In August 2006, the International Astronomical Union (IAU) set out formal definitions as what conditions were required to be met for a celestial body to be considered a planet. Prior to this date, there had been no formal definition as to what was or wasn’t a planet but given the greater knowledge about the celestial body (and former planet) known as Pluto, it was felt that a greater degree of precision was required in the way that celestial objects are defined. At the same time, the IAU also introduced a new category of celestial objects, that of dwarf planet.
A planet, according to the current orthodoxy, is a celestial body that satisfies three conditions:
It must be in orbit around the Sun;
It must possess sufficient mass of itself so that its own gravitational force allows it to assume a hydrostatic equilibrium shape that is to say its gravitational force is sufficient to make it a sphere-shaped object; and
It must have cleared its orbital neighborhood.
Having defined what constitutes a planet, the IAU went on to define the conditions that needed to be satisfied for a celestial object to fall within the new category of dwarf planet. A dwarf planet is a celestial object that:
Satisfies the first condition as for a planet; i.e. it is in orbit around the Sun;
Satisfies the second condition as for a planet; i.e. it has assumed a hydrostatic equilibrium state as a result of its own gravitational force;
Fails to satisfy the third condition as for a planet; i.e. it has failed to clear its orbital neighborhood; and
It is not a satellite.
As a consequence of these new definitions, the former planet Pluto was degraded from being a planet and was reclassified as a dwarf planet (on 24th August 2006) along with certain other celestial objects such as the scattered disc[1] object Eris (on 13th September 2006); the asteroid belt[3] (on 13th September 2006); and the Kuiper belt[4] objects Makemake (on 11th July 2008) and Huamea (17th September 2008). But, what do the conditions set out by the IAU really mean?
The object must be in orbit around the Sun; thus, offhand, every object in the solar system, other than the Sun, is a candidate for dwarf planet as, one way or the other, every such body orbits the Sun.
The requirement for hydrostatic equilibrium shape drastically reduces the numbers of the Sun’s entourage that qualify for the vast majority of celestial objects do not begin to approach the mass requirements that will result in hydrostatic equilibrium. It seems safe to assert that there is no upper mass limit for a body that falls into the class of dwarf planets; certainly the IAU has not specified limits either way. But, as a purely theoretical proposition, if a body as large as Earth say, or even Jupiter has failed to meet the third requirement specified for being a planet, then it seems logical to assert that such a body ought to be classified as a dwarf planet. As to the lower mass limits, it would depend upon the composition of the body in question. Studies indicate that bodies whose composition is mainly of rigid silicates, such as the rocky asteroids, will have to attain a diameter of about 600 kilometers with a mass of about 3.4 x 1020 kilograms; for bodies composed of less rigid materials, say water ice, a diameter of about 320 kilometers with a mass of about 3.25 x 1019 kilograms. As time goes on, we are likely to get a clearer understanding of these matters. It is thought that without meeting these minimum size/mass requirements, a body is unlikely to have the required internal pressure that brings about the process known as plasticization : i.e. the body’s own gravitational force is sufficient to plasticize its constituent matter so that hollows are filled and high grounds brought low, thereby forcing the body into a hydrostatic equilibrium shape.
What is it that is meant by clearance of the body’s neighborhood? Every celestial body has, what may be described as, some amount of debris that litter its orbital path; however, some bodies have more debris along their pathway than others have. Earth, a planet, for example, is a celestial body that has cleared its neighborhood; Earth’s mass is 1.7 million times the combined mass of the other objects that litter its pathway. When we consider Pluto and Ceres, the dwarf planets of which we are presently most knowledgeable, the situation is quite drastically different. Ceres, for instance, masses just about a third the combined mass of the other objects that litter its pathway; as for Pluto, although more massive than Ceres, it masses just 0.7% of the combined mass of the objects that share its pathway.
That the object in question is not a satellite (i.e. a satellite of a body other than the Sun) seems self-explanatory. A satellite is not in direct orbit around the Sun; its journey around the Sun being in conjunction with the journey of its primary. However, it was probably necessary to include this particular point as at least nineteen[5] of the satellites that orbit the planets of the solar system are (known) to have achieved a hydrostatic equilibrium shape as a result of their own internal gravitational forces.
The IAU recognizes five dwarf planets, to wit: Ceres (discovered in 1801); Pluto (1930); Huamea (2004); Makemake (2005); and Eris (2005). In addition, astronomers have marked out dozens of celestial bodies that are more or less likely to qualify to membership of this class of celestial bodies, including Quaoar (2002) and Sedna (2003).
[1] The scattered belt is a somewhat disorganized region comprising of icy bodies that orbit the Sun and lies just beyond the Kuiper belt (see note 4, below).
[2] The asteroid belt, a ring of celestial bodies that orbit the Sun between the orbits of Mars and Jupiter.
[3] The classification of Ceres presents some interesting parallels with that of Pluto. For half a century following its discovery on 1st January 1801, it was classified as a planet. Subsequently, it was reclassified as an asteroid under which designation it spent the next one and a half centuries. Now, it is a dwarf planet.
[4] The Kuiper belt is a large, more or less stable, ring of icy objects that orbit the sun beyond the orbit of Neptune at a distance roughly between 30 and 50 Astronomical Units (AU). The belt is named for the Dutch-born US astronomer, G. Kuiper (1905 – 1973) who first predicted its existence. An AU is a measure of celestial distances and is equivalent to 142.6 million kilometers (92.9 million miles), i.e. the mean distance between the Earth and the Sun. Pluto, itself, is a Kuiper belt object.
[5] In addition to Luna (Earth’s satellite), they are Io, Europa, Ganymede and Callisto (Jupiter); Mimas, Enceladus, Tethys, Dione, Rhea, Titan and Impetus (Saturn); Miranda, Ariel, Umbriel, Titania and Oberon (Uranus); Triton (Neptune); and Charon (Pluto).
