On 24 August 2006, the 26th General Assembly of the International Astronomical Union in Prague came up with one of the most controversial rulings in the history of astronomy:
The IAU therefore resolves that planets and other bodies, except satellites, in our Solar System be defined into three distinct categories in the following way:
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.
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, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
All other objects, except satellites, orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”.
The three glaring questions arising were (1) what is meant by “cleared the neighbourhood around its orbit” since nearly all the recognised planets, including Jupiter and Earth, share their orbit with other bodies (2) what of planets orbiting stars other than the Sun, which by 2006 had already been detected in large numbers (3) why was a definition of a planet needed at all?
The last of these is the easiest to answer: the Pluto question. The status of Pluto, the smallest and outermost planet, had been questioned for some time. Pluto was discovered by American astronomer Clyde Tombaugh in 1930 after a search for an undiscovered planet that was supposedly affecting the orbit of Uranus. Similar considerations had led to the discovery of Neptune in 1846, but subsequent observations suggested that more than one planet was involved. The problem was that instead of another planet the size of Neptune or Uranus, Pluto appeared to be no larger than Earth. Even this turned out to be a gross overestimate, and by the 1980s Pluto had been determined to be considerably smaller than the Moon – far too small to have any significant affect on the orbit of Uranus. It was eventually discovered that Neptune was more massive than had been believed, and there was no need for a second planet. That Pluto was found close to where the supposed planet lay was a pure coincidence.
By the 1990s, it was becoming clear that Pluto was merely the largest member of a region known as the Kuiper Belt, extending from the orbit of Neptune at 30 au from the Sun to around 50 au from the Sun. The region is populated by icy objects left over from the formation of the Solar System. The first was identified in 1992, and other discoveries soon followed. Although most of these were far smaller than Pluto, after 2000 much larger objects began to turn up, including Haumea, Makemake, Varuna, Quaoar, Orcus, and Sedna. These new objects were all named for mythological creation deities of various traditions. Though they were still rather smaller than Pluto, all were comparable in size to Ceres or larger, and it seemed likely that even larger objects might be found.
Sure enough, on 5 January 2005, an object was discovered by a team at Mount Palomar on plates taken some 15 months earlier. The new body was given the provisional nickname ‘Xena’ after the TV character. On 29 July 2005, the Palomar team went public, by which time they suspected that Xena was slightly larger than Pluto. A few months after the announcement, Xena was found to have a moon, enabling its mass to be determined. Regardless of its diameter (it eventually turned out to be fractionally smaller than Pluto), Xena was 27 percent more massive than Pluto, bringing to a head the whole debate on the latter’s status as a planet. It was obvious that if Pluto was a planet, then a more massive object like Xena had to be also. Conversely, if Xena wasn’t a planet, then neither was Pluto.
Even before the discovery of Xena, the International Astronomical Union had set up a committee to consider possible definitions of a planet. On 16 August 2006, a draft proposal was published at the IAU’s Prague conference. It stated: “A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.” This concise and readily testable definition of a planet would have recognised Xena as a planet and retained Pluto’s status. Under this proposal, the asteroid Ceres, originally recognised as a planet, stood to regain its planetary status. In addition, it was proposed to elevate Pluto’s major moon Charon to planetary rank. At 1207 km (750 miles) in diameter, Charon is very large in comparison to Pluto, with 11.6 percent its mass. The Pluto-Charon system would be considered a binary planet, the only such entity in the Solar System.
However, the 16 August proposal was rejected, and the proposal of 24 August adopted instead. Pluto, Ceres, and Xena were denied planetary status. The latter was given the official name Eris a few days later, and the moon was named Dysnomia. Eris is the Greek goddess of discord and Dysnomia means ‘lawless’, an oblique reference to Lucy Lawless, who played Xena in the TV series. All three were given the confusing designation of ‘dwarf planet’.
The most confusing aspect of the definition was the phrase meant by “cleared the neighbourhood around its orbit”. Jupiter shares its orbit with two clusters of asteroids known as the Trojans at its Lagrange points. The asteroids are named for participants in the Trojan War, but the term ‘trojan’ is now used for any asteroid in a similar relationship with other planets. Neptune, Uranus, Mars, and Earth also share their orbits with trojan asteroids.
The term ‘gravitationally dominant’ is often used to explain the concept. Trojan asteroids and the Kuiper Belt objects in orbital resonance with Neptune have all been marshalled into their current orbits by a gravitationally dominant planet. The term does not form part of the 2006 definition, but even if it did, a planet will still be partially defined in terms of its location, which raises a problem. In 1949, Kuiper estimated the diameter of Pluto to be 10,300 km (6,400 miles), only slightly smaller than Earth. If that figure had turned out to be correct, Pluto would still not be a planet on the new definition, despite being larger than Mars or Mercury.
