Uranus is the third-largest planet in the solar system, and the seventh-farthest away from the Sun. This large, pale blue planet orbits between Saturn and more distant Neptune. Discovered by astronomers in 1781, this planet, along with its neighbour Neptune, is still a relative mystery to us. Only the Voyager probes have flown close by Uranus, and no space probe missions to the planet are currently being planned by any of the major space agencies.
DAYS AND YEARS ON URANUS
Uranus weighs the equivalent of about 14.5 Earths. However, like all gas giants, most of its volume is filled by lightweight hydrogen and helium. This means that it is far larger than Earth – about sixty times as large.
At 1.8 billion miles (3 billion kilometres) from the Sun, Uranus also has an orbit about 20 times as far from the Sun as the Earth’s. This makes its year very long: it takes Uranus 84 Earth-years to circle the Sun just once. In addition, at such great distances, only a fraction of sunlight actually reaches the planet. All of the other gas giants actually give off more heat from their exceptionally stormy atmospheres than they receive from the sun; Uranus, for unknown reasons, is much colder than any other planet in the solar system.
Uranus’s rotation is also very curious. A day lasts just 17 hours – less than one on Earth. However, whereas the Earth rotates around north and south poles which are only slightly tilted (causing the seasons here on Earth), Uranus’s axis is tilted so extremely that it faces the sun almost dead-on. For this reason, regions of the planet alternate between decades of constant sunlight and decades of constant darkness, similar to but far more extreme than the short annual periods of long daylight and long night in Earth’s own Arctic and Antarctic.
Possibly because of its strange axis and its cold temperatures, the atmosphere of Uranus does not feature the same remarkable storms as Neptune, Saturn, or Jupiter (in the latter case, for example, the extraordinarily large and seemingly permanent thunderstorm known as the Great Red Spot). The Hubble Space Telescope has been able to chart high-atmosphere weather systems on the planet, but none of them are as stable and long-lived as the great formations on the other gas giants.
COMPOSITION
All gas giants are characterized by a very small, rocky core – roughly Earth-sized – surrounded by enormous layers of liquids and gases. Uranus is no exception: beyond its small rocky core lies a large layer of ice, followed by an even larger atmosphere of hydrogen and helium gas. Because no space probes have ever visited the probe, unfortunately we know relatively little about precisely how these components of the planet work together. We do know that, because it is much farther from the sun, frozen water and ammonia form larger portions of this planet than they do on Jupiter and Saturn, which is why Uranus and Neptune are sometimes referred to as “ice giants” rather than true gas giants.
RINGS AND MOONS
Another feature the gas giants share are planetary ring systems, thin, beautiful bands of rock and ice fragments. However, where Saturn’s are extremely large and highly visible (in fact, Saturn is visually defined more by its rings than any feature of the planet itself), Uranus’s rings are darker, narrower, and therefore considerably less spectacular. A total of eleven rings have been identified around Uranus, which are reddish in colour. There was relatively little knowledge of these rings until the Voyager probes flew by.
Uranus has twenty-seven moons. Unlike the larger systems of Jupiter and Saturn, none of its moons stand out as being especially interesting from an astronomical perspective: they do not have unusual water content (e.g. Europa), volcanic activity (Io), or an atmosphere (Titan). The five largest moons orbiting Uranus are Miranda, Ariel, Umbriel, Titania, and Oberon. All of these are smaller than Earth’s own Moon.
DISCOVERY
Unlike the more distant Neptune, which had to be discovered through theoretical mathematics, Uranus is just visible from Earth, although it took telescopes fto confirm that it was a planet rather than a star (until then, it was known by the much plainer “34 Tauri”). Even the discoverer William Herschel initially believed that it was probably just a comet, since it was so dim and far away. Still, within a matter of years, observations by himself and other astronomers confirmed that it must be a planet.
Not that it was actually named Uranus for some while yet. Herschel wanted to name the planet after his patron, the king of England. Others suggested it be named Herschel’s Planet, in his honour. This sparked a considerable debate among astronomers. The other known planets were named after Roman deities because they had been discovered and named by ancient astronomers. Ultimately, astronomers agreed to stay with this tradition. The name “Uranus” was chosen, after Ouranos, god of the sky in ancient Greece.
