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A planet, as 2006 definition of planet|most recently defined by the International Astronomical Union (IAU), is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, not massive enough to cause thermonuclear fusion in its core, and has cleared its neighbouring region of planetesimals.

  • IAU 2006 General Assembly: Result of the IAU Resolution votes [1] .
  • Working Group on Extrasolar Planets (WGESP) of the International Astronomical Union, [2]

The term planet is an ancient one, with ties to history, science, myth and religion. The planets were originally seen as a divine presence; as emissaries of the gods. As scientific knowledge improved, the human perception of the planets changed over time, incorporating a number of disperate objects. Even now there is no unconstested definition of what a planet is. In 2006, the IAU officially adopted a resolution defining planets within the Solar System. This definition has been both praised and criticised, and remains disputed by some scientists.

Since the dawn of the space age, probes have been sent to every planet in the Solar System, and the discoveries they have made have shifted planetary science from the realm of astronomy to the realms of geography and geology. The planets have been found to share many characteristics, such as volcanism, hurricanes, tectonics and even hydrology, previously only known on Earth. Since 1992, and the discovery of hundreds of extrasolar planets, scientists are beginning to observe similar features across the galaxy.

Under IAU definitions, there are eight planets in the Solar System (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune) and also at least three dwarf planets (Ceres, Pluto, and Eris). Many of these planets are orbited by one or more moons, which can be larger than small planets. There have also been more than two hundred planets discovered orbiting other stars."Encyclopaedia"> Interactive Extra-solar Planets Catalog, The Extrasolar Planets Encyclopaedia, Planets are generally d. ivided into two main types: large, low-density gas giants and smaller, rocky terrestrials. Dwarf planets, a separate category, can either be terrestrials or frozen ice dwarfs.


In ancient times, astronomers noted how certain lights moved across the sky in relation to the other stars. These objects were believed to orbit the Earth, which was considered to be stationary. The lights were first called "πλανήται" (planētai), See romanization of Greek for the transcription scheme meaning "wanderers", by the ancient Greeks, and it is from this that the word "planet" was derived. Definition of planet Merriam-Webster OnLine, Words For Our Modern Age: Especially words derived from Latin and Greek.

The Greeks gave the planets names: the farthest was called Phainon, the shiner, while below it was Phaethon, the bright one. The red planet was known as Pyroeis, "fiery", while the brightest was known as Phosphoros, the light bringer, and the fleeting final planet was called Stilbon, the gleamer. However, the Greeks also made each planet sacred to one of their pantheon of gods, the Olympians: Phainon was sacred to Kronos, the Titan who fathered the Olympians, while Phaethon was sacred to Zeus, his son who deposed him as king. Ares, son of Zeus and god of war, was given dominion over Pyroeis, while Aphrodite, goddess of love, ruled over bright Phosphoros, and Hermes ruled over Stilbon. The History and Practice of Ancient, James Evans, Oxford University Press

The Greek practice of grafting of their gods' names onto the planets was almost certainly borrowed from the Babylonians, a contemporary civilisation in what is now Iraq, from whom they had begun to absorb astronomical learning, including constellations and the zodiac, by 600 BCE. A Chronological History of Babylonian Astronomy, Gary D. Thompson,] The Babylonians had in turn inherited the practice from their predecessors, the Sumerians, who flourished around 2500 years before. The Babylonians named Phosphoros after their goddess of love, Ishtar, Pyroeis after their god of war, Nergal, and Phaethon after their chief god, Marduk, nergal> The Days of The Friesian School, There are too many concordances between Greek and Babylonian naming conventions for them to have arisen separately. There does, however, appear to have been some confusion in translation. For instance, the Babylonian Nergal was a god of war, and the Greeks, seeing this aspect of Nergal's persona, identified him with Ares, their god of war. However, Nergal, unlike Ares, was also a god of the dead and a god of pestilence.<ref name= nergal.

Today, most people in the western world know the planets by names derived from the Olympian pantheon of gods; however, because of the influence of the Roman Empire and, later, the Catholic Church, they are known by their Roman (or Latin) names, rather than the Greek. The Romans, who, like the Greeks, were Indo-Europeans, shared with them a common pantheon under different names but lacked the rich narrative traditions that Greek poetic culture had given their gods. During the later period of the Roman Republic, Roman writers borrowed much of the Greek narratives and applied them to their own pantheon, to the point where they became virtually indistinguishable. When the Romans studied Greek astronomy, they gave the planets their own gods' names. To the Greeks and Romans, there were five known planets; each presumed to be circling the Earth according to the complex laws laid out by Claudius Ptolemy in the 2nd century. They were, in increasing order from Earth (according to Ptolemy): Mercury (Hermes), Venus (Aphrodite), Mars (Ares), Jupiter (Zeus), and Saturn (Kronos). Although strictly the term "planetai" referred only to those five objects, the term was often expanded to include the Sun and the Moon. Theoi Project, The Greeks still use their original names for the planets.

Some Romans, following a belief imported from Mesopotamia into Hellenistic Egypt, 5: Planetary Linguisticw,, believed that the seven gods after whom the planets were named took hourly shifts in looking after affairs on Earth. The order of shifts began with Jupiter and worked inwards; as a result, a list of which god had charge of the first hour in each day became Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn, i.e. the usual weekday name order. name="weekdays". Astronomical Names for the Days of the Week, Journal of the Royal Astronomical Society of Canada Sunday, Monday, and Saturday are straightforward translations of these Roman names. In English the other days were renamed after Tiw, (Tuesday) Wóden (Wednesday), Thunor (Thursday), and Fríge (Friday), Anglo-Saxon gods considered similar or equivalent to Mars, Mercury, Jupiter, and Venus respectively.

