While William Herschel officially discovered Uranus in 1781, he wasn’t the first to observe it. Others thought it was a star and Herschel himself called it a ‘comet’ before deciding that it was, in fact, a planet – and the first one discovered by telescope.
Although Uranus can be seen from Earth with the naked eye, it’s so dim and has such a slow orbit compared to the other known planets that it didn’t register as one. It just looks like a faint pinpoint of greenish or bluish light.
Uranus’s acceptance as a new planet overturned beliefs that had been held for millennia about the size of our Solar System, and kicked off a flurry of planetary discovery. But despite Uranus’s significance, we haven’t spent much time visiting the planet. A flyby by Voyager 2 in 1986 marks the only time we’ve explored it. Because of this, we simply don’t know a great deal about Uranus. Until telescope observations in the past few decades, we thought of it as a rather bland planet: dark, cold, slow and with few interesting features.
Uranus is the third-largest planet by radius and the fourth-largest by mass. It’s about 3 billion kilometres (1.86 billion miles) from the Sun, which means that it receives 0.0025 per cent of the sunlight that the Earth gets. Uranus is a gas giant, along with Jupiter, Saturn and Neptune, with the latter planet sometimes being referred to as its twin. It is the least massive of the four, but still more than 14 times more massive than Earth.
Uranus has a diameter four times that of Earth’s. It also has the coldest atmosphere of any other planet in the Solar System, with a mean temperature of approximately -197 degrees Celsius (-322 degrees Fahrenheit). Uranus also has a multilayered cloud system, although without the flashy variations of colour seen on planets such as Jupiter and Saturn. However, it does have a lot in common with the other gas giants. It has a magnetosphere that is very similar to Jupiter’s. It has 27 moons, and a system of 13 rings that was discovered not long after Saturn’s ring system. It’s most like Neptune in terms of composition, mostly hydrogen and helium with icy volatiles. Sometimes Uranus and Neptune are referred to as the ‘ice giants’.
But Uranus can’t just be lumped in with the other gas giants because the planet has its own unique twist. Literally, as Uranus’s 97.77-degree axial tilt means that it is parallel with the plane of the Solar System – its poles are on either side. While other planets have extreme tilts, none are so perfectly perpendicular to the plane of its orbit. Recent studies show that Uranus is probably tilted due to at least two violent, massive collisions one after another, with objects larger than Earth. These impacts likely occurred early in the planet’s life, even before its moons formed, and have made astronomers rethink how the other gas giants formed as well.
The tilt has impacted just about everything about Uranus. It has extreme seasons and weather fluctuations since each hemisphere experiences either full Sun or deep space. Its magnetosphere is tilted and asymmetric. The ring system is also on its ‘side’, and comes close to rivalling Saturn’s in complexity. The moon system is less massive than any other gas giant’s system. So, in many ways, Uranus is unique.
Uranus inside and out
The term ‘gas giant’ implies that Uranus is solely composed of gases, but studies indicate that it actually has a core of silicate rock, encased in ices and topped with a gaseous layer. The core must be very small, since Uranus is the second-least dense planet. It likely takes up only 20 per cent of the planet’s radius. The ice mantle surrounding the core is fluid, with volatiles like methane, ammonia and water. In fact, this electrically conducive fluid is often called an ammonia-water ocean. The outer layer is mostly helium and hydrogen.
Uranus is also much cooler inside than the other gas giants – it’s actually the coldest planet in the Solar System. Neptune radiates 2.61 times the heat that Uranus does. We aren’t sure why Uranus is so cold in comparison, but it may have been struck by a large body that forced it to expel most of the heat it had when formed, or there could be a complex system at work in the atmosphere that keeps core heat from getting out.
The atmosphere contains three layers: the thermosphere, the stratosphere and the troposphere. The lowest layer, the troposphere, is the most interesting and is rich in volatile ices like methane and ammonia. It has four cloud layers: methane, hydrogen sulphide and ammonia, ammonium hydrosulphide, and water clouds at the upper limit. We’ve only observed the top two layers, along with a hazy layer above them. The stratosphere sits between the troposphere and the outermost layer, the thermosphere. Uranus tends to look light bluish or greenish in colour, and it has faint darker bands. The overall colour is due to the way that methane absorbs visible and near-infrared light.
