Definition of the planets in the Solar System
The following is reproduced in part from the
resolutions of 26th General
Assembly of the
International Astronomical Union (IAU,
the world's organisation of professional astronomers) held in Prague (Czech Republic) from
2006 August 14th to 25th.
Apparently the discussions were vigorous with supporters divided roughly
into two groups. One group supported a "physical" definition - that a
planet is a near spherical body in hydrostatic equilibrium (gravitational
compression is balanced by an outward force such as "gas pressure"). Using
this definition the planets total 12. The other group supported
a "dynamical" definition
where a planet is an object that gravitationally dominates its neighbourhood
- in this case there are 8 planets.
The resolution adopted by the IAU General Assembly (see below) is a compromise which also
defines two categories of bodies different from the planets, and modifies
the former rules. The naming "minor planets" disappears and is replaced by either
"dwarf planets" which designates Solar System bodies big enough
to be in nearly spherical, or "small Solar System
bodies" which designates the other objects including the comets. The planetary satellites
(moons) are not concerned by these new definitions.
An IAU process will be established to assign
borderline objects into either dwarf planet and other categories.
 |
| Diagram showing relative sizes of TNOs, Pluto, Pluto's moon Charon, and
the Moon. Note: the surface detail and shape is only accurate for the
Moon and Pluto, for the others it's an artistic impression (from Sky & Telescope
magazine). |
Background
Since 1992, numerous celestial bodies orbiting around the Sun beyond Neptune's
orbit have been discovered. Due to its physical and dynamical characteristics,
Pluto appears associated with these trans-Neptunian bodies. In 2003, the discovery
of 2003 UB313 (now named (136199)
Eris), a trans-Neptunian object (TNO) bigger than Pluto, raised serious questions
about the definition of a planet: if Pluto is a planet, Eris is another one...
IAU Resolution: Definition of a Planet in the Solar System
Contemporary observations are changing our understanding of planetary systems,
and it is important that our nomenclature for objects reflects our current
understanding. This applies, in particular, to the designation
"planets". The word planet originally described "wanderers" that were known only
as moving lights in the sky. Recent discoveries lead us to create a new definition,
which we can make using currently available scientific information.
The IAU therefore resolves that planets and other bodies in our Solar System
be defined into three distinct categories in the following way.
1. Planets
| A planet is a celestial body that: |
| (a) |
is 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. |
The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
2. Dwarf Planets
| A dwarf planet is a celestial body that: |
| (a) |
is 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 NOT cleared the neighbourhood around its orbit, and |
| (d) |
is not a satellite |
Objects with sufficiently known characteristics to be classified as dwarf
planets are (1) Ceres, (134340) Pluto,
and (136199) Eris. The number in parentheses is the minor planet catalogue
number - a standard astronomical cataloguing/naming convention. The following
is the IAU's Minor Planet Center (MPC) explanation from 2006 September 8 for
the numbering.
As more observations are acquired it is highly probably that other objects will be classified as dwarf planets.
Since at least one of the "dwarf planets" is already included in the catalogue of numbered
"minor planets", and since the MPC Terms of Reference emphasize the need for the MPC to
maintain a database of the astrometric observations of such bodies observed
beyond the confines of the earth's atmosphere, Pluto and (other bodies) are now being added
to this list of objects with reliable orbit determinations under the numbers (134340) and
(136199), respectively. It should be noted that, just as some of the numbered objects that
have exhibited cometary activity also have designations in the catalogue of numbered periodic
comets, the numbering of "dwarf planets" does not preclude their having dual designations in
possible separate catalogues of such bodies.
Note: This numbering does NOT mean that Pluto is an asteroid or a minor planet (a now defunct
definition, see above).
3. Small Solar System Bodies
All other objects, except satellites, orbiting the Sun shall be referred to
collectively as "small
Solar System bodies". This includes: the classical asteroids (except the largest
one, 1 Ceres) that are not dwarf planets; the Centaurs and Neptune Trojans; the
smaller TNOs (except dwarf planets such as Pluto and Eris);
and all comets.
