URANUS

Uranus, seventh planet in distance from the Sun and the least massive of the solar system’s four giant, or Jovian, planets, which also include Jupiter, Saturn, and Neptune. At its brightest, Uranus is just visible to the unaided eye as a blue-green point of light. It is designated by the symbol ♅.

Uranus is named for the personification of heaven and the son and husband of Gaea in Greek mythology. It was discovered in 1781 with the aid of a telescope, the first planet to be found that had not been recognized in prehistoric times. Uranus actually had been seen through the telescope several times over the previous century but dismissed as another star. Its mean distance from the Sun is nearly 2.9 billion km (1.8 billion miles), more than 19 times as far as is Earth, and it never approaches Earth more closely than about 2.7 billion km (1.7 billion miles). Its relatively low density (only about 1.3 times that of water) and large size (four times the radius of Earth) indicate that, like the other giant planets, Uranus is composed primarily of hydrogen, helium, water, and other volatile compounds; also like its kin, Uranus has no solid surface. Methane in the Uranian atmosphere absorbs the red wavelengths of sunlight, giving the planet its blue-green colour.

Planetary data for Uranus

*Time required for the planet to return to the same position in the sky relative to the Sun as seen from Earth.

**Calculated for the altitude at which 1 bar of atmospheric pressure is exerted.

mean distance from Sun= 2,870,658,000 km (19.2 AU)

eccentricity of orbit= 0.0472

inclination of orbit to ecliptic= 0.77°

Uranian year (sidereal period of revolution)= 84.02 Earth years

visual magnitude at mean opposition= 5.5

mean synodic period*= 369.66 Earth days

mean orbital velocity= 6.80 km/sec

equatorial radius**= 25,559 km

polar radius**= 24,973 km

mass= 8.681 × 1025 kg

mean density= 1.27 g/cm3

gravity**= 887 cm/sec2

escape velocity**= 21.3 km/sec

rotation period (magnetic field)= 17 hr 14 min (retrograde)

inclination of equator to orbit = 97.8°

magnetic field strength at equator= 0.23 gauss

tilt angle of magnetic axis= 58.6°

offset of magnetic axis= 0.31 of Uranus's radius

number of known moons= 27

planetary ring system= 13 known rings

Most of the planets rotate on an axis that is more or less perpendicular to the plane of their respective orbits around the Sun. But Uranus’s axis lies almost parallel to its orbital plane, which means that the planet spins nearly on its side, its poles taking turns pointing toward the Sun as the planet travels in its orbit. In addition, the axis of the planet’s magnetic field is substantially tipped relative to the rotation axis and offset from the planet’s centre. Uranus has more than two dozen moons (natural satellites), five of which are relatively large, and a system of narrow rings.

Uranus has been visited by a spacecraft only once—by the U.S. Voyager 2 probe in 1986. Before then, astronomers had known little about the planet, since its distance from Earth makes the study of its visible surface difficult even with the most powerful telescopes available. Earth-based attempts to measure a property as basic as the planetary rotation period had produced widely differing values, ranging from 24 to 13 hours, until Voyager 2 finally established a 17.24-hour rotation period for the Uranian interior. Since Voyager’s encounter, advances in Earth-based observational technology have added to knowledge of the Uranian system.

Basic astronomical data

At Uranus’s distance from the Sun, the planet takes slightly more than 84 Earth years, essentially an entire human life span, to complete one orbit. The eccentricity of its orbit is low—that is, its orbit deviates little from a perfect circle—and the inclination of the orbit to the ecliptic—the plane of Earth’s orbit and nearly the plane of the solar system in general—is less than 1°. Low orbital eccentricity and inclination are characteristic of the planets of the solar system, with the notable exceptions of Mercury and Pluto. Scientists believe that collisions and gaseous drag removed energy from the orbits while the planets were forming and so reduced the eccentricities and inclinations to their present values. Thus, Uranus formed with the other planets soon after the birth of the Sun nearly 4.6 billion years ago (see solar system: Origin of the solar system).

Uranus and its neighbour Neptune, the next planet outward from the Sun, are nearly twins in size. Measured at the level of the atmosphere at which the pressure is one bar (equivalent to Earth’s sea-level pressure), Uranus’s equatorial radius of 25,559 km (15,882 miles) is 3.2 percent greater than that of Neptune. But Uranus has only 85 percent the mass of Neptune and thus is significantly less dense. The difference in their bulk densities—1.285 and 1.64 grams per cubic cm, respectively—reveals a fundamental difference in composition and internal structure. Although Uranus and Neptune are significantly larger than the terrestrial planets, their radii are less than half those of the largest planets, Jupiter and Saturn. For additional orbital and physical data about Uranus, see the table.

Because Uranus’s spin axis is not perfectly parallel to the ecliptic, one of its poles is directed above the ecliptic and the other below it. (The terms above and below refer to the same sides of the ecliptic as Earth’s North and South poles, respectively.) According to international convention, the north pole of a planet is defined as the pole that is above the ecliptic regardless of the direction in which the planet is spinning. In terms of this definition, Uranus spins clockwise, or in a retrograde fashion, about its north pole, which is opposite to the prograde spin of Earth and most of the other planets. When Voyager 2 flew by Uranus in 1986, the north pole was in darkness, and the Sun was almost directly overhead at the south pole. In 42 years, or one-half the Uranian year, the Sun will have moved to a position nearly overhead at the north pole. The prevailing theory is that the severe tilt arose during the final stages of planetary accretion when bodies comparable in size to the present planets collided in a series of violent events that knocked Uranus on its side. An alternate theory is that a Mars-sized moon, orbiting Uranus in a direction opposite to the planet’s spin, eventually crashed into the planet and knocked it on its side.

Uranus’s rotation period of 17.24 hours was inferred when Voyager 2 detected radio wave emissions with that period coming from charged particles trapped in the planet’s magnetic field. Subsequent direct measurements of the field showed that it is tilted at an angle of 58.6° relative to the rotation axis and that it turns with the same 17.24-hour period. Because the field is thought to be generated in the electrically conducting interior of the planet, the 17.24-hour period is assumed to be that of the interior. The relatively fast rotation causes an oblateness, or flattening of the planet’s poles, such that the polar radius is about 2.3 percent smaller than the equatorial radius. Winds in the atmosphere cause cloud markings on the visible surface to rotate around the planet with periods ranging from 18 hours near the equator to slightly more than 14 hours at higher latitudes.