Uranus is the seventh planet from the Sun, unknown to the Greeks, Egyptians and all ancient civilizations, since it is at the limit of detection with the naked eye under dark skies and therefore undistinguishable from a dim star. Uranus was named by Johan Bode (the same as in Titius-Bode law). In Greek mythology, Uranus was the personification of Heaven and ruler of the world, regarded as the husband and son of Gaea (Earth) and father of the Titans, Furies and Cyclopes; he was overthrown by his son Cronus (Saturn), who was also Jupiter's father.

William Herschel discovered this planet in 1781 when he was doing a survey of the stars in the constellation Gemini with his 6-inch telescope. He saw a greenish disk that suggested a "nebulous star" or a comet, but after charting its motion in the sky, its orbit was calculated and it was proven to be a planet. Herschel lived in England and was a musician by profession and an avid amateur astronomer in his spare time. After discovering Uranus, he was made a knight and received a royal pension. Thereafter he dedicated his life to astronomy and made numerous contributions to the field.

Infrared image of Uranus highlighting several of its moons and its ring system. NASA HST.

The five outermost and largest satellites were all discovered from ground observations. Herschel discovered Titania and Oberon in 1787, Lassell discovered Ariel and Umbriel in 1851, and Kuiper discovered Miranda in 1948. Their sizes range from 485 to 1550 km, about the same size as the medium-size moons of Saturn. The densities are between 1.3 and 1.6 g/cm3, suggesting that their composition is about half rock and half ice.

Miranda is the smallest and innermost of the five largest moons, and also the best studied because it had to be used as a gravitational booster for Voyager to reach Neptune, so it was photographed with a resolution better than 1 km. To the surprise of planetary geologists, Miranda has valley systems as wide as 50 km and as deep as 10 km that were produced by large-scale tectonic stresses. Mountain ranges cover about half the surface of this moon. One characteristic geological feature is a chevron, or a number 7 shaped feature that may be a localized upwelling of material from inside.

Uranus is best remembered as the "tilted planet," or the "planet that rotates on its side," because of the inclination of its rotation axis with respect to its orbital plane. With an inclination slightly larger than 90 degrees, Uranus rotation is retrograde (like Venus and Pluto), which means that if you were to look at all the planets from "above" the ecliptic (i.e. from the north as defined by Earth's north), Uranus, Venus and Pluto rotate on their axes in the opposite direction as the rest of the planets.

The rotation period of Uranus could not be measured from Earth because the planet's disk is nearly featureless. It was finally measured in 1986 when the Voyager 2 encounter took place. Voyager took pictures through an orange filter which revealed the presence of clouds and measured a rotation period of about 16 hours. As with Jupiter and Saturn, this rotation period is not the rotation of the planet's interior, but includes the rotation of the winds up in the atmosphere, so the true rotation period is measured from the rotation of the magnetic field.

Physical Data

Uranus's diameter is four times larger than Earth's and its mass is 14.5 times the Earth's mass.

The atmosphere of Uranus is 84% molecular hydrogen gas (H₂), 14% helium gas (He) and 2% methane (CH₄), with trace constituents of acetylene (C₂H₂), hydrogen cyanide (HCN) and carbon monoxide (CO). The upper atmosphere of Uranus is very clear. The greenish-bluish color that you see through a telescope results from the absorption of red light by methane. Using special filters, Voyager 2 photographed a banded polar haze in the sunlit side of the planet. This haze is formed when solar ultraviolet light irradiates the atmosphere.

Once again, the make-up of Uranus's interior is inferred by its density and from observations of the atmospheric composition from Voyager data. Uranus and Jupiter have about the same density, yet Jupiter's diameter is about three times larger. If the two planets had the same composition, the density of Uranus would be less than Jupiter's because of Uranus's smaller gravity. So the greater density of Uranus means that it is made up of a higher proportion of heavier materials than Jupiter. Scientists think that the interior of Uranus consists of a massive core of rock and ice; first a rocky core (mostly silicon and oxygen) surrounded by a core consisting of a mixture of water ice and rock. It is suspected that around these cores is an envelope of liquid hydrogen, and then another one of gaseous hydrogen with helium, methane, ammonia and water.

Uranus's magnetic field (in blue) is offset from the axis of rotation (green arrow).

Unlike all other planets that we have studied so far, the magnetic field of Uranus is inclined 60° to the rotation axis, and it is offset from the center of the planet by about 1/3 of the radius. Thus, you would not be able to find your way to the north with a compass because the direction of the compass needle and the strength of the field vary with location on the planet. For comparison, Earth's field strength is about 0.3 gauss anywhere on the surface, whereas in Uranus it varies from 0.1 to 1.1 gauss. Most of the atomic particles in Uranus's magnetosphere are protons and electrons that escape from the atmosphere. The escaping hydrogen atoms glow in the sunlit side of the planet in ultraviolet light, this is called electroglow and would be seen at latitudes near 30°, because that is the position of the magnetic poles.

Imagine what the seasons are like in Uranus with its axis tilted about 90°. Since the orbital period is about 84 years, the north and south poles alternate between periods of 42 years of light and 42 years of darkness. The summer solstices happen when the north or south poles point directly towards the sun, while the winter solstices occur when they point away from the sun. During the spring and autumn equinoxes, the equator faces the sun directly. Each season on Uranus lasts 21 years.

Voyager 2 snapshot of the Uranian ring system. NASA JPL.

As the moons, the rings lie on the plane of the planet's equator, and as seen from Earth, their appearance alternates every 21 years from face-on to edge-on. The rings are almost circular and very narrow. Some of them are 2 km wide, while the wider is less than 100 km. Their distances from Uranus's atmosphere vary from 19,000 to 26,000 km. The smallest ring particles are a few centimeters in diameter, and the biggest are 10 meters or so. Contrast this with some of the particles in Saturn's E ring, which are on the order of micrometers in size.

Moons and Planets, W. Hartmann, 1993, 3rd edition, Wadsworth Publishing Co.
Astrophysical Data: Planets and Stars, 1992, K. R. Lang, Springer-Verlag New York, Inc.
The Planetary System, D. Morrison and T. Owen, 1996, 2nd edition, Addison-Wesley Publishing Co., Inc.

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