The definition of the term planet has changed several times since the word was coined by the ancient Greeks. Greek astronomers employed the term (), 'wandering stars', for star-like objects which apparently moved over the sky. Over the millennia, the term has included a variety of different celestial bodies, from the Sun and the Moon to satellites and asteroids.
In modern astronomy, there are two primary conceptions of a planet. A planet can be an astronomical object that dynamically dominates its region (that is, whether it controls the fate of other smaller bodies in its vicinity) or it is defined to be in hydrostatic equilibrium (it has become gravitationally rounded and compacted). These may be characterized as the dynamical dominance definition and the geophysical definition.
The issue of a clear definition for planet came to a head in January 2005 with the discovery of the trans-Neptunian object Eris, a body more massive than the smallest then-accepted planet, Pluto. In its August 2006 response, the International Astronomical Union (IAU), which is recognised by astronomers as the international governing body responsible for resolving issues of nomenclature, released its decision on the matter for the IAU definition of planet during a meeting in Prague. This definition, which applies only to the Solar System (though exoplanets had been addressed in 2003), states that a planet is a body that orbits the Sun, is massive enough for its own gravity to make it round, and has "cleared its neighbourhood" of smaller objects approaching its orbit. Pluto fulfills the first two of these criteria, but not the third and therefore does not qualify as a planet under this formalized definition. The IAU's decision has not resolved all controversies. While many astronomers have accepted it, some planetary scientists have rejected it outright, proposing a geophysical or similar definition instead.
History
Planets in antiquity
While knowledge of the planets predates history and is common to most civilizations, the word planet dates back to ancient Greece. Most Greeks believed the Earth to be stationary and at the center of the universe in accordance with the geocentric model and that the objects in the sky, and indeed the sky itself, revolved around it (an exception was Aristarchus of Samos, who put forward an early version of heliocentrism). Greek astronomers employed the term (), 'wandering stars', to describe those starlike lights in the heavens that moved over the course of the year, in contrast to the (), the 'fixed stars', which stayed motionless relative to one another. The five bodies currently called "planets" that were known to the Greeks were those visible to the naked eye (from brightest to dimmest): Venus, Jupiter, Mercury, Mars, and Saturn. Graeco-Roman cosmology commonly considered seven planets, with the Sun and the Moon counted among them (as is the case in modern astrology); however, there is some ambiguity on that point, as many ancient astronomers distinguished the five star-like planets from the Sun and Moon. As the 19th-century German naturalist Alexander von Humboldt noted in his work Cosmos,
In his Timaeus, written in roughly 360 BCE, Plato mentions, "the Sun and Moon and five other stars, which are called the planets". His student Aristotle makes a similar distinction in his On the Heavens: "The movements of the sun and moon are fewer than those of some of the planets". In his Phaenomena, which set to verse an astronomical treatise written by the philosopher Eudoxus in roughly 350 BCE, the poet Aratus describes "those five other orbs, that intermingle with [the constellations] and wheel wandering on every side of the twelve figures of the Zodiac."
In his Almagest written in the 2nd century, Ptolemy refers to "the Sun, Moon and five planets." Hyginus explicitly mentions "the five stars which many have called wandering, and which the Greeks call Planeta." Marcus Manilius, a Latin writer who lived during the time of Caesar Augustus and whose poem Astronomica is considered one of the principal texts for modern astrology, says, "Now the dodecatemory is divided into five parts, for so many are the stars called wanderers which with passing brightness shine in heaven."
The single view of the seven planets is found in Cicero's Dream of Scipio, written sometime around 53 BCE, where the spirit of Scipio Africanus proclaims, "Seven of these spheres contain the planets, one planet in each sphere, which all move contrary to the movement of heaven." In his Natural History, written in 77 CE, Pliny the Elder refers to "the seven stars, which owing to their motion we call planets, though no stars wander less than they do." Nonnus, the 5th century Greek poet, says in his Dionysiaca, "I have oracles of history on seven tablets, and the tablets bear the names of the seven planets."
Planets in the Middle Ages
Medieval and Renaissance writers generally accepted the idea of seven planets. The standard medieval introduction to astronomy, Sacrobosco's De Sphaera, includes the Sun and Moon among the planets,Sacrobosco, "On the Sphere", in Edward Grant, ed. A Source Book in Medieval Science, (Cambridge: Harvard University Press, 1974), page 450. "every planet except the sun has an epicycle." the more advanced Theorica planetarum presents the "theory of the seven planets,"Anonymous, "The Theory of the Planets," in Edward Grant, ed. A Source Book in Medieval Science, (Cambridge: Harvard University Press, 1974), page 452. while the instructions to the Alfonsine Tables show how "to find by means of tables the mean motuses of the sun, moon, and the rest of the planets."John of Saxony, "Extracts from the Alfonsine Tables and Rules for their use", in Edward Grant, ed. A Source Book in Medieval Science, (Cambridge: Harvard University Press, 1974), page 466. In his Confessio Amantis, 14th-century poet John Gower, referring to the planets' connection with the craft of alchemy, writes, "Of the planetes ben begonne/The gold is tilted to the Sonne/The Mone of Selver hath his part...", indicating that the Sun and the Moon were planets.
