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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.