Artist's depiction of the extrasolar planet HD 209458 b orbiting its star

A planet, as 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, and has cleared its neighbouring region of planetesimals. An extrasolar planet, or exoplanet, is a Planet beyond the Solar System, orbiting around other Stars As of September 2008 312 HD 209458 b, also unofficially named Osiris, is an Extrasolar planet that orbits the Solar twin Star HD 209458 in the constellation In Physics, an orbit is the gravitationally curved path of one object around a point or another body for example the gravitational orbit of a planet around a star A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth Stellar evolution is the process by which a Star undergoes a sequence of radical changes during its lifetime Gravitation is a natural Phenomenon by which objects with Mass attract one another In Physics and Nuclear chemistry, nuclear fusion is the process by which multiple- like charged atomic nuclei join together to form a heavier nucleus Planetesimals are solid objects thought to exist in Protoplanetary disks and in Debris disks A widely accepted theory of planet formation the so-called planetesimal ^[a][1][2]

The term planet is an ancient one having ties to history, science, myth, and religion. The planets were originally seen as a divine presence; as emissaries of the gods. Even today, many people continue to believe the movement of the planets affects their lives, although such a causation is rejected by the scientific community. Astrology (from Greek grc ἄστρον astron, "constellation star" and grc -λογία -logia) is a group of Systems Astrology and astronomy are historically one and the same discipline ( Latin: astrologia) and were only gradually recognized as separate in western As scientific knowledge advanced, the human perception of the planets changed over time, incorporating a number of disparate objects. Even now there is no uncontested definition of what a planet is. In 2006, the IAU officially adopted a resolution defining planets within the Solar System. The Solar System consists of the Sun and those celestial objects bound to it by Gravity. This definition has been both praised and criticized, and remains disputed by some scientists.

The planets were initially thought to orbit the Earth in circular motions; after the development of the telescope, the planets were determined to orbit the Sun, and their orbits were found to be elliptical. As observational tools improved, astronomers saw that, like Earth, the planets rotated around tilted axes and some share such features as ice-caps and seasons. Historically Astronomy was more concerned with the classification and description of phenomena in the sky while Astrophysics attempted to explain these phenomena Since the dawn of the Space Age, close observation by probes has found that Earth and the other planets share characteristics such as volcanism, hurricanes, tectonics and even hydrology. The Space Age is a contemporary period encompassing the activities related to the Space Race, Space exploration, space technology and the cultural developments Since 1992, through the discovery of hundreds of extrasolar planets (planets around other stars), scientists are beginning to observe similar features throughout the Milky Way Galaxy. An extrasolar planet, or exoplanet, is a Planet beyond the Solar System, orbiting around other Stars As of September 2008 312 The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply

Under IAU definitions, there are eight planets in the Solar System (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune) and 277 known extrasolar ones. The VENUS ( V ictoria E xperimental N etwork U nder the S ea project is a cabled sea floor observatory operated by the University EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 Neptune ( English|AmE] ] is the eighth and farthest Planet from the Sun in the Solar System. ^[3] The Solar System also contains at least three dwarf planets (Ceres, Pluto, and Eris). A dwarf planet, as defined by the International Astronomical Union (IAU is a Celestial body Orbiting the Sun that is massive enough to be rounded Ceres (ˈsɪəriːz Many of these planets are orbited by one or more moons, which can be larger than small planets. A natural satellite or moon is a Celestial body that Orbits a Planet or smaller body which is called the primary. Planets are generally divided into two main types: large, low-density gas giants and smaller, rocky terrestrials. A gas giant (sometimes also known as a Jovian planet after the planet Jupiter, or giant planet) is a large Planet that is not primarily A terrestrial planet, telluric planet or rocky planet is a Planet that is primarily composed of Silicate rocks Within our

History

Main articles: History of astronomy and Definition of planet

The idea of planets has evolved over its history, from the divine wandering stars of antiquity to the earthly objects of the scientific age. Astronomy is the oldest of the Natural sciences dating back to antiquity, with its origins in the religious, Mythological, and Astrological From its beginnings denoting the "wandering stars" of the classical world the definition of " Planet " has been fraught with ambiguity In Astronomy, the naked-eye planets are the five Planets of our Solar system that can be discerned with the Naked eye without much difficulty The concept has also now expanded to include worlds not only in our Solar System, but in hundreds of other extrasolar systems. The ambiguities inherent in defining planets have led to much scientific controversy.

