Definition of planet

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The definition of planet has proven elusive despite the term being one of the best-known terms in astronomy. The term planet has existed for thousands of years, not only in science but as part of wider culture, applied in its long history to everything from divination to environmentalism. That the solar system consists of nine planets is a widely-held and often repeated idea. To date, however, no actual scientific definition of the word "planet" exists. Until the beginning of the 1990s, there was little need for a definition, as astronomers had only a single sample within the solar system to work from, and the sample was small enough for its many irregularities to be dealt with individually.

However, since 1992 and the discovery of myriad tiny worlds beyond the orbit of Neptune, the size of the sample has risen from nine to at least several dozen. Following the further discovery of the first extrasolar planet beyond our solar system in 1995, the number of samples is now approaching hundreds. These new discoveries have not only increased the number of potential planets, but, in their variety and peculiarity (some large enough to be stars, others smaller than our Moon) challenged long received notions of what a planet could be.

The issue of a correct definition for planet came to a head in 2005 with the discovery of the trans-Neptunian object 2003 UB₃₁₃, a body larger than the smallest accepted planet, Pluto. The International Astronomical Union, or IAU, which is the body responsible for resolving issues of astronomical nomenclature, has stated that it intends to release its final decision on the matter in September 2006.

History and etymology

There has never been a single, precise definition for the word "planet". When originally coined by the ancient Greeks, a planet was any object that appeared to wander against the field of fixed stars that made up the night sky (asteres planetai "wandering stars"). This included not only the five "classical" planets, that is, Mercury, Venus, Mars, Jupiter and Saturn, but also the Sun and the Moon (the "seven heavenly objects"). However, a distinction was occasionally made in terminology; the "five planets" (excluding the Sun and the Moon) were referred to alongside the "seven planets" (including the Sun and the Moon), so that the term "planet", even at this early stage, had acquired ambiguity.

Eventually, when the heliocentric model was accepted over the geocentric, Earth was placed among their number and the Sun was dropped, and after Galileo discovered his four satellites of Jupiter, the Moon was also eventually reclassified. However, the Galilean satellites of Jupiter (in 1610), Saturn's satellite Titan in 1659, and Iapetus and Rhea in 1673 were initially described as "planets", not "moons"; the word "moon" at that time only referred to Earth's Moon.

In 1781, the astronomer William Herschel was searching the sky for binary stars when he observed what he termed a comet in the constellation of Taurus. That his strange object might have been a planet simply did not occur to him; the five planets beyond Earth had been part of humanity's conception of the universe since antiquity. However, unlike a comet, his object's orbit was circular and within the ecliptic plane. Eventually it was recognised as the seventh planet and named Uranus.

Gravitationally induced irregularities in Uranus's observed orbit led eventually to the discovery of Neptune in 1841, and perceived irregularities in Neptune's orbit led to the search which ultimately located Pluto in 1930. Pluto was later discovered to be too small to have caused those irregularities, which Voyager 2 determined were due to a slight overestimation of Neptune's mass.

Today, such prior considerations as circular orbit, orbit-perturbing mass, and lying within the ecliptic have been rendered obsolete by Pluto, to which none of them apply. Astronomers are therefore looking elsewhere in their search for a precise definition. While there is much disagreement between current definitions of planet, most focus on three general criteria: that it must orbit a star, be above a certain size (usually large enough for its own gravity to make it round), and yet not be large enough to commence nuclear fusion. Each of these criteria has been challenged by various discoveries, outlined below.

Minor planets

The solar system consists of many more objects than the nine currently accepted as planets. Objects in orbit around the Sun include such diverse bodies as comets, asteroids and cosmic dust. The term "minor planet" or "planetoid" is often used to describe those objects that, while they orbit the Sun, are seen to not fulfill certain criteria common to the "major" planets. What these criteria are, or even if they should exist at all, is the subject of some debate.