Earth When Copernicus's heliocentric model was accepted over the geocentric, Earth was placed among the planets and the Sun and Moon were reclassified, necessitating a conceptual revolution in the understanding of planets. As the historian of science Thomas Kuhn noted in his 1962 book, The Structure of Scientific Revolutions:Thomas S. Kuhn, (1962) The Structure of Scientific Revolutions, 1st. ed., (Chicago: University of Chicago Press), pages 115, 128–9.
Copernicus obliquely refers to Earth as a planet in De Revolutionibus when he says, "Having thus assumed the motions which I ascribe to the Earth later on in the volume, by long and intense study I finally found that if the motions of the other planets are correlated with the orbiting of the earth..." Galileo also asserts that Earth is a planet in the Dialogue Concerning the Two Chief World Systems: "[T]he Earth, no less than the moon or any other planet, is to be numbered among the natural bodies that move circularly."
Modern planets
In 1781, the astronomer William Herschel was searching the sky for elusive stellar parallaxes when he observed what he termed a comet in the constellation of Taurus. Unlike stars, which remained mere points of light even under high magnification, this object's size increased in proportion to the power used. That this strange object might have been a planet simply did not occur to Herschel; the five planets beyond Earth had been part of humanity's conception of the universe since antiquity. As the asteroids had yet to be discovered, comets were the only moving objects one expected to find in a telescope. However, unlike a comet, this object's orbit was nearly circular and within the ecliptic plane. Before Herschel announced his discovery of his "comet,” his colleague, British Astronomer Royal Nevil Maskelyne, wrote to him, saying "I don't know what to call it. It is as likely to be a regular planet moving in an orbit nearly circular to the sun as a Comet moving in a very eccentric ellipsis. I have not yet seen any coma or tail to it." The "comet" was also very far away, too far away for a mere comet to resolve itself. Eventually it was recognized as the seventh planet and named Uranus after the father of Saturn.
Gravitationally induced irregularities in Uranus's observed orbit led eventually to the discovery of Neptune in 1846, and presumed irregularities in Neptune's orbit subsequently led to a search which did not find the perturbing object (it was later found to be a mathematical artifact caused by an overestimation of Neptune's mass) but did find Pluto in 1930. Initially believed to be roughly the mass of the Earth, observation gradually shrank Pluto's estimated mass until it was revealed to be a mere five hundredth as large; far too small to have influenced Neptune's orbit at all. In 1989, Voyager 2 determined the irregularities to be due to an overestimation of Neptune's mass.
Satellites
When Copernicus placed Earth among the planets, he also placed the Moon in orbit around Earth, making the Moon the first natural satellite to be identified. When Galileo discovered his four satellites of Jupiter in 1610, they lent weight to Copernicus's argument, because if other planets could have satellites, then Earth could too. However, there remained some confusion as to whether these objects were "planets"; Galileo referred to them as "four planets flying around the star of Jupiter at unequal intervals and periods with wonderful swiftness." Similarly, Christiaan Huygens, upon discovering Saturn's largest moon Titan in 1655, employed many terms to describe it, including "planeta" (planet), "stella" (star), "luna" (moon), and "satellite" (attendant), a word coined by Johannes Kepler. Giovanni Cassini, in announcing his discovery of Saturn's moons Iapetus and Rhea in 1671 and 1672, described them as Nouvelles Planetes autour de Saturne ("New planets around Saturn"). However, when the "Journal de Scavans" reported Cassini's discovery of two new Saturnian moons (Dione and Tethys) in 1686, it referred to them strictly as "satellites", though sometimes Saturn as the "primary planet". When William Herschel announced his discovery of two objects in orbit around Uranus in 1787 (Titania and Oberon), he referred to them as "satellites" and "secondary planets". All subsequent reports of natural satellite discoveries used the term "satellite" exclusively,See primary citations in Timeline of discovery of Solar System planets and their moons though the 1868 book "Smith's Illustrated Astronomy" referred to satellites as "secondary planets".
Minor planets
One of the unexpected results of William Herschel's discovery of Uranus was that it appeared 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, astronomer Giuseppe Piazzi found a miniature new world, Ceres, lying 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 fact that two planets could occupy the same orbit was an affront to centuries of thinking. In 1804, another world, Juno, was discovered in a similar orbit. In 1807, Olbers discovered a fourth object, Vesta, at a similar orbital distance.