Antiquity

See also: Geocentric model

Early printed rendition of a geocentric cosmological model

In ancient times, astronomers noted how certain lights moved across the sky in relation to the other stars. In Astronomy, the geocentric model of the Universe is the superseded theory that the Earth is the center of the universe and other Ancient Greeks called these lights "πλάνητες ἀστέρες" (planetes asteres: wandering stars) or simply "πλανήτοι" (planētoi: wanderers),^[4] from which the today's word "planet" was derived. ^[5][6]

In ancient Greece as well as in ancient China, ancient Babylon and indeed all pre-modern civilisations,^[7][8] it was almost universally believed that Earth was in the centre of the Universe and that all the "planets" circled the Earth. The term ancient Greece refers to the period of Greek history lasting from the Greek Dark Ages ca Astronomy in China has a very long history Oracle bones from the Shang Dynasty ( 2nd millennium BC) record eclipses and novae Babylon was a City-state of ancient Mesopotamia, the remains of which can be found in present-day Al Hillah, Babil Province, Iraq The Universe is defined as everything that Physically Exists: the entirety of Space and Time, all forms of Matter, Energy The reasons for this perception was that stars and planets appeared to revolve around the Earth each day,^[9] and the apparently common sense perception that the Earth was solid and stable and that it is not moving but at rest. Common sense (or when used attributively as an Adjective, commonsense, common-sense, or commonsensical) based on a strict construction

The Greek cosmological system was taken from that of the Babylonians,^[10] a contemporary Mesopotamian civilisation from whom they began to acquire astronomical learning from around 600 BC, including the constellations and the zodiac. Babylonia was an Amorite state in lower Mesopotamia (modern southern Iraq) with Babylon as its capital Mesopotamia (from the Greek meaning "land between the rivers" is an area geographically located between the Tigris and Euphrates rivers largely corresponding In common usage a constellation is a group of celestial bodies that are connected together in some arrangement typically stars to form a visible figure or picture Zodiac denotes an annual cycle of twelve stations along the Ecliptic, the apparent path of the sun across the heavens through the Constellations that divide the ecliptic ^[11] In the 6th century BC, the Babylonians had a highly advanced level of astronomical knowledge, and had a theory of the planets centuries before the ancient Greeks. The oldest planetary astronomical text that we possess is the Babylonian Venus tablet of Ammisaduqa, a 7th century BC copy of a list of observations of the motions of the planet Venus that probably dates as early as the second millennium BC. The Venus tablet of Ammisaduqa recovered from the library at Nineveh, is a 7th century BC cuneiform tablet that bears much older ( Bronze Age) The 7th century BC started the first day of 700 BC and ended the last day of 601 BC. ^[12] The Babylonians also laid the foundations of what would eventually become Western astrology. Western astrology is the system of Astrology most popular in Western countries ^[13] The Enuma anu enlil, written during the Neo-Assyrian period in the 7th century BC,^[14] comprises a list of omens and their relationships with various celestial phenomena including the motions of the planets. Enuma Anu Enlil (EAE (literal translation When the gods Anu and Enlil) (meaningful translation In the days of Anu and Enlil The Neo-Assyrian Empire was a period of Mesopotamian history which began in 934 BC and ended in 609 BC An omen (also called portent or presage) is a phenomenon that is believed to foretell the Future, often signifying the advent of change ^[15] Sumerians, predecessors of Babylonians which are credited as one of the first civilizations and the inventors of writing, had identified at least Venus by 1500 BC. Sumer ( Sumerian: sux-Latn [[Ki (earth ki]]-[[EN (cuneiform en]]-'''ĝir15''', Akkadian: Šumeru; possibly Biblical Shinar The cradle of civilization is any of the possible locations for the emergence of Civilization. Inanna ( D INANNA B153ellstpng|100x20px|INANNA]]) is the Sumerian goddess of sexual love fertility and warfare ^[10] Conversely, there is no evidence of knowledge of the planets in the earliest written Greek sources, such as the Iliad and the Odyssey. The Iliad ( Greek: Ἰλιάς (Ancient Ιλιάδα (Modern is together with the Odyssey, one of two ancient The Odyssey ( Greek: Ὀδύσσεια or Odússeia) is one of two major ancient Greek epic poems attributed to Homer. ^[13]

