Reading Comprehension Sheet #9: Pluto
Pluto is the last of the outer planets (that we know of!) and is the ninth planet from the sun. Scientists did not discover Pluto until 1930, because it is so small and hard to see even with a telescope. In 1930, it was discovered by an amateur astronomer! Scientists have interesting theories about why Pluto is so small and where it may have come from.
Some astronomers think Pluto may have come into our Solar System from an area called the Kuiper Belt. The Kuiper Belt is located past Pluto's orbit and appears to be an asteroid belt which circles the outermost part of our Solar System. Astronomers have recently studied this using very powerful telescopes such as the Hubble Space telescope. They have found 32 objects so far which appear to be asteroids. They estimate there may be as many as 35,000 objects in the Kuiper Belt! Maybe Pluto is an asteroid that escaped from the belt and was drawn into orbit in our Solar System by the sun's gravity. Other scientists think that Pluto might even be be an old moon of Neptune that escaped its orbit and moved further out into its own orbit. No one is sure what the truth about Pluto is, so for now it is still officially called a planet!
Pluto is extremely small and has one moon, Charon, which is almost the same size as the planet. Pluto is only 1,430 miles across. It spins fairly slowly. One Pluto day is 6 Earth days and nine hours long. It takes a long time to orbit the sun, however. One Pluto year is 248 earth years long!
Scientists have not discovered much about
Pluto because it is so far away from the
Earth and hard to see even with very powerful
telescopes. They think it is made of rock
and ice, and might have a thin atmosphere
made of gases such as nitrogen, carbon monoxide and methane. They do know it
is smaller than our moon and must be very
cold because it is so far away from the sun. One thing scientists have discovered is
that Pluto has a very odd orbit. Sometimes
it switches places with Neptune and becomes
the eighth planet from the sun. Other times
it is the ninth planet from the sun. From
1979 until February 11, 1999 Pluto was the
eighth planet from the sun. It crossed Neptune's
orbit on February 11, 1999 and once again
became the ninth planet from the sun. It
will stay in this position for the next 228
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