Shared orbit

One possible criterion to distinguish a major planet from a minor planet is whether its orbit is unique, or shared by other objects of similar size. Herschel's discovery of Uranus seemed to validate Bode's law, a mathematical function which generates the size of the semimajor axis of planetary orbits. Astronomers had considered the Law a meaningless coincidence, but Uranus fell at very nearly the exact distance it predicted. Since Bode's Law also predicted a body between Mars and Jupiter that at that point had not been observed, astronomers turned their attention to that region in the hope that it might be vindicated again. Finally, in 1801, a new world, Ceres, was found to lie at just the correct point in space. The object was hailed as a new planet.

Then in 1802, Heinrich Olbers discovered Pallas, a second "planet" at roughly the same distance from the Sun as Ceres. The idea that two planets could occupy the same orbit was an affront to millennia of thinking. Some years later, another world, Juno, was discovered in a similar orbit. Over the following decades, several more were discovered, all within relatively the same orbital distance.

William Herschel suggested that these worlds be given their own separate classification, asteroids, (meaning "starlike" since they were too small for their disks to resolve and thus resembled stars), though most astronomers preferred to refer to them as planets. Science textbooks in 1828, after Herschel's death, still numbered the asteroids among the planets. By 1851, the number of asteroids had increased to 15, and a new method of classifying them, by adding a number before their names, was adopted, inadvertently placing them in their own distinct category. By the 1860s, observatories in Europe and America began referring to them as "minor planets", or "small planets", though it took the first four asteroids longer to be grouped as such.

The long road from planethood to reconsideration undergone by Ceres is mirrored in the story of Pluto, which was named a planet soon after its discovery in 1930. Pluto was an anomaly: a tiny, icy world in a region of gas giants with an orbit that carried it high above the plane of the ecliptic or even inside that of Neptune. However, it was, as far as anyone could tell, unique. Then, beginning in 1992, astronomers began to detect large numbers of icy bodies beyond the orbit of Neptune that were similar in composition and size to Pluto. They concluded that they had discovered the long-hypothesized Kuiper Belt (sometimes called the Edgeworth-Kuiper Belt), a band of icy debris that is the source for "short-period" comets—those, like Halley, with orbital periods of up to 200 years.

Pluto's orbit lay right in the middle of this band and thus its planetary status was thrown into question; the precedent set by Ceres in downgrading an object from planet status because of a shared orbit has led many to conclude that Pluto must be downgraded to a minor planet as well. Mike Brown of Caltech has suggested that a "planet" should be redefined as "any body in the solar system that is more massive than the total mass of all of the other bodies in a similar orbit". The eight planets over that mass limit would be referred to as "major planets". There has been outcry at the prospect of Pluto's "demotion", and in 1999 the International Astronomical Union officially voted to retain Pluto's classification as a planet.

The discovery of several objects approaching the size of Pluto, such as 50000 Quaoar and 90377 Sedna, continued to erode arguments that Pluto was exceptional from the rest of the trans-Neptunian population. On July 28, 2005, Mike Brown and his team announced the discovery of a trans-Neptunian object that was confirmed to be larger than Pluto, designated 2003 UB₃₁₃. Although its discoverers (and many in the news media) immediately referred to it as the tenth planet, it is officially designated as a minor planet—the provisional designation 2003 UB₃₁₃ referring to its official listing in the minor-planet archive as the 7827th object first identified in observations made in the second half of October 2003.

However, the criterion of a shared orbit is not without ambiguity; it does not define a planet by composition or formation, but, effectively, by its location. Hence, by this definition, a body of Pluto's size or smaller orbiting in isolation would be called a planet, whereas larger objects in close proximity to one another would be termed "minor planets".

