With the advent of powerful new telescopes on the ground and in space, recent new discoveries have been made of objects in the outer regions of our Solar System that have sizes comparable to and larger than Pluto.

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With the advent of powerful new telescopes on the
ground and in space, recent new discoveries have
been made of objects in the outer regions of our
Solar System that have sizes comparable to and
larger than Pluto. These discoveries have
rightfully called into question whether or not
they should be considered as new planets.'
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The International Astronomical Union (IAU), have
concluded two years of work defining the
difference between planets and the smaller
solar system bodies such as comets and
asteroids. The astronomers gathered August
14-25, 2006 at the IAU General Assembly in
Prague, where they announced voting results on
the definition of a planet.
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• The new definition of a planet has 3 parts
• A Planet is a celestial body that is in orbit
  around the Sun.
• It has sufficient mass that its self-gravity
  forms the body into a (nearly) spherical shape.
• It has cleared the neighborhood around its orbit.
  This means that it has enough mass to sweep
  out debris, such as small asteroids, from its
  orbital path.

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According to this definition, the Solar System
contains 8 classical planets Mercury, Venus,
Earth, Mars, Jupiter, Saturn, Uranus, and
Neptune. In addition, the IAU voted to create a
new class of objects called Dwarf Planets.
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The first 3 objects to be classified as Dwarf
Planets are Pluto, Ceres and 2003 UB313 (also
known as Xena). Dwarf Planets are defined as

• A celestial body in orbit around the Sun.
• Has sufficient mass for its self-gravity to form
  it into a (nearly) spherical shape.
• Has not cleared the neighborhood around its
  orbit.
• Is not a satellite.

Artist concept of 2003 UB313. Image credit
NASA/JPL-Caltech
Color View of Ceres. Image credit NASA, ESA,
J. Parker
Pluto and its moons. Credit NASA
The name 2003 UB313 is provisional, as a "real"
name has not yet been assigned to this object.
The nickname for this object is Xena. A decision
and announcement of a new name will by made by
the IAU at a later date.
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Because Plutos moon, Charon, is about half the
diameter of Pluto, there was much discussion of
classifying the system as a double planet. The
IAU did not re-classify Charon, so for now it
remains a satellite of Pluto. Charon may later
be classified as a Dwarf Planet.
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Currently a dozen candidate Dwarf Planets" are
listed on IAU's "watchlist" which keeps changing
as new objects are found and the physics of the
existing candidates becomes better known.
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Small Solar System Bodies is the classification
name of all other objects in the Solar System
except satellites. This includes most comets,
asteroids, and Kuiper Belt objects.
Credit NASA
Credit Lynette Cook and W. M. Keck Observatory
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Dwarf Planets and Small Solar System Bodies
typically have orbits that are highly tilted with
respect to the classical planets (technically
referred to as a large orbital inclination).
They also typically have orbits that are far
from being perfectly circular (technically
referred to as having a large orbital
eccentricity). All of these distinguishing
characteristics are scientifically interesting in
that they suggest a different origin from the 8
classical planets.
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Most Dwarf Planets, including Pluto, reside in
the Kuiper Belt, in the outer region of our Solar
System. These objects, which orbit beyond
Neptune, are known as Trans-Neptunian Objects
(TNOs) Pluto has much more in common with other
recently discovered TNOs than with the planets,
which led to its designation as a Dwarf
Planet. It is expected that many more
Trans-Neptunian Objects will be discovered in the
coming years.
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These 3 newly defined classification systems are
not without controversy in the astronomy
community!
The International Astronomical Union (IAU) is an
organization of over 8,500 professional
astronomers from all over the world. The IAU is
recognized as the official authority in
astronomy, and is responsible for naming all
planets, asteroids, and other celestial bodies
and phenomena.
Despite the large number of members, only about
5 of members were involved in voting on the 2006
resolution which narrowed the definition of a
planet.
Many astronomers have pointed out ambiguities in
the definition. For instance, several planets,
including Earth, have not fully cleared their
neighborhood of debris. Many astronomers are
also clearly disappointed that Pluto was not
classified as a double planet.
What Do You Think??