Download Pluto was discovered on February 18th 1930 by Clyde

Student 6: High Not Achieved
Finding dwarf planets:
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There are 8 planets in our Solar System, but lots of smaller bodies that are now
being called dwarf planets. Examples of dwarf planets are Pluto, Ceres and Eris.
Astronomers used to think that the Solar System ended at Pluto but in 1992 the
Kuiper Belt was discovered beyond Neptune. Many dwarf planets are being
discovered in the Kuiper Belt.
Planets and dwarf planets are similar in two ways but different in one other way.
Planets and dwarf planets both orbit the Sun and are round. This means that they
have sufficient mass to have the right amount of gravity to be round.
Planets also dominate their orbits which means that they have cleared their
neighbourhood of smaller objects. Dwarf planets haven’t cleared their
neighbourhood. The dwarf planets do not dominate their orbit as they are not the
main gravitational body in the orbit.
Many objects in our solar system have been classified as dwarf planets, the main
ones being Pluto, Ceres and Eris.
Pluto was discovered on February 18th 1930 by Clyde Tombaugh. Pluto is usually
the farthest from the sun out of any of the main planets, however, due to its
erratic orbit, it sometimes comes inside Neptune. Since Pluto is so far from Earth,
little is known about the planet’s size or surface conditions.
Ceres is the smallest identified dwarf planet in the solar system and the only one
in the asteroid belt. The asteroid belt sits between the planets Mars and Jupiter.
It is a region where millions of rocks can be found circling the sun. Some rocks
are huge like Ceres itself but some only as small as a baseball. The surface of
Ceres is a mixture of water, ice and various minerals such as carbonates and
clays. The mass of Ceres is small, but it is large enough to give it a shape with is
nearly spherical. No other asteroids are dwarf planets because none of them have
enough mass to be round.
The Kuiper Belt is like the asteroid belt except that the objects are icier. There
are many objects in the Kuiper belt, many of which may be larger than Pluto. Eris
is the largest dwarf planet as is the ninth biggest body known to orbit the sun
directly. Because Eris is bigger than Pluto, it was initially considered to be a tenth
planet but as the definition for a planet changed, both Pluto and Eris became
known as dwarf planets. Eris’s surface is described as icy and rocky and is similar
to Pluto’s surface.
Pluto is the only dwarf planet to once have been considered a major planet. Once
thought of as the ninth planet and the one most distant from the sun, Pluto is
now seen as one of the largest known objects of the Kuiper belt though a number
of other objects were almost as large. The Kuiper belt is a region beyond the orbit
of Neptune that contains hundreds of icy objects. Detecting these objects is
difficult because they are very faint and have very erratic orbits. Modern
telescopes are revealing a lot more of these objects in the Kuiper belt as the
technology is becoming a lot more powerful.
Pluto is considered for one main reason. Objects still remain in Pluto’s path which
means Pluto does not have a cleared neighbourhood. The new definition of a
planet is an object that orbits the sun and is large enough to overcome the force
of its own gravity. In addition to this the object must clear its neighbourhood
around its orbit. Under this definition, Pluto is not considered one of the planets in
our solar system. This is because Pluto isn’t the dominant gravitational body in its
orbit.
In the future telescopes may discover even more dwarf planets in the Kuiper Belt.
It will be interesting to see if any of these are even bigger than Pluto and Eris.