Sphericity

Indeed, a number of astronomers, such as Alan Stern, contend that size, not a unique orbit, is the proper criterion for defining a minor planet. Objects in orbit round the Sun range in size from Jupiter to dust particles, so obviously there would need to be a lower limit. The most oft-mooted potential limit is when an object becomes spherical under its own gravity. Many astronomers favour this definition, because it would allow Pluto to retain its status as a planet. Such a definition would overturn conventional notions of our solar system; Ceres, once a mere point of light, is now known to be spherical, and thus, under this definition, would regain its lost planet status.

However, deciding which objects in the Solar system are spherical or spheroid is more complicated than it seems. In mathematical terms, spheroids consist of an ellipse rotated around one axis. Consequently they have two axes of equal length and one that is either longer or shorter; they resemble spheres that have been deformed (by stretching or squashing) in one dimension. A section through one axis will produce a circle, and a section through the other two axes will produce an ellipse.

Ellipsoid is a general term for bodies including spheres and spheroids, but is used here for scalene ellipsoids, bodies in which all three axes differ in length. Every section through a scalene ellipsoid produces an ellipse.

All ellipsoids, however, have the points on their surfaces joined by smooth curves (which form the elliptical or circular sections). On a topographically irregular body this can only be an approximation; however, taking such irregularity into account, a definite contrast exists between bodies, such as Enceladus, which are essentially ellipsoidal, and irregular bodies, like Neptune's moon Proteus, whose limbs do not show smooth curvature.

If one uses this mathematical basis to define a spheroid, then the boundary between spheroidal and irregular objects within the solar system frays noticeably, as this table illustrates:

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Object	Dimensions	Mass	Density *	Shape
{{mp	~1960 × 1520 × 1000 km	(4.2±0.1) x1021 kg	2.6–3.3 g/cm3	Ellipsoid
1 Ceres	975 × 909 km	9.5 ×1020 kg	2.08 g/cm3	Spheroid
4 Vesta	578 × 560 × 478 km	2.7 ×1020 kg	3.4 g/cm3	Spheroid
2 Pallas	570 × 525 × 500 km	2.2 ×1020 kg	2.8 g/cm3	Irregular,
Enceladus	505 km	1.08 ×1020 kg	1.61 g/cm3	Spheroid
10 Hygiea	500 × 385 × 350 km	1.0×1020 kg	2.76 g/cm3	Irregular
Miranda	471.6 km	6.59 ×1019 kg	1.20 g/cm3	Spheroid
Proteus	436 × 416 × 402 km	5.0 ×1019 kg	1.3 g/cm3	Irregular
Mimas	397.2 km	3.84 ×1019 kg	1.17 g/cm3	Spheroid
511 Davida	326.1 km	3.6 ×1019 kg	2.0 g/cm3	Irregular
704 Interamnia	316.6 km	3.3 ×1019 kg	2.0? g/cm3	Irregular
Nereid	340 km	3.1 ×1019 kg	? g/cm3	Irregular
3 Juno	290 × 240 × 190 km	3.0 ×1019 kg	3.4 g/cm3	Irregular

^*The density of an object is a rough guide to its composition: the lower the density, the higher the fraction of ices, and the lower the fraction of rock. The most dense of these objects, Vesta and Juno, are composed almost entirely of rock with very little ice, and have a density close to the Moon's, while the less dense, such as Proteus and Enceladus, are composed mainly of ice. _________________________________________________________________________________

Plainly, there is no clear divide between those objects in the solar system which could be considered "spheroids" and those which are obviously irregular. The irregular objects Pallas, Hygeia and Proteus are all larger than regular objects, such as Miranda and Mimas. Also, as demonstrated by the dimensions listed in the table, the term "spheroid" is, in any case, fairly loose. Vesta, by the above formulation, is a spheroid, yet it is not, by any commonly held definition of the term, spherical. (see image)

However, even if we limit our sample to approximate spheres, gravity alone is not the sole determiner of their shape. Objects made of ices, such as Enceladus and Miranda, assume a spherical shape more easily than those made of rock, such as Vesta and Pallas. Heat energy, from gravitational collapse, impacts, tidal forces, or radioactive decay also factors into whether an object will be spherical or not; Saturn's icy moon Mimas is spherical, but Neptune's larger moon Proteus, which is similarly composed but colder because of its greater distance from the Sun, is not.

