What is a planet ?

The International Astronomical Union (IAU) hold its General Assembly in Prague on August 14-25, 2006 and discussed about the definition of a planet. Since 1992, many celestial bodies orbiting beyond Neptune have been discovered. Because of its physical and dynamical characteristics, Pluto appeared to belong to this new family of Trans-Neptunian Objects (TNO) or "Kuiper belt" (suggested by Kuiper several decades ago). The discovery in 2003 of such a body larger than Pluto (2003 UB313) was the beginning of the discussion for a new definition of a planet. If Pluto was a planet, why not 2003 UB313 ?

Discussions were lively: two opinions were opposed: a physical definition: a planet is a spherical body in hydrostatic equilibrium (12 bodies entered such a classification) a dynamical definition: a planet is a body dominating gravitationally its environment, its orbit being then quasi circular in a plane close to the ecliptic (8 bodies entered such a classification).

The text of the adopted resolution (see below) is a compromise describing also the other categories of objects modifying the old definitions. The name "minor planet" disappears and two new definitions appear: the "dwarf planets" sufficiently large to be in hydrostatic equilibrium (but not enough powerful "gravitationally") and the "small bodies" of the solar system including all other asteroids and comets. The satellites are not concerned by the new definitions.

Recent observations have modified our vision of the planetary systems and it is important to have now definitions in agreement with our present understanding of the solar system. This has to be first applied to the definition of a "planet". The word "planet" was first used for "wandering star" opposite to fixed stars. The progresses of our knowledge of these wandering stars led to new definitions.

As a consequence, The International A stronomical Union (IAU) decided to divide solar system bodies into three categories as follows:

A planet (note 1) is a celestial body, which (a) is in orbit around the Sun, (b) has a sufficient mass implying that its gravity outweighs the cohesive forces of the solid body and keeps it in hydrostatic equilibrium, under a near spherical shape, (c) has eliminated any body moving on a close orbit;
A "dwarf planet" (note 2) is a clestial body, which (a) is in orbit around the Sun, (b) has a sufficient mass implying that its gravity outweighs the cohesive forces of the solid body and keeps it under a near spherical shape, (c) has not eliminated all bodies moving on a close orbit, (d) is not a satellite.
all other objects (note 3) in orbit around the Sun, except satellites, are named "small bodies of the solar system".

Notes:

1 - the eight planets are : Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
2 - a specific action will be organized by the IAU in order to decide for the case of ambiguous bodies.
3 - this includes most of the astéroids of the solar system, most of the trans-neptunian objetcs (TNO), comets and all other bodies.

According to the above definition, Pluto is a "dwarf planet". It is identified as the prototype of a new class of objects trans-Neptunian whose name remains to be defined.

The major achievement of the resolution is classification into three categories of body system solar.
The old categories:
"planets" (the nine objects given usually as planets),
"satellites" (orbiting around planets),
"asteroids or minor planets" (others pebbles can be spherical or not)
"comets" (small body of ice with highly eccentric orbits)
are replaced by:
"planets" (body gravitationally dominant and heavy enough to be in equilibrium hydrostatic)
"satellites" (no change)
"dwarf planets" (spherical bodies in hydrostatic equilibrium with a weak gravitational influence on their environment)
"small bodies of the solar system" (the rest i.e. both nonspherical asteroids and comets which are
sometimes difficult to discern from each other, some asteroids becoming comets...).

The new definition is very different (the "minor planets" disappear) and is more in agreement with our
present knowledge. To explain the new definition, it is important to see the historical side, history of humanity and also history of the solar system.

The first astronomers easily saw the existence of seven wandering stars in the sky, opposite to fixed stars (Mercury, Venus, Mars, Jupiter and Saturn and also the Sun and the Moon. These bodies remained the only known solar system bodies during centuries. from the XVIIth century, the Copernician system will change our understanding of the system. The planets are now six: Mercury, Venus, the Earth, Mars, Jupiter and Saturn. Galileo's telescope will put into evidence the existence of satellites around Jupiter and Saturne but the number of planets will remain the same until the discovery of Uranus, by chance, in 1781. Celestial mechanics is sufficiently advanced so that we have a pretty good view of the solar system: the Sun surrounded by seven planets, accompanied by some satellites. In 1801, the discovery of Ceeres between Mars and Jupiter let think that an eighth planet was founded but many small bodies that will be discovered in the nineteenth century will lose its status as a planet to Ceres. In addition to planets, satellites and comets, a new family of body appeared: the asteroids. When Le Verrier discovered Neptune by calculation in 1846, it was seen immediately that it was an eighth planet, massive, orbiting in the same plane as almost others on a circular path. Searching a ninth planet was then a challenge but it was only in 1930 that a new distant body (Pluto) was discovered and was considered as a ninth planet because astronomers thought that it was more massive and also because it was believed to have completed an inventory of the solar system. Pluto will, however, always considered a little apart, its orbit intersecting that of Neptune, which does not correspond to the idea that there is a planet ... The discovery of a larger body than Pluto in 2003 (2003 UB313) and a multitude of "small plutos" and other small asteroids in a second belt beyond Neptune required definitively a new rule on the concept of planet.

Now consider the history of the solar system. Although its formation is not yet fully explained, it is assumed that there was formation of aggregates of dust particles, enlarging gradually. From a certain mass, gravitation establishes a hydrostatic equilibrium which gives a spherical shape to large celestial bodies. The evolution of such a body is however not complete: its orbit is eccentric and multiple shocks will happen (which will still make the body growing) which will give it a nearly circular orbit. The body will be at its minimum energy (see the proof from the two-body problem) and has absorbed most of the small bodies in its environment unless they were on stable orbits (satellites of Jupiter, Trojan asteroids,...). This is in this final state that it can be considered that a body acquires its status as a planet. It is not impossible that a ninth planet exists beyond the second asteroid belt but it must have been able to complete its formation (motions are very slow) and we must be able to observe it.