Download Student 5: Low Achieved

Student 5: Low Achieved
What’s the difference between a planet and a dwarf planet?
Modern land and space observatories have meant that much more is known about the
solar system today. New objects in the outer solar system are often being identified.
In our solar system Pluto has always been the odd one out, mainly because that it has a
tilted orbit which for 20 years comes inside of Neptune and for the rest of the 248 years
orbits outside Neptune, and also because it is a very small rocky object near to an
enormous gas giant.
In 2005 a team of astronomers discovered an object further out than the orbit of Pluto
which was probably the same size or larger than Pluto. This object was named Eris and it
is in fact larger than Pluto. A debate began as to whether it was another planet or not
and what exactly was a planet anyway. Astronomers decided they would need to make
some sort of final decision about how to classify objects in the solar system. At the 2006
General Assembly of the International Astronomers Union (IAU) astronomers debated
how to classify objects that may or may not be planets. They decided on three
categories: planets, dwarf planets and small solar system bodies. Although not everyone
agreed the outcome was that the IAU has defined a planet as having to meet the
following criteria:1. It is in orbit around the sun
2. It needs to have enough gravity to pull itself into a round shape
3. It needs to orbit in a clear path around the sun i.e.; it needs to have ‘cleared the
neighbourhood’ of similar sized objects. This means that the largest object in an
orbit, and therefore the one with the most gravity, either consumes smaller
objects or flings them out of the orbit. A dwarf planet is defined by the IAU as a
body that:1. It is in orbit around the sun
2. Has enough gravity to pull itself into a spherical shape
3. Has not cleared the neighbourhood around its orbit, and therefore hasn’t
consumed smaller objects or flung them out of the orbit.
4. Is not a satellite (a moon)
The final category defined by the IAU is Small Solar System Bodies and these are all
other objects, except satellites, orbiting the sun. This includes asteroids, comets and
most Trans Neptunian Objects (TNOs) or as they are often called, Kuiper Belt Objects
(KBOs).
Pluto is different to the 8 planets of our Solar System:
Pluto was discovered in 1930 and it is located in an area in the outer solar system called
the Kuiper Belt which extends from the orbit of Neptune out to 55 astronomical units
which is 55 times the distance from earth to the sun.
Since 2006 Pluto has been called a dwarf planet. It is only approximately 2390 km in
diameter and its largest moon, Charon, is large in comparison to Pluto. Charon is
approximately half the size of Pluto where as other planet’s moons are proportionally
much smaller than them. Pluto has a tilted orbit which is at an angle from the plane of
the eight planets. Pluto has five known moons which shows that Pluto hasn’t cleared its
neighbourhood.
Other Dwarf Planets:
Currently the IAU recognises five dwarf planets; Pluto, Eris, Ceres, Makemake and
Haumea. The IAU designates dwarf planets that orbit the sun beyond Neptune as
‘Plutoids’. Other dwarf planets that are Plutoids are Eris, Makemake and Haumea. Ceres
is a dwarf planet that orbits in the main asteroid belt between Mars and Jupiter.
Ceres is the smallest of the dwarf planets, having a diameter of about 950km. It is the
only dwarf planet in the asteroid belt. Ceres was first discovered in 1801 and its orbit
around the sun takes 4.6 earth years.
Eris is the largest of the dwarf planets being approximately 2400km in diameter. It is
roughly three times further out in the solar system than Pluto and may even be beyond
the Kuiper Belt and in the start of the Oort Cloud. Eris’s orbit is highly tilted and it takes
557 earth years to orbit around the sun. Eris has one moon.
After Eris and Pluto, Makemake is the third largest dwarf planet and it is also located in
the Kuiper Belt. It was discovered in 2005 and takes 310 earth years to orbit the sun.
Haumea was first noticed in the Kuiper Belt in 2004 and recognised by the IAU in 2008
as a dwarf planet. Its orbit around the sun takes 285 earth years and it has two moons.
It has an unusual oval shape which may be due to a collision or rapid spinning but it
does have enough mass to be a dwarf planet.
Further discoveries in the Kuiper Belt:
Astronomers and scientists now know that the Kuiper Belt is home to a large collection of
objects, maybe up to 70000 KBOs. It is estimated this may include 20 or more further
dwarf planets of similar composition to Pluto. Further observations will determine their
size, shape and properties allowing their status to be classified.
Studies are conducted through powerful telescopes on land and also in space. Space
probes, spectroscopes and space crafts also collect valuable information. In 2005 NASA
launched the spacecraft New Horizons and this will reach Pluto in 2015 providing new
insights.
New dwarf planets in the Kuiper Belt are hard to find for many reasons. They are a long
distance away and they can be very hard to detect. Even when they are seen it may be
hard to find out much about them or even to tell how big they are. Also, KBOs have very
tilted orbits which means that they are hard to see in the sky.