Space Forum / Astronomy / July 2008

In recent times it has become necessary for the IAU to
reevaluate and reclassify certain objects in our Solar
System. This process has been going on for a long time,
and received a special kick the year Isaac Asimov died,
1992.  While scientists had thought for many years that
there were other objects way out there with Pluto, it
wasn't until 1992 that the first, (15760) 1992 QB1, was
actually discovered.  Then it took eleven more years for
the first authoritative and official statement to form...

          Modern definition of "planet" formed by
        the International Astronomical Union (IAU)

Ref.:  Main Wiki article:  "Planet"

With the discovery during the latter half of the 20th
century of more objects within the Solar System and
large objects around other stars, disputes arose over
what should constitute a planet.  There was particular
disagreement over whether an object should be
considered a planet if it was part of a distinct population
such as a belt, or if it was large enough to generate
energy by the thermonuclear fusion of deuterium.

In 2003, the IAU Working Group on Extrasolar Planets
made a position statement on the definition of a planet
that incorporated a working definition:

1) Objects with true masses below the limiting mass for
thermonuclear fusion of deuterium (currently calculated
to be 13 times the mass of Jupiter for objects with the
same isotopic abundance as the Sun) that orbit stars or
stellar remnants are "planets" (no matter how they
formed).  The minimum mass and size required for an
extrasolar object to be considered a planet should be
the same as that used in our Solar System.

2) Substellar objects with true masses above the
limiting mass for thermonuclear fusion of deuterium are
"brown dwarfs", no matter how they formed or where
they are located.

3) Free-floating objects in young star clusters with
masses below the limiting mass for thermonuclear
fusion of deuterium are not "planets", but are "sub-
brown dwarfs" (or whatever name is most appropriate).

This definition has since been widely used by many
astronomers when publishing discoveries in academic
journals.  Although temporary, it remains an effective,
working definition until a more permanent one is
formally adopted.  Nevertheless, it did not address the
dispute over the lower mass limit, and steered clear of
the controversy regarding objects within the Solar
System.

This matter was finally addressed during the 2006
meeting of the IAU's General Assembly.  After much
debate and one failed proposal, the assembly voted to
pass a resolution that defined planets within the Solar
System as:

A celestial body that is

  (a) in orbit around the Sun,

  (b) has sufficient mass for its self-gravity to overcome
        rigid body forces so that it assumes a hydrostatic
        equilibrium (nearly round) shape, and

  (c) has cleared the neighbourhood around its orbit.

Under this definition, the Solar System is considered to
have NINE planets.  Bodies which fulfill the first two
conditions but not the third (such as Pluto and Eris) are
classified as dwarf planets, providing they are not also
natural satellites of other planets.  Originally an IAU
committee had proposed a definition that would have
included a much larger number of planets as it did not
include (c) as a criterion.  After much discussion, it was
decided via a vote that those bodies should instead be
classified as dwarf planets.

This definition is based in modern theories of planetary
formation, in which planetary embryos initially clear
their orbital neighborhood of other smaller objects.

The end product of secondary disk accretion is a small
number of relatively large bodies (planets) in either non-
intersecting or resonant orbits, which prevent collisions
between them.  Asteroids and comets, including KBOs,
differ from planets in that they can collide with each
other and with planets.

In the aftermath of the IAU's 2006 vote, there has been
criticism of the new definition, and some astronomers
have even stated that they will not use it.  Part of the
dispute centers around the belief that point (c)
(clearing its orbit) should not have been listed, and that
those objects now categorised as dwarf planets should
actually be part of a broader planetary definition.  The
next IAU conference is in 2009, when modifications
could be made to the definition, also possibly including
extrasolar planets.

BY DEFINITION, the nine planets in our Solar System are...

           Mercury
                Venus
                      Earth
                            Selene
                                  Mars
                                        Jupiter
                                              Saturn
                                                    Uranus
                                                          Neptune

Planet Selene (the Moon) is a major terrestrial planet as
defined both by the fact that Selene demonstrates itself
to be a planet, and by the official definition agreed upon
by the members of the IAU!

Since the IAU definition of "planet" includes our Moon, it
is now "official".  Earth and Selene form a double- or
binary-planet system.

Great and powerful Selene, which gives Earth its tides,
and served to empower the formation and progress of
life, has finally come full circle. Selene is now officially a

                    Full-Fledged Major Planet

(in its own right!)

happy days and...
  starry starry nights!