Uranus
The seventh planet from the Sun is Uranus. It is one of the giant outer planets with no solid surfaces. Although Uranus is not as big as Jupiter or Saturn, more than 60 Earths would fit inside it. The planet is most similar in size and composition to Neptune, its outer neighbor. Like Neptune, Uranus is blue-green because of the small amount of methane in its atmosphere, or surrounding layer of gases.
Basic Planetary Data
Size, Mass, and Density Uranus is the third largest planet in the solar system, after Jupiter and Saturn. Uranus is about four times bigger than Earth. Its diameter at the equator is about 31,763 miles (51,118 kilometers), as measured at the level of the atmosphere where the pressure is the same as at sea level on Earth. The planet is slightly larger than Neptune, but Neptune is about 1.2 times more massive. Uranus' density is quite low—only about 1.3 times that of water, compared with 1.6 for Neptune and 5.5 for rocky Earth.
Orbit and Spin Like all the planets, Uranus orbits the Sun in a slightly elliptical, or oval-shaped, orbit. With an average distance from the Sun of about 1,783,950,000 miles (2,870,990,000 kilometers), Uranus is about 19 times farther from the Sun than Earth is. The closest the planet ever gets to Earth is some 1.7 billion miles (2.7 billion kilometers) away. It takes Uranus about 84 Earth years to complete just one trip around the Sun. This means that a year on Uranus is about 84 times as long as a year on Earth.
A day on Uranus, however, is shorter than one on Earth. Uranus completes one rotation on its axis in about 17 Earth hours, compared with about 24 hours for Earth. This rapid rotation causes its polar regions to flatten slightly and its equator to bulge. The diameter at its poles is about 2 percent smaller than that at its equator.
Technically, Uranus spins on its axis in retrograde motion, or the direction opposite that of most other planets. However, it is a bit misleading to describe its rotation that way, since Uranus lies nearly on its side. Unlike in any other planet, its rotational axis is tilted an unusually large 97.9 degrees relative to the plane in which it orbits. Scientists think that Uranus may have been knocked into this alignment early in its history by one or more violent collisions with other bodies.
Each season on Uranus lasts about 21 Earth years. Since the planet is nearly tipped on its side, as it orbits it points first one pole toward the Sun, then its equator, and then the other pole. As a result, summers and winters are extreme, with one hemisphere bathed in sunlight for many years during its summer, while the other hemisphere is plunged in constant darkness for its long winter. The sunshine is more evenly distributed during spring and fall, when the equator is pointed toward the Sun. However, heat seems to be fairly evenly distributed year-round. The two hemispheres are probably always about the same temperature, probably because the atmosphere transfers and stores heat well. (See also planet, “Years, Days, and Seasons.”
Atmosphere Like the other outer planets, Uranus has a massive atmosphere with a composition similar to that of the Sun and other stars. Scientists think it is roughly three quarters hydrogen and a quarter helium by mass, plus a small amount of methane and probably trace amounts of water, ammonia, and other substances.
The highest clouds are very bright and are formed of frozen methane. Farther down, there are perhaps clouds of frozen water and ammonium hydrosulfide. The lower parts of the atmosphere, in which clouds form, are quite cold, and the temperature there decreases with increased altitude. The coldest part of the atmosphere is about − 366° F (− 221° C). The temperature rises remarkably in the upper atmosphere, however, reaching 890° F (480° C).
Unlike Jupiter and Saturn, Uranus appears nearly featureless in visible light. Faint bands of clouds are revealed in images taken at other wavelengths of light or processed to show extreme contrast. The bands of clouds are parallel to the equator. As on Earth and Neptune, winds travel west in a zone near the equator and east in zones at higher latitudes. The winds are several times stronger than Earth's but weaker than Neptune's. The atmosphere of Uranus seems to be calmer than those of the other outer planets. Spots observed on the planet are thought to be storms, but they are smaller and fewer than those seen on Jupiter, Saturn, and Neptune.
Interior Pressures and temperatures are very high inside the planet, so its interior must be liquid. Scientists think that Uranus is composed mainly of melted ices of water, methane, and ammonia, with some molten silicate rock and metals, and a smaller amount of hydrogen and helium. At its center the planet might have a core of rock and metal. However, scientists think that the rock and metal are more likely to be spread throughout the fluid interior than in a separate layer.
The interior of Uranus is more like that of Neptune than like the interiors of Jupiter and Saturn, which are mostly hydrogen and helium. As in Neptune, melted ices, rock, and metal make up a much greater part of the mass. For some unknown reason, Uranus does not seem to generate as much internal heat as the three other outer planets. Those planets radiate almost twice as much heat as they receive from the Sun, but Uranus emits just a bit more heat than it receives.
Like most other planets in the solar system, Uranus produces its own magnetic field. It is similar to a bar magnet, with a north pole and a south pole. As on Earth, a compass would point north. However, Uranus' magnetic north pole is tilted an exceptionally great 58.6 degrees from its rotational north pole, compared with an 11.5-degree inclination for Earth. Only Neptune's magnetic field is similarly tilted.