By the first century BC, the Greeks had begun to develop their own mathematical schemes for predicting the positions of the planets. These schemes, which were based on geometry rather than the arithmetic of the Babylonians, would eventually eclipse the Babylonians' theories in complexity and comprehensiveness and account for much of the astronomical movements observed from Earth with the naked eye. These theories would reach their fullest expression in the Almagest written by Ptolemy in the 2nd century AD. Almagest is the Latin form of the Arabic name ( الكتاب المجسطي, al-kitabu-l-mijisti, i Claudius Ptolemaeus ( Greek: Klaúdios Ptolemaîos; after 83 &ndash ca So complete was the domination of Ptolemy's model that it superseded all previous works on astronomy and remained the definitive astronomical text in the Western world for 13 centuries. ^[16][12]

To the Greeks and Romans there were seven known planets; each presumed to be circling the Earth according to the complex laws laid out by Ptolemy. In Astronomy, the geocentric model of the Universe is the superseded theory that the Earth is the center of the universe and other They were, in increasing order from Earth (in Ptolemy's order): the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. The Sun (Sol is the Star at the center of the Solar System. ^[16][17][6]

Modern times

See also: Heliocentrism

The five naked-eye planets have been known since ancient times, and have had a significant impact on mythology, religious cosmology, and ancient astronomy. In Astronomy, heliocentrism is the theory that the Sun is at the center of the Solar System. In Astronomy, the naked-eye planets are the five Planets of our Solar system that can be discerned with the Naked eye without much difficulty The word mythology (from the Greek grc μυθολογία mythología, meaning "a story-telling a legendary lore" See Cosmology (disambiguation. Religious cosmologies are ways of explaining the history and evolution of the Universe based Astronomy (from the Greek words astron (ἄστρον "star" and nomos (νόμος "law" is the scientific study As scientific knowledge progressed, however, understanding of the term "planet" changed from something that moved across the sky (in relation to the star field); to a body that orbited the Earth (or that were believed to do so at the time); and in the 16th century to something that directly orbited the Sun when the heliocentric model of Copernicus, Galileo and Kepler gained sway. The fixed stars (from the Latin stellae fixae) are celestial objects that do not seem to move in relation to the other stars of the night sky In Astronomy, heliocentrism is the theory that the Sun is at the center of the Solar System. Galileo Galilei (15 February 1564 &ndash 8 January 1642 was a Tuscan ( Italian) Physicist, Mathematician, Astronomer, and Philosopher Johannes Kepler (ˈkɛplɚ ( December 27 1571 &ndash November 15 1630) was a German Mathematician, Astronomer

Heliocentrism (lower panel) in comparison to the geocentric model (upper panel)

Thus the Earth became included in the list of planets,^[18] while the Sun and Moon were excluded. At first, when the first satellites of Saturn were discovered at the end of the 17th century, the terms "planet" and "satellite" were used interchangeably – although the latter would gradually become more prevalent in the following century. ^[19] Until the mid-19th century, the number of "planets" rose rapidly since any newly discovered object directly orbiting the Sun was listed as a planet by the scientific community.

In the 19th century astronomers began to realize that recently discovered bodies that had been classified as planets for almost half a century (such as Ceres, Pallas, and Vesta), were very different from the traditional one. TemplateInfobox Planet. --> 2 Pallas (ˈpæləs, or as Παλλάς) is one of the largest TemplateInfobox Planet. --> 4 Vesta (ˈvɛstə Vesta is the second most massive object in the Asteroid belt These bodies shared the same region of space between Mars and Jupiter (the Asteroid belt), and had a much smaller mass; as a result they were reclassified as "asteroids". The asteroid belt is the region of the Solar System located roughly between the orbits of the Planets Mars and Jupiter. Asteroids, sometimes called Minor planets or planetoids', are bodies—primarily of the inner Solar System —that are smaller than planets but In the absence of any formal definition, a "planet" came to be understood as any "large" body that orbited the Sun. Since 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, there was no apparent need to have a formal definition. ^[20]

However, in the 20th century, Pluto was discovered. After initial observations led to the belief it was larger than Earth,^[21] the object was immediately accepted as the ninth planet. Further monitoring found the body was actually much smaller: in 1936, Raymond Lyttleton suggested that Pluto may be an escaped satellite of Neptune,^[22] and Fred Whipple suggested in 1964 that Pluto may be a comet. Raymond Arthur Lyttleton ( May 7, 1911 – May 16, 1995) was a British Astronomer. Neptune ( English|AmE] ] is the eighth and farthest Planet from the Sun in the Solar System. Fred Lawrence Whipple ( November 5, 1906 – August 30, 2004) was an American Astronomer. ^[23] However, as it was still larger than all known asteroids and seemingly did not exist within a larger population,^[24] it kept its status until 2006.