While accepting that the IAU was under pressure to define a planet, the question must still be asked: do we actually need a formal definition for a word that once simply referred to bright star-like objects that, unlike the fixed stars, moved over a period of time? The word ‘continent’ lacks a formal definition, yet most would list the continents as Europe, Asia, Africa, North America, South America, Australia, and Antarctica. Similarly, Sir Patrick Moore came up with the common-sense definition that Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune are planets and everything else isn’t. He was long of the opinion that Pluto was not a planet, but just as some would combine Europe and Asia into Eurasia, or the Americas into a single continent, we could add Pluto and Eris nee Xena if we wished.
A major concern was that the 16 August proposal opened up the way to planethood for objects the size of Ceres, of which there are many in the Kuiper Belt. Haumea, Makemake, Varuna, Quaoar, Gonggong (discovered in 2007), and Sedna are larger than Ceres, and Orcus is only slightly smaller. Nor is Ceres the lower limit: the asteroid Hygiea, the Saturnian moons Enceladus and Mimas, and the Uranian moon Miranda are all around half the diameter of Ceres, but they are spherical and under the 24 August ruling the moons would qualify as dwarf planets if they orbited the Sun.
However, matters are not entirely clear-cut. The asteroids Vesta and Pallas are larger than any of these four bodies, and the Neptunian moon Proteus is larger than Mimas – yet all are irregular in shape. The Ceres/Orcus-Mimas size range also includes 67 known trans-Neptunian objects and the moons Dysnomia (Eris I) and Vanth (Orcus I).
Currently, in addition to Pluto, Eris, and Ceres, Haumea and Makemake are officially recognised as dwarf planets. Varuna, Quaoar, Gonggong, Sedna, Orcus, and Hygiea are probably dwarf planets, as are at least some of the trans-Neptunian objects intermediate between Ceres/Orcus and Mimas. If we were to adopt the 16 August proposal, the Solar System would comprise at least 14, probably 20, and potentially as many as 88 planets (including Charon).
Could we have a Solar System with 88 planets? Generations of schoolchildren have been able to name the nine (now only eight) planets. Even seasoned astronomers would struggle to remember 88. But is that a reason? How many chemists could name all 118 currently known chemical elements; how many geographers could name all the 193 members of the United Nations?
I now intend to go even further than the 16 August proposal, which proposed to elevate Charon to planethood, even though it is only the twelfth largest moon in the Solar System. Two of these moons – Ganymede and Titan – are larger, albeit less massive, than Mercury. If we accept that location cannot be a determinant of whether something is or isn’t a planet, then being a moon should not disbar it. Indeed, planetary scientists often refer to Ganymede, Titan, and other large moons as planets. The seven large moons, in order of size, are Ganymede, Titan, Callisto, Io, the Moon, Europa, and Triton. In addition, there are eleven medium-sized spherical moons: Rhea, Tethys, Dione, Enceladus, and Mimas (Saturn), Ariel, Umbriel, Titania, Oberon, and Miranda (Uranus), and Charon (Pluto). This adds a further 17 planets to the roster, not counting Charon which has already been included.
At a conservative estimate, that would give us a fifty-planet Solar System. Again, just as chemical elements are grouped in accordance with the periodic table, so planets can be categorised by type. Traditionally, planets were divided between rocky Earth-type planets (Mercury, Venus, Earth, Mars) and gas giants (Jupiter, Saturn, Uranus, Neptune), but Uranus and Neptune are now classed as ice giants, composed predominantly of methane, ammonia, and water rather than hydrogen and helium. The ‘new’ planets are of many types, including:
Silicate (rocky) surface and mantle, metal core e.g. the Moon
Icy surface, silicate mantle, metal core e.g. Ganymede
Icy surface, icy mantle, silicate and metal core, e.g. Triton
Icy surface, icy mantle, silicate core, e.g. Titan
Icy surface, icy mantle, icy core e.g. Mimas
Some of the icy moons are believed to have deep subsurface oceans, including Europa, Ganymede, and Enceladus.
The IAU has no plans to revisit its definition of a planet, but nobody is under any obligation to abide by it. Pluto’s demotion was especially unpopular in Clyde Tombaugh’s home state of Illinois. The Illinois Senate passed a resolution decreeing that Pluto “be reestablished with full planetary status, and that March 13, 2009 be declared “Pluto Day” in the State of Illinois in honor of the date its discovery was announced in 1930″.