Note also that Ceres is spherical, but the Kuiper belt object 2003 EL₆₁, which is several times more massive and the largest known non-spherical object in the solar system, has been elongated into an ellipsoid by its faster rotation. Jupiter and Saturn have also been rendered highly oblate by their rapid rotations. Mimas, Enceladus, and Miranda have been stretched into prolate spheroids by tidal forces.

Other astronomers have suggested that, to overcome this uncertainty, the diameter limit for planethood should be arbitrarily pinned at that of Pluto, thus preserving the traditional nine planets while allowing the possibility of future additions, while others have suggested that it be fixed at 1000 km, which would potentially define at least three smaller KBOs as planets alongside Pluto.

Double planets

Pluto and its largest satellite, Charon, are characterized by their barycenter lying outside the surface of either body. This means that both orbit each other like the tips of a spinning baton. Since neither can be said to be orbiting the other, it is common for astronomers to refer to Pluto/Charon as a double planet: two objects orbiting the Sun in tandem. Charon is not importantly affected by its orbit—unlike the synchronous orbital radius, for instance, which does have profound consequences for the orbiting body (see Phobos, for example). Even our own Moon could be considered a partner in a double planet system. Though it orbits the Earth, the timing of its orbit is in tandem with the Earth's own orbit around the Sun—looking down on the ecliptic, the Moon never actually loops back on itself, and in essence it orbits the Sun in its own right.

This is true of any moon sufficiently far enough away from its parent body that its orbital speed round the planet is slower than the planet's speed round the Sun. The required distance from the planet to the moon depends on the mass of the planet, and the distance from the planet to the Sun, but not the mass of the moon. If the distance from the Sun to the planet increases, or the planet's mass decreases, then the required distance between the planet and moon increases. Consequently, the same argument could be used that Jupiter and Callisto or Saturn and Iapetus form double planets.

Also, many moons, even those that do not orbit the Sun directly, often exhibit features in common with true planets. Jupiter's moon Ganymede and Saturn's moon Titan are both larger in terms of diameter (though not mass) than Mercury, and Titan even has a substantial atmosphere, thicker than the Earth's. Moons such as Io and Triton demonstrate obvious and ongoing geological activity, and Ganymede has a magnetic field. It could be argued that, just as stars in orbit around other stars are still referred to as stars, thus objects in orbit around planets that share all their characteristics could also be called planets.

Extrasolar planets

The boundary between "star" and "planet" has blurred considerably since 1995, with the discovery to date of nearly 200 extrasolar planets: planet-sized objects in orbit around other stars. Many of these planets are of considerable size, approaching the mass of small stars, while many newly-discovered stars, conversely, are small enough to be considered planets.

Brown and white dwarfs

Traditionally, the defining characteristic for starhood has been an object's ability to fuse hydrogen in its core. However, stars such as brown dwarfs have always challenged that distinction. Too small to commence sustained hydrogen fusion, they have been granted star status on their ability to fuse deuterium. However, due to the relative rarity of that isotope, this process lasts only a tiny fraction of the star's lifetime, and hence most brown dwarfs would have ceased fusion long before their discovery. Binary stars and other multiple-star formations are common, and many brown dwarfs orbit other stars. Therefore, since they would not be producing energy through fusion, they could be described as planets. Indeed, astronomer Adam Burrows of the University of Arizona claims that "from the theoretical perspective, however different their modes of formation, extrasolar giant planets and brown dwarfs are essentially the same." Similarly, an orbiting white dwarf, such as Sirius B, since it too has ceased fusion, could be considered a planet. However, the current convention among astronomers is that any object massive enough to have possessed the capability to fuse during its lifetime should be considered a star.

