Definition of planet
Greek astronomers in antiquity used the term planetes to describe star-like objects that moved across the sky over the course of a year. They contrasted these wandering lights with fixed stars, which appeared motionless relative to one another. Five bodies visible to the naked eye were known as planets: Venus, Jupiter, Mercury, Mars, and Saturn. Ancient cosmology often counted seven planets, including both the Sun and the Moon alongside the five wandering stars. Cicero wrote in his Dream of Scipio around 53 BCE that seven spheres contained the planets, each moving contrary to the movement of heaven. Pliny the Elder noted in Natural History from 77 CE that they called them planets because of their motion, even though no stars wander less than they do. Plato mentioned the Sun and Moon along with five other stars in Timaeus written roughly 360 BCE. Aristotle made a similar distinction in On the Heavens regarding the fewer movements of the sun and moon compared to some planets.
When Copernicus published De Revolutionibus, he separated the sun, moon, planets, and stars into distinct categories within the text itself. However, in his dedication to Pope Paul III, he referred to the motion of the sun and the moon alongside the five other planets. Galileo asserted in Dialogue Concerning the Two Chief World Systems that Earth is numbered among natural bodies that move circularly just like the moon or any other planet. Thomas Kuhn observed in The Structure of Scientific Revolutions from 1962 that Copernicans changed the meaning of planet to maintain useful distinctions in a new worldview. A convert to Copernicanism might say I once took the moon to be a planet but was mistaken. This shift placed Earth among the planets while removing the Sun and Moon from that category. The heliocentric model required this conceptual revolution as all celestial bodies were seen differently from before.
William Herschel observed what he termed a comet in Taurus in 1781 when searching for stellar parallaxes. Unlike stars which remained points of light even under high magnification, this object's size increased with power used. His colleague Nevil Maskelyne wrote that it was as likely to be a regular planet moving in an orbit nearly circular to the sun as a comet. 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. Presumed irregularities in Neptune's orbit subsequently led to a search which found Pluto in 1930. Voyager 2 determined in 1989 that the irregularities were due to an overestimation of Neptune's mass. Initially believed to be roughly the mass of Earth, observation gradually shrank Pluto's estimated mass until it was revealed to be a mere five hundredth as large.
Mike Brown announced on the 29th of July 2005 the discovery of Eris, a trans-Neptunian object confirmed to be more massive than Pluto. NASA issued a press release describing it as the tenth planet immediately following the announcement. The International Astronomical Union held its General Assembly meeting in Prague in August 2006 to vote on the matter. On August 24, the IAU put a final draft to a vote with 424 astronomers taking part in the ballot. The decision created a medial classification called dwarf planet and placed Pluto in it along with Ceres and Eris. The IAU resolved that planets and dwarf planets are two distinct classes of objects meaning dwarf planets would not be considered planets. On the 13th of September 2006, the union placed Eris, its moon Dysnomia, and Pluto into their Minor Planet Catalogue giving them official designations like 134340 Pluto.
Steven Soter wrote in Scientific American from January 2007 about how winning bodies outstripped others by a wide margin during formation. Mars is larger than all other collected material in its orbital zone by a factor of 5100 according to his calculations. Ceres accounts for only one third of the material in its orbit while Pluto's ratio is even lower at around 7 percent. Alan Stern argues that since neither Earth nor Jupiter have entirely cleared regions of debris none could properly be considered planets under the definition. Jean-Luc Margot proposed an October 2015 metric based on whether an object can clear an orbital zone within a specific time scale. An Earth-mass body clears its orbit at distances up to 400 astronomical units from the star. Mike Brown notes that abandoning the clearing criterion could raise the number of planets from eight to more than 50 with hundreds potentially discovered later.
Galileo discovered four satellites of Jupiter in 1610 which lent weight to Copernicus's argument because if other planets had satellites then Earth could too. Christiaan Huygens employed many terms including planeta and satellite when discovering Titan in 1655. Giovanni Cassini described Iapetus and Rhea as Nouvelles Planetes autour de Saturne in 1671 and 1672 before reports used satellite exclusively. There are 20 moons in the Solar System massive enough to achieve hydrostatic equilibrium known as planetary-mass moons. Both Ganymede and Titan are larger than Mercury while Titan has a substantial atmosphere thicker than Earth's. Alan Stern proposes the term satellite planet for these objects since location should not matter according to his view. Mike Brown states it is hard to make a consistent argument that a 400 km iceball counts as a planet while a 5000 km satellite with methane lakes shouldn't be put into the same category.
The discovery since 1992 of extrasolar planets widened the debate on planethood in unexpected ways. Many brown dwarfs orbiting stars have been granted star status on their ability to fuse deuterium though this process lasts only a tiny fraction of the lifetime. Adam Burrows claims from the University of Arizona that extrasolar giant planets and brown dwarfs are essentially the same theoretically. Objects below roughly 13 Jupiter masses discovered by María Rosa Zapatero Osorio were described as free floating planets because they may be ejected from systems. The IAU released a working definition in 2003 which was amended in August 2018 stating an object in orbit around a fusing star is a planet even if identical to a sub-brown dwarf elsewhere. A 2016 study shows no noticeable difference between gas giants and brown dwarfs in mass-radius trends from one Saturn mass to about 75 Jupiter masses.
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Common questions
What objects did Greek astronomers call planets in antiquity?
Greek astronomers called Venus, Jupiter, Mercury, Mars, and Saturn planets. They also counted the Sun and Moon as planets to make a total of seven wandering lights distinct from fixed stars.
When was Pluto discovered and why was it reclassified?
Pluto was discovered in 1930 after irregularities in Neptune's orbit prompted a search. It was reclassified as a dwarf planet on the 24th of August 2006 when the International Astronomical Union voted to separate it from the eight official planets.
How many moons are massive enough to be considered planetary-mass moons?
There are 20 moons in the Solar System massive enough to achieve hydrostatic equilibrium known as planetary-mass moons. Both Ganymede and Titan are larger than Mercury while Titan has an atmosphere thicker than Earth's.
What criteria does Mike Brown use to define a planet?
Mike Brown notes that abandoning the clearing criterion could raise the number of planets from eight to more than 50 with hundreds potentially discovered later. He argues that location should not matter if an object is large enough to have features like methane lakes.
When did the IAU release its working definition for extrasolar planets?
The International Astronomical Union released a working definition in 2003 which was amended in August 2018 stating an object in orbit around a fusing star is a planet even if identical to a sub-brown dwarf elsewhere.