Pluto
Pluto was named after the god of the underworld in Greek and Roman mythology. The planet is normally th
e outermost in the solar system. Its orbit, however, is much more elliptical, or oval, than those of the other planets, so its distance from the sun varies considerably. At its aphelion, or greatest distance from the sun, it is about 4.53 billion miles (7.30 billion kilometers) away from the sun. About every 248 years when Pluto reaches its perihelion, or nearest point to the sun, it is within some 2.76 billion miles (4.44 billion kilometers). Pluto is then closer to the sun than Neptune is for a 20-year period, as it was in 1979–99. The two planets do not collide, however. Pluto's orbit is tilted more than any other planet's; it is inclined 17° from the ecliptic, which is an imaginary plane passing through the Earth and the sun. As Pluto nears perihelion, it is always much above or below the plane of Neptune's orbit. In addition, Pluto revolves twice around the sun in the same time that Neptune revolves three times, in a configuration such that the planets never pass each other closely.
Because of its great distance from the Earth, Pluto appears relatively faint even when viewed with a telescope. Its brightness level varies regularly by about 12 percent in a period of 6.387 days. This variation indicates that some areas of the planet's surface reflect much more light than others and that it completes one rotation on its axis every 6.387 days. Pluto's axis is tipped about 122° relative to its orbital plane, so that, like Uranus, it lies nearly on its side. Both planets spin in retrograde motion, or in the direction opposite that of most of the other planets. An observer on Pluto would see the sun rise in the west and set in the east.
Pluto's unusual physical characteristics make it difficult to classify. It is neither one of the inner, rocky planets like Earth, nor one of the outer, gaseous planets like Jupiter. Some astronomers believe that Pluto should be considered a minor planet or the largest member of the Kuiper
belt, a region of small, icy cometlike bodies beyond Neptune. In size, density, and surface composition, Pluto resembles Triton, an icy moon of Neptune, more than it does the other planets. Pluto is the smallest planet in the solar system by far. Its diameter of approximately 1,485 miles (2,390 kilometers) is less than half that of the next-smallest planet, Mercury. Several moons, including the Earth's, are larger. The planet's density of about 110 pounds per cubic foot (1,750 kilograms per cubic meter) is similar to Triton's but less than a third of the Earth's.
Because of this density value, astronomers theorize that the planet may be composed of about 50–75 percent rock, with the remainder water ice. The bright regions of Pluto's icy surface consist largely of frozen nitrogen, with some solid methane, carbon monoxide, and ethane. Little is known about the darker portions of the surface, but it is thought that they might contain organic compounds. Observations made with infrared telescopes suggest strongly that Pluto has polar caps composed of methane ice. These ice caps sometimes appear to extend halfway to Pluto's equator.
Because it is so far from the sun, the planet receives only about 1/1600 the sunlight that the Earth does, and its surface is very cold. The average surface temperature is thought to be about −373° F (−225° C), with the brighter areas generally colder than the darker ones. The temperature varies considerably along with the planet's distance from the sun. Since one year on Pluto equals about 248 Earth years, its seasons are very long. The planet's elliptical orbit makes them uneven; winter, for example, probably lasts about 100 Earth years. Pluto has a thin atmosphere, likely composed of nitrogen with smaller amounts of methane and carbon monoxide. As the planet moves away from the sun, the atmosphere freezes and falls to the surface like snow.
Pluto's eccentric orbit and its physical similarities to icy satellites originally led some astronomers to believe that Pluto did not have the same origin as the other planets. One theory suggested that Pluto and its moon, Charon, may once have been satellites of Neptune but were pulled away from Neptune's gravitational field. Most scientists, however, no longer believe this model is physically plausible. Current models suggest that Pluto and its satellite instead formed as two independent bodies in the solar nebula. Pluto may have collided with a proto-Charon, with the present satellite developing from the resulting debris.
e outermost in the solar system. Its orbit, however, is much more elliptical, or oval, than those of the other planets, so its distance from the sun varies considerably. At its aphelion, or greatest distance from the sun, it is about 4.53 billion miles (7.30 billion kilometers) away from the sun. About every 248 years when Pluto reaches its perihelion, or nearest point to the sun, it is within some 2.76 billion miles (4.44 billion kilometers). Pluto is then closer to the sun than Neptune is for a 20-year period, as it was in 1979–99. The two planets do not collide, however. Pluto's orbit is tilted more than any other planet's; it is inclined 17° from the ecliptic, which is an imaginary plane passing through the Earth and the sun. As Pluto nears perihelion, it is always much above or below the plane of Neptune's orbit. In addition, Pluto revolves twice around the sun in the same time that Neptune revolves three times, in a configuration such that the planets never pass each other closely.Because of its great distance from the Earth, Pluto appears relatively faint even when viewed with a telescope. Its brightness level varies regularly by about 12 percent in a period of 6.387 days. This variation indicates that some areas of the planet's surface reflect much more light than others and that it completes one rotation on its axis every 6.387 days. Pluto's axis is tipped about 122° relative to its orbital plane, so that, like Uranus, it lies nearly on its side. Both planets spin in retrograde motion, or in the direction opposite that of most of the other planets. An observer on Pluto would see the sun rise in the west and set in the east.
Pluto's unusual physical characteristics make it difficult to classify. It is neither one of the inner, rocky planets like Earth, nor one of the outer, gaseous planets like Jupiter. Some astronomers believe that Pluto should be considered a minor planet or the largest member of the Kuiper
belt, a region of small, icy cometlike bodies beyond Neptune. In size, density, and surface composition, Pluto resembles Triton, an icy moon of Neptune, more than it does the other planets. Pluto is the smallest planet in the solar system by far. Its diameter of approximately 1,485 miles (2,390 kilometers) is less than half that of the next-smallest planet, Mercury. Several moons, including the Earth's, are larger. The planet's density of about 110 pounds per cubic foot (1,750 kilograms per cubic meter) is similar to Triton's but less than a third of the Earth's.
Because of this density value, astronomers theorize that the planet may be composed of about 50–75 percent rock, with the remainder water ice. The bright regions of Pluto's icy surface consist largely of frozen nitrogen, with some solid methane, carbon monoxide, and ethane. Little is known about the darker portions of the surface, but it is thought that they might contain organic compounds. Observations made with infrared telescopes suggest strongly that Pluto has polar caps composed of methane ice. These ice caps sometimes appear to extend halfway to Pluto's equator.
Because it is so far from the sun, the planet receives only about 1/1600 the sunlight that the Earth does, and its surface is very cold. The average surface temperature is thought to be about −373° F (−225° C), with the brighter areas generally colder than the darker ones. The temperature varies considerably along with the planet's distance from the sun. Since one year on Pluto equals about 248 Earth years, its seasons are very long. The planet's elliptical orbit makes them uneven; winter, for example, probably lasts about 100 Earth years. Pluto has a thin atmosphere, likely composed of nitrogen with smaller amounts of methane and carbon monoxide. As the planet moves away from the sun, the atmosphere freezes and falls to the surface like snow.
Pluto's eccentric orbit and its physical similarities to icy satellites originally led some astronomers to believe that Pluto did not have the same origin as the other planets. One theory suggested that Pluto and its moon, Charon, may once have been satellites of Neptune but were pulled away from Neptune's gravitational field. Most scientists, however, no longer believe this model is physically plausible. Current models suggest that Pluto and its satellite instead formed as two independent bodies in the solar nebula. Pluto may have collided with a proto-Charon, with the present satellite developing from the resulting debris.
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