천왕성 Uranus

Uranus is the seventh planet from the Sun, and was the first to have been discovered in recorded history. It is unique because its rotational axis is radically tilted towards its orbital plane, and its moons also have orbits that are tilted with respect to Uranus's orbital plane.

Observation and Exploration

Uranus orbits the sun at an average distance of 1.8 billion miles (2.9 billion km), or 19.20 AU; its orbital period is just over 84 years.

From Earth, Uranus is visible with the unaided eye under very dark sky conditions. Its apparent magnitude varies from about 5.6 to 5.9, and its size varies from about 3.4 to 3.7 arc seconds. In small telescopes, its surface is featureless. Even in the largest Earth-bound telescopes, it appears as a small blue-green disk. The first (and so far, the only) spacecraft to visit Uranus was Voyager 2, in 1986; it returned a wealth of new data and high resolution images of the planet and its moons.



Composition, Atmosphere, Rotation, and Magnetism

Uranus is the third largest planet in the solar system, just slightly bigger than Neptune. Like Neptune, Uranus is a "gas giant" planet because it is composed mostly of hydrogen and helium, without any solid, rocky surface. Although Uranus is four times larger than Earth, its low density makes its surface gravity slightly less than Earth's.

In visible light, Uranus is a featureless bluish-green orb. The blue-green color is primarily due to the presence of methane in the its atmosphere. Uranus is a very cold planet, with an effective temperature of -360°F (-220°C) at the cloud tops. Unlike Jupiter and Saturn, Uranus appears to have no internal heat source. The bulk of Uranus's mass is contained in an extended liquid or icy core of primarily water, methane, and ammonia. Uranus possesses a magnetic field about 48 times more powerful than Earth's, produced by convection inside the planet's electrically-conducting interior.

Uranus's rotational axis is nearly horizontal, compared to the plane of its orbit, as though Uranus has been knocked on its side. This unusual orientation may be the result of a collision with a planet-sized body early in the planet's history. Additionally, Uranus' magnetic field also is "tipped over": its magnetic axis is tilted nearly 60 degrees from its axis of rotation, and is offset from the planet's center by one-third of its radius.

The Rings and Moons of Uranus

Uranus has a faint system of thin rings, discovered in 1977 by astronomer James Elliot, when they occulted a background star. In visible light, Uranus's rings are so dark as to be all but invisible. However, at infrared wavelengths, the rings are actually prominent. The rings of Uranus, unlike Saturn's, are faint and narrow. They are confined into nine narrow bands by the gravitational action of small "shepherd" moons orbiting within the rings. Working outward from the planet, the rings named 6, 5, 4, Alpha, Beta, Eta, Gamma, Delta, and Epsilon.
Uranus has five medium-sized moons which were discovered before the Voyager 2 spacecraft's visit in 1986. The largest is Titania, at about 1578 km (960 mi) in diameter; the smallest is Miranda at 471 km (287 mi). They are all massive enough to have become spherical in shape. Miranda is the strangest-looking Uranian moon, appearing as though it were jumbled together from spare parts. It may have reassembled itself after being shattered by a very large impact. Its high cliffs and winding valleys may indicate partial melting of the interior, with icy material rising to the surface.

Ten small additional moons were discovered the Voyager 2 spacecraft in 1986. Their sizes range from about 25 miles (40 kilometers) for Bianca to 100 miles (150 kilometers) for Puck. These satellites revolve around Uranus in less than a day, and act as "shepherds", herding the ring material into narrow bands.


Earth-based observations after 1997 have brought the total number of Uranus's known moons to 27. All of the recently-discovered moons are small - under 160 km (100 mi) in diameter - and have irregular orbits; many are retrograde. They may be captured minor planets since they have similar densities and albedos to the asteroids. There may be even more small moons in irregular orbits that have yet to be discovered.