What is a planet revisited

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 the picture was incomplete, and that more than one planet was involved. But 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. Its diameter is now known to be 2376.6 km (1467.8 miles) and its mass about 18 percent that of the Moon. It was far too small to have any significant affect on the orbit of Uranus. In the event, it was eventually discovered that Neptune was more massive than had been believed, and there was no need for a second planet. That Pluto should have happened to be close to where the supposed planet lay was a pure coincidence. Furthermore, Pluto is in an eccentric orbit ranging from 50 astronomical units (au) at aphelion to 30 au at perihelion, when it is closer to the Sun than Neptune. The orbital inclination to the ecliptic is 17.6 degrees. For comparison, the orbital inclination of Mercury is 7 degrees, and no other planet has an inclination greater than 2.5 degrees.

By the 1990s, it was becoming clear that Pluto was merely the largest denizen of a region known as the Kuiper Belt (more accurately the Edgeworth-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 trans-Neptunian objects (TNOs) left over from the formation of the Solar System. It was first conjectured to exist by Irish astronomer Kenneth Edgeworth in 1943 and by Dutch astronomer Gerald Kuiper in 1951. Kuiper suggested that periodically, one of these objects entered the inner Solar System to become a comet.

However, it was not until 1992 that the first Kuiper Belt object was located. It was given the provisional designation of 1992 QB1. The respectably-sized 200 km (125 mile) diameter body lies just outside the orbit of Pluto. Its discoverers wanted to name it ‘Smiley’ after John le Carré’s fictional intelligence officer, but the name had already been allocated to an asteroid to honour the American astronomer Charles Smiley. QB1 remained unnamed until 2018, when it received the name 15760 Albion (the mythological name for Britain, now associated with several football teams). Other discoveries soon followed, and over a two thousand Kuiper Belt objects are now known. These are thought to be no more than a fraction of the total.

Kuiper Belt objects fall into two main classes: ‘classical’ objects with orbits of fairly low eccentricity; and ‘resonant’ objects that are in ‘orbital resonance’ with Neptune, that is to say their orbital period and that of Neptune can be expressed as a simple numerical ratio. For example, an object in a 2:3 resonance will complete two orbits of the Sun while Neptune is completing three. The resonant objects are in rather more eccentric orbits than the classical objects, and it is believed that they were perturbed into their present orbits by Neptune as it migrated outwards from the Sun during the early history of the Solar System.

Astronomers began to take the view that Pluto was nothing more than a Kuiper Belt object in a 2:3 orbital resonance with Neptune, along with around 200 other objects. To be sure, Pluto was much larger than these other object, but that didn’t necessarily make it a planet. Neptune’s major moon Triton almost certainly started life as a Kuiper Belt object. Triton’s orbit around Neptune is retrograde, i.e. in the opposite direction to that of Neptune and indeed the majority of Solar System bodies. Retrograde satellites cannot have formed from the primordial solar nebula alongside their parent bodies and must therefore have been captured from elsewhere.  Most are small, but at 2,707 km (1,682 miles) in diameter, Triton is rather larger than Pluto.

The debate over Pluto’s status was not without precedent. On the first day of the nineteenth century Italian astronomer Giuseppe Piazzi found a star-like object that moved from hour to hour. Piazzi, who was compiling a star catalogue, thought he had discovered a comet, but soon began to suspect it might be a new planet. In the weeks that followed he observed it 24 times, but before he could complete his observations he was taken ill. By the time he had recovered, the object had disappeared into the evening twilight. However, the German mathematician Carl Friedrich Gauss was able to calculate an orbit from Piazzi’s interrupted observations and on New Year’s Eve 1801, the object was recovered by German astronomer Heinrich Olbers. The object was named Ceres after the Roman goddess of agriculture, and it was at once recognised as a new planet between the orbits of Mars and Jupiter. Three more objects were soon found, and named Pallas, Juno, and Vesta. All were recognised as planets for several decades, but then six more were discovered in the 1840s, and thereafter discoveries came in thick and fast. The four new ‘planets’ were eventually downgraded to asteroids – even Ceres, which at 960 km (597 miles) in diameter was twice the size of any of the others.

Although early Kuiper Belt discoveries were all 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 names associated with creation in various traditions. Though they were still rather smaller than Pluto, all were comparable in size to Ceres or larger. It was speculated that even larger objects might be found – and so it proved.

