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Uranus (planet), major planet in the solar system, equivalent in brightness to a sixth-magnitude star. It ranks seventh in order of distance from the sun, revolving outside the orbit of Saturn and inside the orbit of Neptune. Uranus was accidentally discovered in 1781 by the British astronomer Sir William Herschel and was originally named the Georgium Sidus (Star of George) in honor of his royal patron King George III of Great Britain. The planet was later, for a time, called Herschel in honor of its discoverer. The name Uranus, which was first proposed by the German astronomer Johann Elert Bode, was in use by the late 19th century.
Uranus has a diameter of 51,120 km (31,771 mi), and its mean distance from the sun is 2.87 billion km (1.78 billion mi). Uranus takes 84 years for a single revolution, or orbit, and 17 hr 15 min for a complete rotation about its axis, which is inclined 98° to the plane of the planet's orbit around the sun. Uranus's atmosphere consists largely of hydrogen and helium, with a trace of methane. Through a telescope the planet appears as a small, bluish-green disk with a faint green periphery. Compared to the earth, Uranus has a mass 14.5 times greater, a volume 67 times greater, and a gravity 1.17 times greater. Uranus's magnetic field, however, is only a tenth as strong as earth's, with an axis tilted 55° from the rotational axis. The density of Uranus is approximately 1.2.
In 1977, while recording the occultation of a star behind the planet, the American astronomer James L. Elliot discovered the presence of five rings encircling the equator of Uranus. Named Alpha, Beta, Gamma, Delta, and Epsilon (starting from the innermost ring), they form a 9400-km- (5840-mi-) wide belt extending to 51,300 km (31,860 mi) from the planet's center. Four more rings were discovered in January 1986 during the exploratory flight of Voyager 2.
In addition to its rings, Uranus has 15 satellites (the last 10 discovered by Voyager 2); all revolve about its equator and move with the planet in an east-west direction. The two largest moons, Oberon and Titania, were discovered by Herschel in 1787. The next two, Umbriel and Ariel, were found in 1851 by the British astronomer William Lassell. Miranda, thought before 1986 to be the innermost moon, was discovered in 1948 by the American astronomer Gerard Peter Kuiper. |
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Neptune (planet), fourth largest of the planets in the solar system, and eighth major planet in order of increasing distance from the sun. Neptune is, on average, about 4.5 billion km (2.8 billion mi) from the sun. It is about 49,400 km (30,700 mi) in diameter, or about 3.8 times as wide as the earth. Even though Neptune's volume is 72 times that of Earth's, its mass is only 17 times Earth's mass. Like Jupiter and Saturn, Neptune is one of the "gas giant" planets, composed of a deep atmosphere around a liquid surface and sometimes a solid core. Neptune's atmosphere consists of mostly hydrogen and helium, but up to three percent of Neptune's atmosphere is made of methane, which gives the planet its striking blue color. Its core contains more rock and metal than the cores of other gas giant planets. Neptune has a magnetic field, which is tilted more than 50° to the rotation axis.
Neptune rotates completely on its axis once every 16 hours and orbits the sun once in 164.79 Earth years. Its albedo, or reflectivity, is high (84 percent of the light falling on it is reflected), but it is so far away from the earth its stellar magnitude (a scale used to describe the brightness of an astronomical object; lower numbers correspond to brighter objects,) is only 7.8, which means it is never bright enough to be seen with the naked eye from earth. When observed through a telescope, it appears as a greenish-blue disk without any definite surface markings. However, images from the Voyager 2 probe, which flew past Neptune in 1989, and the Hubble Space Telescope, which observed Neptune in 1994, have revealed dynamic bright and dark spots in Neptune's atmosphere that are thought to be huge storms caused by the difference in temperature between the heat-producing core and the frigid cloud tops. Voyager 2 measured wind speeds of 2400 km/h (1500 mph), the highest on any planet.
Neptune is orbited by five thin rings and eight known satellites, two of which are observable from Earth. The largest and brightest is Triton, discovered in 1846, the same year Neptune was first observed. Triton, with a diameter of 2705 km (1680 mi), is only slightly smaller than Earth's moon. It has a retrograde orbit-that is, opposite the direction of rotation of the body that it orbits. Despite its extreme coldness, Triton has a nitrogen atmosphere with some methane and some form of haze, and it displays an active surface of geysers that spout an unknown subsurface material. Triton is slowly spiraling in toward Neptune; in 10 million to 100 million years it is expected to be so close to Neptune that it will be pulled apart by gravitational forces, its remnants adding to Neptune's five rings. Nereid, the second satellite (discovered in 1949), has a diameter of only about 320 km (about 200 mi). Six more satellites were discovered by the Voyager 2 planetary probe in 1989.
The discovery of Neptune was one of the triumphs of mathematical astronomy. To account for perturbations in the orbit of the planet Uranus, British astronomer John Couch Adams, by 1845, and French astronomer Urbain Jean Joseph Leverrier, by 1846, independently calculated the existence and position of a new planet. Using information from Leverrier, German astronomer Johann Gottfried Galle first observed the planet in 1846. |
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Pluto (planet), ninth planet from the sun and outermost known member of the solar system. Pluto was discovered as the result of a telescopic search inaugurated in 1905 by the American astronomer Percival Lowell, who postulated the existence of a distant planet beyond Neptune as the cause of slight perturbations in the motions of Uranus. Continued by members of the Lowell Observatory staff, the search ended successfully in 1930, when the American astronomer Clyde William Tombaugh found Pluto near the position Lowell had predicted.
Pluto revolves about the sun once in 247.7 Earth years at an average distance of 5.9 billion km (3.67 billion mi) from the sun. The orbit is so eccentric that at certain points along its path Pluto is closer to the sun than is Neptune. No possibility of collision exists, however, because Pluto's orbit is inclined more than 17.2° to the plane of the ecliptic and never actually crosses Neptune's path.
Visible only through large telescopes, Pluto appears to have a yellowish color. For many years very little was known about the planet, but in 1978 astronomers discovered a relatively large moon orbiting Pluto at a distance of only about 19,000 km (about 12,000 mi) and named it Charon. The orbits of Pluto and Charon caused them to pass repeatedly in front of one another from 1985 through 1990, enabling astronomers to determine their sizes fairly accurately. The Hubble Space Telescope allowed astronomers to determine the sizes of Pluto and Charon even more accurately in 1994. Pluto is about 2320 km (1440 mi) in diameter, about two-thirds the size of Earth's moon. Charon is about 1270 km (790 mi) in diameter, making Pluto and Charon the planet-satellite pair closest in size in the solar system. Pluto was also found to have a thin atmosphere, probably of methane, exerting a pressure on the planet's surface that is about 100,000 times weaker than the Earth's atmospheric pressure at sea level.
In 1994 the Hubble Space Telescope imaged 85 percent of Pluto's surface, revealing bright and dark areas of startling contrast. Astronomers believe that the bright areas are shifting fields of nitrogen ice and the dark areas are fields of methane ice colored by interaction with sunlight. Some of the dark areas may also be valleys or fresh impact craters. These images support the theory that extensive ice caps form on Pluto's poles, especially when the planet is farthest from the sun.
With a density about twice that of water, Pluto is apparently made of much rockier material than are the other planets of the outer solar system. This may be the result of the kind of chemical combinations that took place during the formation of the planet under cold temperatures and low pressure. Many astronomers think Pluto may be a former satellite of Neptune, knocked into a separate orbit during the early days of the solar system. Charon could be an accumulation of the lighter materials resulting from the collision. |
