When viewed through Earth-based telescopes, Uranus looks like a tiny blue green disk. Uranus, however, looks tiny only because it is so far away from Earth. Uranus has a diameter of about 32,000 miles (51,500 kilometers). This is about four times the diameter of Earth. In fact, Uranus is the third largest planet in the solar system.
Uranus is one of the solar system’s four gas giants. These are massive planets that consist mostly of flowing matter. A gas giant lacks a solid surface. That is, it lacks any solid matter, even at the core. Or it has a solid core that stays hidden beneath a thick atmosphere. Besides Uranus, the gas giants include Neptune, Saturn, and Jupiter. Uranus is slightly larger than Neptune. Uranus’s diameter is less than half the diameter of Saturn or Jupiter.
In 1781, British astronomer William Herschel surveyed the night sky with a 6-inch (15-centimeter) reflecting telescope he had built himself. He carefully scrutinized every star, looking for something unusual. One night in March he found a tiny shining disk that was clearly not a star. At first he thought it was a comet. But other astronomers soon realized that Herschel had discovered a new planet. Some people suggested naming the new planet after Herschel. Astronomers rejected this idea. Instead, they named the planet Uranus, after the Greek god of the heavens.
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.
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.
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.
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.
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.