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Student 2: High Merit
Discovering Dwarf Planets
Dwarf Planets
A dwarf planet is a celestial body that is in orbit around the Sun, is massive enough
for its own gravity to make it round, is not a moon but has not cleared the
neighbourhood around its orbit. The latter point means that there are bodies of a
similar size in the orbit of the dwarf planet. For example, Pluto shares its orbit with
Kuiper Belt objects of a similar size called plutinos.
There are currently five dwarf planets Pluto, Ceres, Eris, Makemake and Haumea, all
of which meet the above criteria.
Ceres is located in the asteroid belt between the orbits of Mars and Jupiter and is
about 950km in diameter. It was discovered in 1801 and was called a planet for about
50 years before becoming an asteroid and then a dwarf planet in 2006.
Eris, Makemake and Haumea are all located in the Kuiper Belt and were discovered in
2005, 2006 and 2004 respectively.
Eris is larger than Pluto and this discovery started the debate as to what was a planet
and what was a dwarf planet. Eris orbits the sun every 560years and its moon is called
The Asteroid Belt
The asteroid belt has more rocky objects compared with the Kuiper’s belts icier and
larger objects. The asteroid belt is found between Mars and Jupiter and has several
million asteroids - lumps of irregular shaped rock much smaller than planets. These
are sometimes known as minor planets and are not visible from Earth by the naked
eye. Anything above 50km in diameter is an asteroid and anything smaller is a
meteoroid. The four largest objects are Ceres, Pallas, Vespa and Hygiea but only
Ceres is considered a dwarf planet. No more large asteroids are liable to be found and
so any more dwarf planets will be found in the Kuiper Belt.
The Kuiper Belt
The Kuiper belt extends from about 30 to 55 AU and contains thousands of icy and
rocky bodies, many of which may become dwarf planets and may even be bigger than
Eris. The Kuiper belt is similar to the asteroid belt but a lot larger. In 1992 the first
Kuiper Belt object (KBO) was discovered. Since then, over 1000 KBOs have been
discovered and there may be as many as 70,000 KBOs.
KBOs are hard to find though for several reasons.
• They are mostly very small.
• They are all a long way away, up to 55 AU. Only the very biggest telescopes
can see that far.
• They have highly erratic orbits which means that the whole sky has to be
searched, rather than just the plane that the planets and asteroid belt are in.
• Their ability to be seen depends on how reflective the surface is. This is called
albedo. An object with a dark surface is much harder to see than an object with
a bright, white surface.
their diameters are very difficult to measure and have to be done by the
estimation of their brightness and how far away they are form the Sun. This
makes it very difficult to classify the objects found.
Pluto was first discovered in 1930 by Clyde W. Tombaugh. It has a diameter of 2400
km across. At that time the Kuiper belt had not been discovered and Pluto was
thought to be at the edge of the Solar System. When the first KBO was discovered in
1992 astronomers then realised that Pluto was probably part of the Kuiper Belt. One of
Pluto’s moons, Charon, is half Pluto’s size and some astronomers believe that the pair
should be treated as a binary system rather than a planet and satellite. Pluto has an
erratic orbit that is at a 17 degree angle to the ecliptic and crosses Neptune’s orbit.
These are just some of the facts that have contributed to a long running debate as to
if Pluto is to be considered a planet or not.
Pluto was considered a planet until 2009 when it was officially downgraded to a dwarf
planet by the International Astronomical Union. This happened because more and
more objects were starting to be discovered in the Kuiper Belt that were only slightly
smaller than Pluto, or in Eris’s case, larger. If Pluto remained as a planet many other
subjects would have to be declared planets as well, including Ceres. The only eight
planets that meet these requirements are the ones from Mercury to Neptune.
In July 2005 a group of astronomers have discovered a KBO that is vaguely larger
than Pluto. It was temporarily named 2003 UB313 and later named Eris. Its orbit
around the sun takes 560 year with its distance stretching from 38 to 98 AU. It also
has a tiny moon called Dysnomia.
Pluto was originally found by using a blink comparator. Photographs of the night sky
would be taken using optical telescopes and then the blink comparator would rapidly
switch from one photograph to the other. The moving object would appear to be
jumping back and forth and so could be identified.
These days, KBOs are looked for by robotically controlled telescopes taking pictures of
the same section of sky at different time intervals. Each day the computers locate
different moving objects that then need to be looked at by astronomers. The billions of
stars and galaxies always appear stationary in sky, which means that anything that
moves could be either a satellite, dwarf planet, asteroid or comet. The objects that are
found to be moving are then be looked at and analysed until they are identified.
In the future even more KBOs will be found. I wonder how many will become dwarf
planets. Our Solar System will keep on growing for a while yet.