Uranus is the seventh planet from the Sun, and has the third-largest diameter in our solar system. It was the first planet found with the aid of a telescope, Uranus was discovered in 1781 by astronomer William Herschel, although he originally thought it was either a comet or a star.
It was two years later that the object was universally accepted as a new planet, in part because of observations by astronomer Johann Elert Bode. Herschel tried unsuccessfully to name his discovery Georgium Sidus after King George III. Instead, the scientific community accepted Bode's suggestion to name it Uranus, the Greek god of the sky, as suggested by Bode.
Voyager 2 is the only spacecraft to fly by Uranus. No spacecraft has orbited this distant planet to study it at length and up.
Uranus is an ice giant. Most of its mass is a hot, dense fluid of "icy" materials – water, methane and ammonia – above a small rocky core.
Uranus orbits our Sun, a star, and is the seventh planet from the Sun at a distance of about 1.8 billion miles (2.9 billion kilometers).
Uranus is about four times wider than Earth. If Earth were a large apple, Uranus would be the size of a basketball.Uranus takes about 17 hours to rotate once (a Uranian day), and about 84 Earth years to complete an orbit of the Sun (a Uranian year).
Uranus’s fluid mass is mostly hydrogen and helium with a small amount of methane. It also contains traces of water and ammonia. On Uranus, all of these substances are mostly in the form of supercritical fluids. Regardless, planetary scientists say that the hydrogen and helium on Uranus are gases and that the other substances are ices. In fact, these scientists sometimes call Uranus an ice giant instead of a gas giant.
They do not mean that Uranus is a body largely consisting of frozen matter. The “ices” on Uranus are substances with relatively high boiling points, compared to those of hydrogen and helium.
Uranus has an almost featureless appearance, at least to human eyes. Beneath the haze, storms rage and bands of clouds form at various latitudes. These features remain hidden in visible light, but they are revealed in infrared light. Infrared instruments distinguish between warmer regions and cooler regions. And these regions may be highlighted in false-color images. Such images have shown a cap of clouds over one of the planet’s poles, as well as subtle clouds formations that resemble the plumes of thunderstorms.
The planet’s bluish haze is due to methane that is present high in the atmosphere. The methane absorbs red and orange light, leaving blue light to be scattered by various substances. These substances, which result from interactions between sunlight and methane, form a photochemical smog.
The temperature at the upper layers of Uranus’s atmosphere is about -355°F (-215°C). Below the uppermost layers of haze and clouds, the atmosphere descends for approximately 6,000 miles (10,000 kilometers). Below the outer layer of the haze lies a strange ocean of water, methane, and ammonia. Scientists are not sure whether this ocean contains any actual liquids or solid ices. But they think that at the very center of Uranus is a rocky core about the diameter of the Earth.
Uranus is blue-green in color, as a result of the methane in its mostly hydrogen-helium atmosphere. The planet is often dubbed an ice giant, since at least 80% of its mass is a fluid mix of water, methane and ammonia ice.
Uranus is a gas giant that is shrouded in haze. The haze is so thick that any details beneath it are obscured. These details include cloud layers that are detected only when special instruments are used.
The few structural details of Uranus that are known have been established by the Voyager 2 probe and the Hubble Space Telescope.
The top of Uranus’s atmosphere consists of wispy gases. Deeper within the planet, temperatures and pressures generally rise. As conditions grow increasingly extreme, the gaseous atmosphere gradually transitions into a supercritical fluid. This is an unusual kind of fluid. It combines the properties of gases and liquids. At greater depths, it becomes increasingly dense. But it never condenses to form a distinct liquid phase. Instead, it retains its strange gas-liquid character all the way to the planet’s core, which appears to consist of rocky material. This rocky core is about the size of the Earth.
The absence of any significant internal heat flow on Uranus means that this planet’s atmosphere is distinctly less active than Neptune’s. In fact the Uranian atmosphere in winter is the coldest planetary atmosphere in the solar system. When Voyager 2 flew past Uranus in 1986, the planet appeared as a largely featureless green-blue disc. In the years since, however, scientists have realised that even this apparently cold, dead world has a surprisingly dynamic atmosphere.
But the new images from the Hubble Space Telescope show a previously unseen huge white cloud likely composed of ammonia or methane ice enveloping the north pole (see top image). Clearly visible at the edge of this huge cloud system is a smaller cloud of methane ice which rotates around the larger cloud edge. These cloud structures may be seasonal, resulting from the current constant sunlight at the north pole.
Around the equator of Uranus we can also see a thin band of cloud (top image), though how this cloud band remains so narrow is not currently understood. Wind speeds on Uranus are so high that they can blow clouds along at up to 560mph, which would spread clouds outwards over a large area. All planetary atmospheres possess a latitudinal circulation system which should, in theory, also distribute this cloud band over wider latitudes. It could be that these methane clouds are somehow constrained by these circulation patterns, due to altitude or chemical instability.
If we could visit Uranus, the winds at a depth equivalent to the atmospheric pressure of Earth’s surface could reach up to 250 metres per second, or roughly three times as fast as a category five hurricane. Be sure to bring your coat, too, as temperatures at this depth are a frigid -200C.
Uranus has five large satellites, or moons. They are named Ariel, Umbriel, Titania, Oberon, and Miranda. Titania‘s diameter is about 1,000 miles (1,600 kilometers). Oberon‘s diameter is about 950 miles (1,530 kilometers). Ariel and Umbriel are almost the same size. Ariel’s diameter is about 725 miles (1,168 kilometers), and Umbriel’s is about 730 miles (1,175 kilometers). Miranda is the smallest of the five. It is about 290 miles (470 kilometers) in diameter.
The planet also has numerous small satellites. Many of these were discovered by the Voyager 2 probe. None has a diameter greater than 101 miles (162 kilometers). They all have dark surfaces. Like Uranus’s rings, they may be coated with dark compounds of hydrogen and carbon. Other small moons have been discovered using ground-based telescopes as well as the Hubble Space Telescope. As of 2004, Uranus had 27 known satellites. More may be found in the future.
The ring closest to Uranus is about 23,600 miles (37,191 kilometers) from the planet’s center. The farthest is about 38,000 miles (51,456 kilometers) away. Unlike Saturn’s rings, which are broad and bright, the rings of Uranus are very narrow and dark. They are made up of billions of tiny particles of ice and rock.
One possible explanation for the darkness of these particles is that they may be coated with compounds of hydrogen and carbon. Astronomers also think that the particles may come from the planet’s satellites. These bodies are less than 6 miles (10 kilometers) in diameter. As they are struck by meteorites, tiny particles are knocked off. These particles may then join the rings.
At a distance of about 1.8 billion miles (2.9 billion kilometers), Uranus is the seventh planet from the sun. It revolves around the sun once every 84 years in a large elliptical (oval-shaped) orbit. This orbit keeps Uranus twice as far from the sun as Saturn and 19 times farther from the sun than the Earth.
The planet rotates once on its axis approximately every 17 hours. The tilt of Uranus’s axis is unique in the solar system. This axis is tilted 98 degrees from the plane of the planet’s orbit. As a result, the planet appears to lie on its side. The north pole of Uranus faces the sun for half of each orbit. And the planet’s south pole faces the sun for the other half of each orbit.
Scientists think that the tilt of Uranus’s axis may have been caused by the impact of a large object during or soon after the planet’s formation. Like the other planets, Uranus formed as a result of the collision of many small bodies called planetesimals. Astronomers think that a particularly large planetesimal may have struck Uranus late in its formation, causing it to tip sideways.