Until Voyager 2 explored Uranus’s atmosphere, we didn’t know much about its features. The probe found a bright polar cap at the south pole, as well as a lighter band called a collar. There were darker bands in the southern hemisphere and about ten lighter clouds around the middle latitudes. The timing of Voyager 2’s arrival meant that it could not fully observe the northern hemisphere. In the Nineties, Hubble and ground-based telescopes like the Keck Observatory began to see more atmospheric features on Uranus. They spotted many more clouds in the northern hemisphere, which are brighter and at a higher elevation than the ones in the southern hemisphere. They also observed in 2007 that the southern collar had nearly disappeared, while one in the north had grown.
Discovering the rings
When William Herschel discovered Uranus, he also claimed to see a faint ring around the planet. It seems unlikely he was truly able to see anything, however. The ring system was officially discovered by a team working with the Kuiper Airborne Observatory (KAO). In 1977 and 1978, the team found a total of nine rings around Uranus. KAO was the first airborne astronomical research observatory, based at the NASA Ames Research Center in California. A military jet aircraft was modified to carry a 91.5cm (36in) reflecting telescope used to conduct infrared astronomy. Why a telescope on a plane? The aircraft was used to fly above 13,716m (45,000ft). This altitude is above the layer of water vapour in the Earth’s atmosphere that absorbs infrared radiation, allowing the team to use the telescope to observe other planets in our Solar System.
Dr James Elliot, an astrophysicist with the KAO team, was an expert in stellar occultation – learning about a planet by measuring the light blocked when it passed between the Earth and a distant star. While working with data recording during an observation of Uranus, Dr Elliot noticed that the light between the planet and the distant star went dim for a few minutes before Uranus appeared. This was the first evidence of the rings, confirmed by Voyager 2 about a decade later.
With just one probe visiting the planet, most of our Uranian exploration has been done via telescope.
Uranus has a special significance as the first planet in the Solar System to be discovered by telescope rather than the naked eye – and that’s mostly how we’ve observed it. Since William Herschel’s findings, astronomers have continued to learn more about the planet as telescopes and technology have improved. French astronomer Pierre-Simon Laplace calculated the planet’s orbit in 1783, but others soon realized that it wasn’t behaving as predicted. Further observation showed that the erratic elements of Uranus’s orbit were caused by a new planet, Neptune. Uranus’s rings were officially discovered nearly 200 years later.
NASA’s Voyager 2 probe encountered Uranus in January 1986, as part of its Planetary Grand Tour mission. The spacecraft passed within 81,500 kilometres (50,600 miles) of the planet’s cloud tops and made numerous discoveries. Voyager 2 studied the Uranian atmosphere and magnetosphere, revealing that the latter was asymmetric. It provided a number of high-resolution images of the planet and its rings, as well as its five major moons. The probe also discovered ten additional moons and two new rings. The Hubble Space Telescope revealed two further rings and two more satellites in the mid-2000s. Ground-based telescopes, such as the Keck Observatory, have continued to reveal more about Uranus, but there are no current plans to send another spacecraft to explore it up close.
One of the obstacles to exploring the outer planets was how to get there, without needing so much fuel that the mission would be too costly. In the Seventies, NASA began planning a Planetary Grand Tour to take advantage of the outer planets’ rare alignment. In 1974, Mariner 10 was the first to use a new technique called gravity assist, or slingshot, to reach Mercury. Spacecraft can use a planet’s movements and gravity to change course or speed to save both fuel and time. After reaching Jupiter, both probes followed a circular trajectory, using each planet’s gravity to travel on to the next. Voyager 2 reached Neptune, then continued on to deep space, while Voyager 1 performed a flyby of Saturn’s moon Titan and was sent out of the plane of the ecliptic and into deep space.