At the time of its discovery, Sedna was the most distant Solar System object ever observed, lying at a distance of three times that of Neptune from the Sun – but it is in a highly eccentric orbit and actually spends most of its 11,400 year orbital period at even greater distances. At perihelion, it is 76 astronomical units from the Sun, but the aphelion distance is a staggering 937 astronomical units.  At that distance, even travelling at the speed of light, it would take more than five days to get there. Even when close to perihelion, Sedna lies well outside the main Kuiper Belt and its motion against the starry background is very slow. Its discovery led a team at Mount Palomar to recalibrate their image-searching software, to see if any similar objects had simply been ignored because they were moving too slowly against the background stars to be picked up. Also, given that Sedna just happened to be close to perihelion, it was likely that there were similar objects at other, further, points of their orbits.

Sure enough, on 5 January 2005, an object was discovered on plates taken some 15 months earlier (in fact ‘precovery’ images going back to 1954 would later be found not only of it but also Quaoar and Varuna). The Palomar team nicknamed the new body ‘Xena’ after the fictional TV character. Like Sedna, Xena was located beyond the main Kuiper Belt. The orbit was again highly eccentric, with a perihelion of 38 astronomical units and aphelion of 98 astronomical units, but unlike Sedna, Xena was fairly close to aphelion. The orbit was steeply inclined at 44 degrees, far more so even than Pluto. Xena was probably perturbed into its present orbit by Neptune, and it is referred to as a ‘scattered disk object’. The team then found themselves caught between the conflicting needs of best practice and priority. An announcement was not made immediately because more observations were required to permit more accurate determinations of the object’s size. Against that were fears that another team might beat them to an announcement. On 29 July 2005, the Palomar team finally 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, Ceres, the only asteroid large enough to qualify, stood to regain the planetary status it had lost a century and half earlier. 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 two are 19,571 km (12,161 miles) apart centre to centre, but crucially Charon is not in orbit around Pluto; instead the two rotate around a common centre of gravity with a period of 6.4 days. The two are tidally locked, meaning that on either, from one hemisphere, the other hangs motionless in the sky while from the other hemisphere it is never seen at all. The Pluto-Charon system would be considered a binary planet, the only such entity in the Solar System. Had the 16 August proposal been accepted, the Solar System’s Premier League would have retained Pluto, regained Ceres, and gained Xena and Charon.

Perhaps the latter was too much for some. How could Charon be a planet when much larger moons,  including our own, were not? At all events, the 16 August proposal was rejected, and the proposal of 24 August adopted instead. Pluto’s 66-year membership of the top-flight was over, and Ceres’s hopes for a return were dashed. Wannabe Xena, having failed to gain a seat at the top table, 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’ – with the proviso that a ‘dwarf planet’ isn’t actually a planet. It is not only confusing, it is ungrammatical. An adjective describes a noun, it doesn’t completely change its meaning. A dwarf star is still a star; a dwarf hippopotamus is still a hippopotamus, not a horse or a tiger. But, apparently, a dwarf planet is not a planet. Ditto an exoplanet, since the definition of a planet required it to orbit the Sun. At least it could be argued that ‘exoplanet’ is a noun in its own right, albeit the meaning is ‘external planet’.

It was noted that only about only four percent of the International Astronomical Union’s membership had voted on the resolution, and that most of these were not planetary scientists. Many attendees at the conference had already left before the vote was taken.

The most troubling 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, with one group (‘the Greeks’) leading by 60 degrees and the other group (‘the Trojans’) trailing by 60 degrees. The asteroids have names taken from the two sides 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 have trojan asteroids.

The term ‘gravitationally dominant’ is often used in ‘explainers’ of orbit clearing. 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. However, the term does not form part of the 2006 definition. But even if we incorporate gravitational dominance into the definition, 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.

While the promotion of Charon might have been the last straw for some, 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).

Is 88 ‘too many’? Generations of schoolchildren have been able to name the eight (previously nine) planets of the Solar System. Even seasoned astronomers would struggle to remember 88, the majority of which do not even yet have names. 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 the following resolution:

RESOLVED, BY THE SENATE OF THE NINETY-SIXTH GENERAL ASSEMBLY OF THE STATE OF ILLINOIS, that as Pluto passes overhead through Illinois’ night skies, that it 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.