Uranus (yŏrāˈnəs, yŏrˈə–) [key], in astronomy, 7th planet from the sun, at a mean distance of 1.78 billion mi (2.87 billion km), with an orbit lying between those of Saturn and Neptune; its period of revolution is slightly more than 84 years. The first planet discovered in modern times with the aid of a telescope, Uranus was detected in 1781 by Sir William Herschel, who originally thought it to be a comet. Because the calculated orbit of Uranus did not compare accurately with the observed orbit, astronomers concluded that a disturbing influence was present. A study of this irregularity led to the discovery of Neptune in 1846. Uranus has a diameter of c.31,760 mi (46,700 km), roughly 4 times that of the earth, and a mass of about 15 times that of the earth. Like the giant planets Jupiter and Saturn, Uranus has a thick atmosphere of hydrogen, helium, and methane; a relatively low density; and a rapid period of rotation of about 17.9 hr, which causes a polar flattening of over 6%. However, its axis of rotation is tilted 98° to the plane of its orbit. The Voyager 2 space probe found that Uranus has the most inclined magnetic field in the solar system, and some astronomers interpret this as evidence that the magnetic field is reversing its polarity. Viewed through a telescope, Uranus appears as a greenish disk, slightly elliptical because of its rapid rotation. Its temperature is estimated to be about - 330°F ( - 200°C), and at this temperature ammonia, the main constituent of the visible cloud cover, would exist in the form of ice crystals. Uranus has 27 known natural satellites with diameters ranging in size from 7 mi (11 km) to 986 mi (1,578 km).
Prior to 1986, only five of Uranus's natural satellites were known: Titania, the largest, and Oberon were discovered by Herschel in 1787; Ariel and Umbriel, by William Lassell in 1851; and Miranda, by Gerard Kuiper in 1948. When Voyager 2 flew by Uranus in 1986, it discovered 10 more natural satellites—Cordelia, Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Belinda, and Puck—and confirmed the existence of 11 rings. Two additional satellites, Caliban and Sycorax, were discovered in 1997, and three more, Prospero, Setebos, and Stephano, were found in 1999. Trinculo, a small irregular satellite, was discovered in 2002; eight other small satellites are also irregular, that is, their motion around Uranus is retrograde (motion opposite to that of the planet's rotation). The moons of Uranus are named after characters found in the works of William Shakespeare and Alexander Pope.
Titania along with Oberon and Umbriel appear geologically to be relatively quiet. Ariel has surface features that indicate past seismic activity. Miranda shows the most dramatic surface of all, with fracture patterns and sudden landscape changes that indicate that the moon fell apart and then reassembled after a collision in its early history. In 1977, during an occultation by Uranus of a star, astronomers detected a system of nine narrow rings of small, dark particles orbiting around the planet; two more rings, many tiny ringlets, and arcs of rings were later found by Voyager 2. Uranus's rings are distinctly different from those of Jupiter and Saturn. For example, Saturn's rings are very bright and easily seen but Uranus's are very dark, with only 5% of the sunlight being reflected back. Uranus's rings also are very narrow and flat. The widest part of Uranus's outermost ring, the epsilon ring, is 60 mi (97 km) across. The others are only 1 to 2 mi (1.5–3.2 km) wide and barely half a mile (0.8 km) deep.