Uranus

The seventh planet from the sun (at a distance of about 19 astronomical units), Uranus is one of the four gas giants, composed primarily of gases rather than solid matter. Its orbit around the sun takes 84.323 years. It is one of only two planets, along with Venus, to have retrograde rotation. It has an extreme axial tilt of 97.77 degrees, putting its axis of rotation almost parallel with the solar system’s plane. It is believed that this was caused by a collision during the formation of the solar system. Its rotational period is about seventeen hours and fourteen minutes, and its surface gravity is close to Earth’s, at 0.886 g.

Uranus is the least massive gas giant, with a mass of 14.5 Earths and a volume of 63 Earths. It is also the second least dense planet in the solar system, after Saturn. Its exact composition is not known, but the standard model posits three layers: a rocky core of iron, nickel, and silicate; a mantle consisting of a hot, dense, and volatile water-ammonia ocean that is confusingly called “icy” in planetary terms; and a gaseous outer layer that resembles those of the other gas giants. However, the model represents only the most plausible possibility based on what is known, and little is known. There are other mysteries surrounding Uranus, the solutions to which may impact the standard model’s plausibility. For instance, although Neptune and Uranus have much in common, Neptune radiates significant amounts of heat into space, while Uranus radiates almost none, a difference which is not explained by anything known about the two planets.

Atmosphere and Climate

Like Neptune, Uranus differs from the other gas giants, Saturn and Jupiter, in that its atmosphere has a higher concentration of water, ammonia, and methane. Neptune and Uranus are sometimes called the ice giants to distinguish them. Still, like the other gas giants, Uranus has an atmosphere made up almost entirely of hydrogen and helium. Methane in the atmosphere may contribute to Uranus’s color, which is somewhat blue, although it is not nearly as blue as Neptune. Experiments have suggested that below Uranus’s atmosphere, extreme pressures may condense atmospheric carbon into diamond crystals that precipitate like hail.

Uranus’s is the coldest planetary atmosphere in the solar system, as cold as –224 degrees Celsius in places. In contrast with the active weather systems of Neptune, its climate is relatively placid, if inhospitable. Clouds of water, ammonium hydrosulfide, ammonia, and methane drift through the troposphere (the lowest layer of the atmosphere), each at different atmospheric pressures.

Uranus’s axial tilt results in unique seasonality unlike anything else. Each pole receives forty-two years of continual light followed by forty-two years of continual darkness, as one pole faces the sun directly during each solstice. Only during the equinoxes, when the equator faces the sun, does the planet experience normal day-night cycles. Over the course of the year, the poles thus receive more solar energy than the equator, though for unknown reasons the planet’s equatorial weather is warmer than the polar weather.

Planetary Rings and Moons

Uranus is surrounded by an unknown number of rings, at least thirteen of them. The rings are very dark and believed to be young, which may mean they are the remnants of a shattered moon. Unlike some ring systems, these appear to contain fairly little dust, being composed instead of macroscopic particles of unknown material. They differ from both Jupiter’s dusty rings and Saturn’s bright ones in this respect, but may consist of large amounts of ice, as some of Saturn’s are, darkened by mixture with some other material. A set of two outer rings was discovered by scientists working with the Hubble Space Telescope in images from 2003 to 2005; the outer rings are even more faint than the inner rings.

Uranus also has twenty-eight known moons, named for characters from William Shakespeare and Alexander Pope. The moons are all small, and the combined mass of Uranus’s moons is less than that of any other planet. Most or all seem to be composed of equal amounts of rock and ice, the ice including water, ammonia, and carbon dioxide. The largest moon, Titania, is about half the size of the Earth’s moon. Some of the moons share names with asteroids, notably Titania, Ophelia, Bianca, and Desdemona. Thirteen of Uranus’s moons are the “inner moons,” all of which are close to the ring system. The theory that the rings were formed by the remnants of a shattered moon is strengthened by the fact that the inner moons are unstable and prone to crossing orbits, making a collision likely.

History of Observation and Exploration

Uranus is barely visible to the naked eye, but is so dim and slow moving that it was not recognized as a planet until the eighteenth century. Uranus is the only planet in the solar system named for a Greek god rather than its Roman equivalent (which in this case would be Caelus). Its discoverer, William Herschel, initially believed it was a comet, despite the lack of apparent tail. Only gradually over the next few years after its 1781 discovery did the scientific community come to see Uranus as a planet. Herschel originally named the planet Georgium Sidus in honor of King George III, but it was not a popular choice. Johann Bode suggested Uranus, on the logic that Saturn had been named for the father of Jupiter and so the new planet should be named for the Titan who conceived Saturn. This name became more popular, and Georgium Sidus died out by the middle of the nineteenth century.

The discovery of Uranus led directly to the discovery of Neptune, since observing Uranus’s orbit revealed discrepancies in its path that could only be explained by the gravitational pull of another planet.

Herschel described a ring around Uranus in 1789, but it was not confirmed until 1977 when the rings were conclusively observed from an airborne observatory. The Voyager 2 probe took images of some of Uranus’s rings in 1986, both those previously observed and some previously unknown, in addition to taking measurements of the planet. Voyager 2’s flyby is the only close-approach investigation of Uranus, conducted from 81,000 kilometers above the clouds. Numerous other Uranus missions have been proposed, as it is one of the least-explored planets in the solar system.

Astronomers continued to observe Uranus in the twenty-first century. For example, in 2005, the Hubble Space Telescope photographed previously undocumented rings and moons in Uranus's orbit. In 2024, HST and New Horizons spacecraft teams simultaneously observed the planet from different vantage points. Astronomers compared their observations of Uranus from different viewpoints to inform future research on exoplanets, or planets beyond the solar system.

Bibliography

Chambers, John, and Jacqueline Mitton. From Dust to Life: The Origin and Evolution of Our Solar System. Princeton UP, 2013.

Cole, George H. A., and Michael M. Woolfson. Planetary Science: The Science of Planets Around Stars. CRC, 2013.

Dick, Steven J. Discovery and Classification in Astronomy. Cambridge UP, 2013.

Encrenaz, Therese. Planets: Ours and Others. World Scientific, 2013.

Esposito, Larry. Planetary Rings: A Post-Equinox View. Cambridge UP, 2014.

Milone, Eugene F., and William J. F. Wilson. Solar System Astrophysics: Planetary Atmospheres and the Outer Solar System. Springer, 2014.

"NASA's Hubble, New Horizons Team Up for a Simultaneous Look at Uranus." NASA Hubblesite, 9 Oct. 2024, hubblesite.org/contents/news-releases/2024/news-2024-033. Accessed 5 Dec. 2024.

"Uranus Facts." NASA, National Aeronautics and Space Administration, Nov. 2024, science.nasa.gov/uranus/facts/. Accessed 5 Dec. 2024.