knowledge of this icy world, its moons, and rings in just six hours, making its closest approach of 50,700 miles above the planet’s top clouds. Other than that, no spacecraft has orbited it. Voyager 2 imaged its large moons, revealing each to be unique with more geological activity than was expected. More than 7,000 images returned by Voyager 2 revealed 11 new moons and two new rings.

The Hubble Space Telescope, launched in 1990, has been observing it and the other outer planets. In 2019, it revealed a vast bright stormy cloud cap across the north pole – providing a fresh look at a long-lived storm circling the planet’s north polar region. Keck Observatory and Hubble Telescope observations show that its outer rings are blue and the inner new ring are reddish in colour. In March 2020, scientists at NASA’s Goddard Space Flight Center, reanalysed Voyager 2 data, finding a giant magnetic bubble known as a plasmoid that may have been slowly throwing the planet’s atmosphere out to space. And Voyager 2 data still keeps on giving. An image it took 13 years ago recorded a moon that went unnoticed until Erich Karkoschka from the University of Arizona noticed Uranus’ 18th moon when he compared the 1986 photo with a recent one taken by Hubble in 1999.

Then this year, on March 31, 2021, astronomers detected X-rays being emitted from Uranus for the first time, using NASA’s Chandra X-ray Observatory observations, taken in 2002 and 2017. While X-rays have been detected on most of the planets in our solar system, they had not been detected on the ice-giants Neptune and Uranus, till this latest study.

The X-rays are probably caused by the Sun’s X-ray light being scattered by the planet, similar to Jupiter and Saturn. However, there is also evidence that at least one other source of X-rays is present on Uranus. This could be the planet’s rings producing the X-rays, like the rings of Saturn do. The charged electrons and protons that surround Uranus space environment could be colliding with the rings, which could cause them to glow in X-rays. Auroras were also observed on Uranus in other wavelengths in 2011. Because Uranus’ magnetic field is inclined 59 degrees to its spin axis, the auroral spots appear far from the planet’s north and south poles – offset from its centre, making them complex and variable. These auroras could also be the reason for the X-rays.

(Jupiter – Solar System’s Giant)

NASA astronomers think that determining the sources of the X-rays from Uranus could help increase understanding of X-ray emissions from other objects in space, such as black holes and neutron stars.

Fun fact: Just eight years after the discovery of Uranus, a radioactive element was discovered in 1789. This element, Uranium, was named after the ice giant.

Summary

• Discovered by Sir William Herschel in 1781
• Average distance from the Sun is 3 billion km
• It circles the Sun once every 84 Earth years
• Its diameter is four times that of our planet
• Hydrogen and helium dominate the atmosphere
• It has at least 13 rings and numerous moons

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