Survey
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Pluto Relatively little is known about Pluto, the farthest planet from the sun. On average, Pluto orbits at a distance of about 3.7 billion miles from the sun. This is so far away that even the Hubble Space Telescope has trouble taking clear photographs of the planet. Pluto is not always the solar system’s outermost planet. Its orbit is so eccentric (oval-shaped) that it is sometimes closer to the sun than Neptune. In fact, from 1979 to 1999, Neptune was the farthest planet from the sun. Pluto was discovered almost by pure luck. In 1930, astronomers thought that the odd orbits of Uranus and Neptune must be caused by some ninth planet that was farther away from the sun. After some calculations, the scientists made a prediction of where they would find the ninth planet: and they found it! This would all seem fine if not for one thing: the astronomers’ calculations turned out to be wrong! At any rate Pluto is not large enough to cause any perturbations in the orbits of other planets, so for a long time astronomers looked for a tenth planet (Planet X) to no avail. Fortunately for astronomers, Pluto has a moon, Charon (pronounced CARE-en). By watching Pluto and Charon pass in front of each other, astronomers have been able to obtain some basic information on the surface features of both objects. Astronomers have also been able to calculate the total mass of both Pluto and Charon together with some ease, but determining their individual masses is much more difficult. Temperatures on Pluto can get as low as -390 degrees Fahrenheit. Astronomers aren’t sure, but they think that Pluto is composed mostly of rock (about 70%) and frozen water (about 30%). Pluto’s atmosphere is most likely composed primarily of nitrogen, with some carbon monoxide and methane. For most of its year, Pluto is so cold that its atmosphere cannot exist as a gas. Pluto’s atmosphere is only gaseous when it is closest to the sun.