Rogue planets

The confusion does not end with brown dwarfs. Zapatario Osorio et al. have discovered many objects in young star clusters of masses below that required to sustain fusion of any sort (currently calculated to be roughly 13 Jupiter masses). These have been described as "free floating planets" because current theories of solar system formation suggest that planets may be ejected from solar systems altogether if their orbits become unstable. One could therefore argue that the original criterion that a planet must orbit a star should instead be amended to indicate that it must have originated in orbit around a star.

Sub-stars

However, it is also possible that these "free floating planets" could have formed in the same manner as stars; thus their discoverers also term them "grey dwarfs" or sub-brown dwarfs. The material difference between a low-mass star and a large gas giant is not clearcut; apart from size and relative temperature, there is little to separate a gas giant like Jupiter from its host star. Both have similar overall compositions: hydrogen and helium, with trace levels of heavier elements in their atmospheres. The generally accepted difference is one of formation; stars are said to have formed from the "top down"; out of the gases in a nebula as they underwent gravitational collapse, and thus would be composed almost entirely of hydrogen and helium, while planets are said to have formed from the "bottom up"; from the accretion of dust and gas in orbit around the young star, and thus should have cores of silicates or ices. As yet it is uncertain whether gas giants possess such cores. If it is indeed possible that a gas giant could form as a star does, then it raises the question of whether such an object, even one as familiar as Jupiter or Saturn, should be considered an orbiting low-mass star rather than a planet.

The IAU has officially released a statement to define what constitutes an extrasolar planet and what constitutes an orbiting star:

1. Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System.
2. Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed nor where they are located.
3. Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).

Like defining a "minor planet" by shared orbit, this definition creates ambiguity by making location, rather than formation or composition, the determining characteristic for planethood. A free-floating object with a mass below 13 Jupiter masses is a "sub-brown dwarf," whereas such an object in orbit round a fusing star is a planet, even if, in all other respects, the two objects may be identical. This ambiguity was highlighted in December 2005, when the Spitzer Space Telescope observed Cha 110913-773444, the least massive brown dwarf yet found, only eight times Jupiter's mass with what appears to be the beginnings of its own star system. Were this object found in orbit round another star, it would have been termed a planet.

The IAU debate

For most astronomers, the issue of what constitutes a planet will be decided by the International Astronomical Union (IAU). According to a published report from Nature magazine, the discovery of 2003 UB₃₁₃ has forced the issue. In October 2005, a group of 19 IAU members, which had already been working on a definition since the discovery of Sedna in 2003, narrowed their choices to a shortlist of three, allowing each member to vote for more than one. The definitions were:

• A planet is any object in orbit round the Sun with a diameter greater than 2000 km. (eleven votes in favour)

• A planet is any object in orbit round the Sun whose shape is stable due to its own gravity (eight votes in favour)

• A planet is any object in orbit round the Sun that is dominant in its immediate neighborbood (six votes in favour)

The first would be an essentially cultural/historical definition, recognising Pluto's historical identity as a planet by setting an arbitrary limit immediately below its diameter. Under this definition, the only known planets in our solar system would be the current nine, plus 2003 UB₃₁₃. The second provides a more scientific basis for a limit, and also avoids the "roundness" cutoff muddied by objects such as 2003 EL₆₁, but still discounts many irregular objects, such as Pallas, that are larger than many regular objects. By its criterion, dozens of objects in our Solar system could be considered planets. The final definition would leave only eight planets in our solar system, relegating Pluto to the status of minor planet. Perhaps for this reason, it proved the least popular.

Since no overall consensus could be reached, the committee decided to put these three definitions to a wider vote, most likely at the IAU General Assembly meeting in Prague in August 2006. The IAU has stated that it will publish a definition early in the following month.

See also

• sub-brown dwarf
• Minor planet
• Mesoplanet
• Planemo
• Planetar

Primary Sources

Secondary sources

 

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