In the 1990s and early 2000s, there was a flood of discoveries of similar objects in the same region of the Solar System (the Kuiper belt). The Kuiper belt (ˈkaɪpɚ to rhyme with "viper" sometimes called the Edgeworth-Kuiper belt, is a region of the Solar System beyond the planets extending 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. From its beginnings denoting the "wandering stars" of the classical world the definition of " Planet " has been fraught with ambiguity 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). A dwarf planet, as defined by the International Astronomical Union (IAU is a Celestial body Orbiting the Sun that is massive enough to be rounded ^[25]

In 1992, astronomers Aleksander Wolszczan and Dale Frail announced the discovery of planets around a pulsar, PSR B1257+12. Aleksander Wolszczan (alɛk'sandɛr 'vɔlʂt͡ʂan) ( Apr 29 1946 in Szczecinek, Poland) is a Polish astronomer. Dale A Frail is a Canadian radio Astronomer working for the National Radio Astronomy Observatory (NRAO in Socorro New Mexico. Pulsars are highly magnetized rotating Neutron stars that emit a beam of Electromagnetic radiation in the form of radio waves PSR B1257+12, sometimes abbreviated as PSR 1257+12, is a Pulsar located 980 Light-years from the Sun. ^[26] This discovery is generally considered to be the first definitive detection of a planetary system around another star. Then, on October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi). Events 105 BC - Battle of Arausio: The Cimbri inflict the heaviest defeat on the Roman army of Gnaeus Mallius Maximus Year 1995 ( MCMXCV) was a Common year starting on Sunday. Events of 1995 Michel G E Mayor (born 12 January, 1942) is a Swiss professor in the Department of Astronomy at the University of Geneva. Didier Queloz (born February 23 1966) is a Geneva -based Astronomer with a prolific record in finding Extrasolar planets He is understudy The University of Geneva (Université de Genève is a university in Geneva, Switzerland. The main sequence is the name for a continuous and distinctive band of stars that appear on a plot of stellar color versus brightness 51 Pegasi is a Sun -like star (although somewhat older than the Sun located 15 ^[27]

The discovery of extrasolar planets led to another ambiguity in defining a planet; the point at which a planet becomes a star. Many known extrasolar planets are many times the mass of Jupiter, approaching that of stellar objects known as "brown dwarfs". Brown dwarfs are sub- stellar objects with a mass below that necessary to maintain Hydrogen -burning Nuclear fusion reactions in their cores as do stars ^[28] Brown dwarfs are generally considered stars due to their ability to fuse deuterium, a heavier isotope of hydrogen. Deuterium, also called heavy hydrogen, is a Stable isotope of Hydrogen with a Natural abundance in the Oceans of Earth Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 While stars more massive than 75 times that of Jupiter fuse hydrogen, stars of only 13 Jupiter masses can fuse deuterium. However, deuterium is quite rare, and most brown dwarfs would have ceased fusing deuterium long before their discovery, making them effectively indistinguishable from supermassive planets. ^[29]

As large Kuiper belt and scattered disc objects were discovered in the late 1990s and early years of the twenty-first century, a number including Quaoar, Sedna and Eris were heralded in the popular press as the 'tenth planet', however none of these received widespread scientific recognition as such, although Eris has now been classified as a Dwarf Planet. The Kuiper belt (ˈkaɪpɚ to rhyme with "viper" sometimes called the Edgeworth-Kuiper belt, is a region of the Solar System beyond the planets extending TemplateInfobox Planet.--> 90377 Sedna (ˈsɛdnə) is a Trans-Neptunian A dwarf planet, as defined by the International Astronomical Union (IAU is a Celestial body Orbiting the Sun that is massive enough to be rounded

Former classifications

The table below lists Solar System bodies formerly considered to be planets:

Modern definition

Main article: 2006 definition of planet

With the discovery during the latter half of the 20th 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. The asteroid belt is the region of the Solar System located roughly between the orbits of the Planets Mars and Jupiter. In Physics and Nuclear chemistry, nuclear fusion is the process by which multiple- like charged atomic nuclei join together to form a heavier nucleus Deuterium, also called heavy hydrogen, is a Stable isotope of Hydrogen with a Natural abundance in the Oceans of Earth