Since Earth was only generally accepted as a planet in the 17th century, there is no tradition of naming it after a god. Many of the Romance languages (including French, Italian, Spanish and Portuguese), which are descended from Latin, retain the old Roman name of Terra or some variation thereof. However, the non-Romance languages use their own respective native words. Again, the Greeks retain their original name, Γή (Ge or Yi); the Germanic languages, including English, use a variation of an ancient Germanic word ertho, "ground," as can be seen in the English Earth, the German Erde, the Dutch Aarde, and the Scandinavian Jorde. The same is true for the Sun and the Moon, though they are no longer considered planets.

Some non-European cultures use their own planetary naming systems. India uses a naming system based on the Navagraha, which incorporates the seven traditional planets (Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn) and the ascending and descending lunar nodes Rahu and Ketu. China, and the countries of eastern Asia subject to Chinese cultural influence, such as Japan, Korea and Vietnam, use a naming system based on the five Chinese elements.


As scientific knowledge progressed, understanding of the term "planet" changed from something that moved across the sky (in relation to the starfield), to a body that orbited the Earth (or that were believed to do so at the time). When the heliocentric model gained sway in the 16th century, it became accepted that a planet was actually something that directly orbited the Sun. Thus the Earth was itself a planet,, Copernican The Galileo Project while the Sun and Moon were not. Since they do not directly "orbit the Sun". At the end of the 17th century, when the first satellites of Saturn were discovered, the terms "planet" and "satellite" were at first used interchangeably, although "satellite" would gradually become more prevalent in the following century. "A Discovery of two New Planets about Saturn, made in the Royal Parisian Observatory by Signor Cassini, Fellow of both the Royal Societys, of England and France; English't out of French. [3] . This journal became the Philosophical Transactions of the Royal Society of London in 1775. There may just be earlier publications within the Until the mid-19th century, any newly discovered object orbiting the Sun was listed with the planets by the scientific community, and the number of "planets" swelled rapidly towards the end of that period.

During the 1800s, astronomers began to realize most recent discoveries were unlike the traditional planets. They shared the same region of space, between Mars and Jupiter, and had a far smaller mass. Bodies such as Ceres, Pallas, and Vesta, which had been classed as planets for almost half a century, became classified with the new designation "asteroid." From this point, a "planet" came to be understood, in the absence of any formal definition, as any "large" body that orbited the Sun. There was no apparent need to create a set limit, as there was a dramatic size gap between the asteroids and the planets, and the spate of new discoveries seemed to have ended after the discovery of Neptune in 1846.<ref> When Did the Asteroids Become Minor Planets? U.S. Naval Observatory

However, in the 20th century, Pluto was discovered. After initial observations led to the belief it was larger than Earth, the recently-created IAU accepted the object as a planet. Further monitoring found the body was actually much smaller, but, as it was still larger than all known asteroids and seemingly did not exist within a larger population, it kept its status for some seventy years. Is Pluto a giant comet?

In the 1990s and early 2000s, there was a flood of discoveries of similar objects in the same region of the Solar System. Like Ceres and the asteroids before it, Pluto was found to be just one small body in a population of thousands. A growing number of astronomers argued for it to be declassified as a planet, since many similar objects approaching its size were found. The discovery of Eris, a more massive object widely publicised as the tenth planet, brought things to a head. The IAU set about creating the definition of planet, and eventually produced one in 2006. The number of planets dropped to the eight significantly larger bodies that had cleared their orbit (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus & Neptune), and a new class of dwarf planets was created, initially containing three objects (Ceres, Pluto and Eris).

Former planets

seealso|List of Solar System bodies formerly regarded as planets In ancient times, astronomers accepted as "planets" the seven visible objects that moved across the starfield: the Sun, the Moon, Mercury, Venus, Mars, Jupiter and Saturn. Since then, many objects have qualified as planets for a time:

Asteroid until at least 2006,

Definition and disputes

With the discovery during the latter half of the twentieth century of more objects within the Solar System and large objects around other stars, disputes arose over what should constitute a planet. There was particular disagreement over whether an object should be considered a planet if it was part of a distinct population such as a belt, or if it was large enough to generate energy by the thermonuclear fusion of deuterium.


  1. Details on the new coding for clickable images is here:
  2. While it may look strange, it's important to keep the codes for a particular system in order. The clickable coding treats the first object created in an area as the one on top.
  3. Moons should be placed on "top" so that their smaller circles won't disappear "under" their respective primaries.

In 2003, The International Astronomical Union (IAU) Working Group on Extrasolar Planets made a position statement on the definition of a planet that incorporated a working definition: Working Group on Extrasolar Planets (WGESP) of the International Astronomical,

  1. Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 times the mass of Jupiter for objects with the same isotopic abundance as the Sun) Saumon, D.; Hubbard, W. B.; Burrows, A.; Guillot, T.; Lunine, J. I.; Chabrier, G. A Theory of Extrasolar Giant Planets, Astrophysical Journal, that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass and 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).