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, Of the seven cosmical bodies which, by their continually varying relative positions and distances apart, have ever since the remotest antiquity been distinguished from the "unwandering orbs" of the heaven of the "fixed stars", which to all sensible appearance preserve their relative positions and distances unchanged, five only—Mercury, Venus, Mars, Jupiter and Saturn—wear the appearance of stars—"
cinque stellas errantes"—while the Sun and Moon, from the size of their disks, their importance to man, and the place assigned to them in mythological systems, were classed apart. 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." the more advanced
Theorica planetarum presents the "theory of the seven planets," 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." 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. Even
Nicolaus Copernicus, who rejected the geocentric model, was ambivalent concerning whether the Sun and Moon were planets. In his
De Revolutionibus, Copernicus clearly separates "the sun, moon, planets and stars"; however, in his Dedication of the work to Pope Paul III, Copernicus refers to, "the motion of the sun and the moon... and of the five other 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: The Copernicans who denied its traditional title 'planet' to the sun ... were changing the meaning of 'planet' so that it would continue to make useful distinctions in a world where all celestial bodies ... were seen differently from the way they had been seen before... Looking at the moon, the convert to Copernicanism ... says, 'I once took the moon to be (or saw the moon as) a planet, but I was mistaken.' 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..."
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. 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, 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. Science textbooks in 1828, after Herschel's death, still numbered the asteroids among the planets.
Pluto 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 by
Clyde Tombaugh in 1930. Uranus and Neptune had been declared planets based on their circular orbits, large masses and proximity to the ecliptic plane. None of these applied to Pluto, a tiny and icy world in a region of
gas giants with an orbit that carried it high above the
ecliptic and even inside that of Neptune. In 1978, astronomers discovered Pluto's largest moon,
Charon, which allowed them to determine its mass. Pluto was found to be much tinier than anyone had expected: only one-sixth the mass of Earth's Moon. However, as far as anyone could yet tell, it was unique. Then, beginning in 1992, astronomers began to detect large numbers of icy bodies beyond the orbit of Neptune that were similar to Pluto in composition, size, and orbital characteristics. They concluded that they had discovered the hypothesized
Kuiper belt (sometimes called the Edgeworth–Kuiper belt), a band of icy debris that is the source for "short-period" comets—those with orbital periods of up to 200 years. Pluto's orbit lays within this band and thus its planetary status was thrown into question. Many scientists concluded that tiny Pluto should be reclassified as a minor planet, just as Ceres had been a century earlier.
Mike Brown of the
California Institute of Technology 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." Those objects under that mass limit would become minor planets. In 1999,
Brian G. Marsden of
Harvard University's
Minor Planet Center suggested that Pluto be given the
minor planet number 10000 while still retaining its official position as a planet. The prospect of Pluto's "demotion" created a public outcry, and in response the
International Astronomical Union clarified that it was not at that time proposing to remove Pluto from the planet list. The discovery of several other
trans-Neptunian objects, such as
Quaoar and
Sedna, continued to erode arguments that Pluto was exceptional from the rest of the trans-Neptunian population. On July 29, 2005, Mike Brown and his team announced the discovery of a trans-Neptunian object confirmed to be more massive than Pluto, named
Eris. In the immediate aftermath of the object's discovery, there was much discussion as to whether it could be termed a "
tenth planet". NASA even put out a press release describing it as such. However, acceptance of Eris as the tenth planet implicitly demanded a definition of planet that set Pluto as an arbitrary minimum size. Many astronomers, claiming that the definition of planet was of little scientific importance, preferred to recognise Pluto's historical identity as a planet by "
grandfathering" it into the planet list. ==IAU definition==