Ring System
Moons
Spacecraft Exploration
- A montage of Voyager 2 photographs and artworksimulates a view of Uranus and its rings as if seen …
Basic Planetary Data
- Uranus' southern hemisphere, ring system, and eight of its small inner moons appear in two …
Size, Mass, and Density Uranus is the third largest planet in the solar system, after Jupiter and Saturn. Uranus is about four times bigger than Earth. Its diameter at the equator is about 31,763 miles (51,118 kilometers), as measured at the level of the atmosphere where the pressure is the same as at sea level on Earth. The planet is slightly larger than Neptune, but Neptune is about 1.2 times more massive. Uranus' density is quite low—only about 1.3 times that of water, compared with 1.6 for Neptune and 5.5 for rocky Earth.
Orbit and Spin Like all the planets, Uranus orbits the Sun in a slightly elliptical, or oval-shaped, orbit. With an average distance from the Sun of about 1,783,950,000 miles (2,870,990,000 kilometers), Uranus is about 19 times farther from the Sun than Earth is. The closest the planet ever gets to Earth is some 1.7 billion miles (2.7 billion kilometers) away. It takes Uranus about 84 Earth years to complete just one trip around the Sun. This means that a year on Uranus is about 84 times as long as a year on Earth.
A day on Uranus, however, is shorter than one on Earth. Uranus completes one rotation on its axis in about 17 Earth hours, compared with about 24 hours for Earth. This rapid rotation causes its polar regions to flatten slightly and its equator to bulge. The diameter at its poles is about 2 percent smaller than that at its equator.
Technically, Uranus spins on its axis in retrograde motion, or the direction opposite that of most other planets. However, it is a bit misleading to describe its rotation that way, since Uranus lies nearly on its side. Unlike in any other planet, its rotational axis is tilted an unusually large 97.9 degrees relative to the plane in which it orbits. Scientists think that Uranus may have been knocked into this alignment early in its history by one or more violent collisions with other bodies.
Each season on Uranus lasts about 21 Earth years. Since the planet is nearly tipped on its side, as it orbits it points first one pole toward the Sun, then its equator, and then the other pole. As a result, summers and winters are extreme, with one hemisphere bathed in sunlight for many years during its summer, while the other hemisphere is plunged in constant darkness for its long winter. The sunshine is more evenly distributed during spring and fall, when the equator is pointed toward the Sun. However, heat seems to be fairly evenly distributed year-round. The two hemispheres are probably always about the same temperature, probably because the atmosphere transfers and stores heat well. (See also planet, “Years, Days, and Seasons.”
Atmosphere Like the other outer planets, Uranus has a massive atmosphere with a composition similar to that of the Sun and other stars. Scientists think it is roughly three quarters hydrogen and a quarter helium by mass, plus a small amount of methane and probably trace amounts of water, ammonia, and other substances.
The highest clouds are very bright and are formed of frozen methane. Farther down, there are perhaps clouds of frozen water and ammonium hydrosulfide. The lower parts of the atmosphere, in which clouds form, are quite cold, and the temperature there decreases with increased altitude. The coldest part of the atmosphere is about − 366° F (− 221° C). The temperature rises remarkably in the upper atmosphere, however, reaching 890° F (480° C).
Unlike Jupiter and Saturn, Uranus appears nearly featureless in visible light. Faint bands of clouds are revealed in images taken at other wavelengths of light or processed to show extreme contrast. The bands of clouds are parallel to the equator. As on Earth and Neptune, winds travel west in a zone near the equator and east in zones at higher latitudes. The winds are several times stronger than Earth's but weaker than Neptune's. The atmosphere of Uranus seems to be calmer than those of the other outer planets. Spots observed on the planet are thought to be storms, but they are smaller and fewer than those seen on Jupiter, Saturn, and Neptune.
Interior Pressures and temperatures are very high inside the planet, so its interior must be liquid. Scientists think that Uranus is composed mainly of melted ices of water, methane, and ammonia, with some molten silicate rock and metals, and a smaller amount of hydrogen and helium. At its center the planet might have a core of rock and metal. However, scientists think that the rock and metal are more likely to be spread throughout the fluid interior than in a separate layer.
The interior of Uranus is more like that of Neptune than like the interiors of Jupiter and Saturn, which are mostly hydrogen and helium. As in Neptune, melted ices, rock, and metal make up a much greater part of the mass. For some unknown reason, Uranus does not seem to generate as much internal heat as the three other outer planets. Those planets radiate almost twice as much heat as they receive from the Sun, but Uranus emits just a bit more heat than it receives.
Like most other planets in the solar system, Uranus produces its own magnetic field. It is similar to a bar magnet, with a north pole and a south pole. As on Earth, a compass would point north. However, Uranus' magnetic north pole is tilted an exceptionally great 58.6 degrees from its rotational north pole, compared with an 11.5-degree inclination for Earth. Only Neptune's magnetic field is similarly tilted.
Ring System
- A false-color infrared image shows Uranus and its rings, based on observations made by the Hubble …
- Two shepherd moons, Cordelia and Ophelia, orbit on either side of Uranus' Epsilon ring, in an image …
Moons
- Oberon is the outermost of Uranus' five major moons. An image taken by the Voyager 2 spacecraft …
- Titania is the largest moon of Uranus. A composite of images taken by the Voyager 2 spacecraft …
- Umbriel is the darkest and most heavily cratered of Uranus' five major moons. An image taken by the …
- Ariel, one of the five major moons of Uranus, appears in a mosaic image made from the most detailed …
- Miranda has the most diverse terrain of Uranus' moons. A mosaic of images taken by Voyager 2 shows …
Spacecraft Exploration