c. 4.5 Billion BCE

Uranus

William Herschel (1738–1822)

Our solar system’s seventh planet, unlike the first six, was not known to the ancients. Uranus (pronounced YUR-uh-nus by astronomers) was discovered in 1781 by telescopic observations of the English astronomer Sir William Herschel. Indeed, it had been observed by many other astronomers as early as 1690, but because of its extremely slow motion across the sky (an 84-year orbit period), it was mistaken for a star. Because Uranus has an average orbital distance of about 19 astronomical units (Saturn, the next closest planet to the Sun, has an average orbital distance of about 9.5 astronomical units), its discovery instantly doubled the size of the solar system.

At 4 times the diameter and 15 times the mass of Earth, Uranus is classified as a giant planet, but it is much smaller than planetary cousins Jupiter and Saturn. Still, the atmosphere of Uranus contains mostly hydrogen and helium, and the planet’s distinctive blue-green color is caused by methane clouds and hazes in the upper atmosphere. Storms on Uranus are rare, and the cloud and haze bands are usually quite faint. Uranus has a different overall planetary composition from Jupiter and Saturn, however, with significant amounts of ice and rock in the deep interior. In fact, the ratio of ice and rock to gas is so much higher in Uranus (and Neptune), as compared to Jupiter and Saturn, that the planet is more appropriately called an ice giant instead of a gas giant.

As discovered by telescopic observations and the Voyager 2 flyby in 1986, Uranus has 5 large moons and 22 smaller moons, all of them dark and icy. The planet also sports a series of about a dozen thin, dark, icy rings, possibly formed from a relatively recent breakup of one or more small moons.

Perhaps the strangest thing about Uranus is that its spin axis is tilted on its side by about 98 degrees relative to the ecliptic (the plane of Earth’s orbit around the Sun). The unusual axial tilt of Uranus may be a result of a grazing giant impact or a close encounter with Jupiter long ago. Whatever the case may be, it is yet another of our solar system’s many unsolved mysteries.

SEE ALSO Earth (c. 4.5 Billion BCE), Saturn (c. 4.5 Billion BCE), Jupiter (c. 4.5 Billion BCE), Neptune (c. 4.5 Billion BCE), Discovery of Uranus (1781), Titania and Oberon (1787), Discovery of Neptune (1846), Ariel and Umbriel (1851), Miranda (1948), Voyager 2 at Uranus (1986).

The Keck telescope in Hawaii obtained this spectacular false-color infrared composite image of Uranus and its rings in 2004. Rare white storm clouds can be seen in the atmosphere.