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:^[2]

The largest Trans-Neptunian objects that prompted the IAU's 2006 decision

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^[32]) that orbit stars or stellar remnants are "planets" (no matter how they formed). The term true mass is synonymous with the term Mass, but is used in Astronomy to differentiate the measured mass of a planet from the lower limit of mass usually obtained In Chemistry, natural abundance (NA refers to the abundance Isotopes of a Chemical element as naturally found on a planet 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 or where they are located. Brown dwarfs are sub- stellar objects with a mass below that necessary to maintain Hydrogen -burning Nuclear fusion reactions in their cores as do stars
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). Star clusters are groups of Stars which are gravitationally bound

This definition has since been widely used by astronomers when publishing discoveries in academic journals. An academic journal is a peer-reviewed Periodical in which scholarship relating to a particular Academic discipline is published ^[33] Although temporary, it remains an effective, working definition until a more permanent one is formally adopted. Nevertheless, it did not address the dispute over the lower mass limit,^[34] and steered clear of the controversy regarding objects within the Solar System.

This matter was finally addressed during the 2006 meeting of the IAU's General Assembly. After much debate and one failed proposal, the assembly voted to pass a resolution that defined planets within the Solar System as^[1]:

A celestial body that is (a) in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. Hydrostatic equilibrium occurs when compression due to Gravity is balanced by a Pressure gradient which creates a Pressure gradient force in the opposite

Under this definition, the Solar System is considered to have eight planets. Bodies which fulfill the first two conditions but not the third (such as Pluto and Eris) are classified as dwarf planets, providing they are not also natural satellites of other planets. A dwarf planet, as defined by the International Astronomical Union (IAU is a Celestial body Orbiting the Sun that is massive enough to be rounded A natural satellite or moon is a Celestial body that Orbits a Planet or smaller body which is called the primary. Originally an IAU committee had proposed a definition that would have included a much larger number of planets as it did not include (c) as a criterion. ^[35] After much discussion, it was decided via a vote that those bodies should instead be classified as dwarf planets. ^[36]

This definition is based in modern theories of planetary formation, in which planetary embryos initially clear their orbital neighborhood of other smaller objects. As described by astronomer Steven Soter:^[37]

The end product of secondary disk accretion is a small number of relatively large bodies (planets) in either non-intersecting or resonant orbits, which prevent collisions between them. Dr Steven Soter, PhD, is an astrophysicist currently holding the positions of scientist-in-residence for New York University 's Environmental Studies Program and of Asteroids and comets, including KBOs, differ from planets in that they can collide with each other and with planets.

In the aftermath of the IAU's 2006 vote, there has been criticism of the new definition,^[38][39] and some astronomers have even stated that they will not use it. ^[40] Part of the dispute centres around the belief that point (c) (clearing its orbit) should not have been listed, and that those objects now categorised as dwarf planets should actually be part of a broader planetary definition. The next IAU conference is not until 2009, when modifications could be made to the definition, also possibly including extrasolar planets. An academic conference is a conference for Researchers (not always Academics to present and Discuss their work

Beyond the scientific community, Pluto has held a strong cultural significance for many in the general public considering its planetary status during most of the 20th century – similarly to Ceres and its kin in the 1800s. The discovery of Eris was widely reported in the media as the "tenth planet" and therefore the reclassification of all three objects as dwarf planets has attracted a lot of media and public attention. "Popular press" redirects here note that the University of Wisconsin Press publishes under the imprint "The Popular Press" ^[41]

Mythology

See also: Days of the week and Naked-eye planet

The gods of Olympus, after whom the Solar System's planets are named

The names for the planets in the Western world are derived from the naming practices of the Romans, which ultimately derive from those of the Greeks and the Babylonians. The names of the days of the week in various world languages can be classified as either numerical or planetary; however in either case the names of one or more days In Astronomy, the naked-eye planets are the five Planets of our Solar system that can be discerned with the Naked eye without much difficulty Mount Olympus (Όλυμπος also transliterated as Ólympos, and on Greek maps Óros Ólimbos) is the highest Mountain in Greece In ancient Greece, the two great luminaries the Sun and the Moon were called Helios and Selene; the farthest planet was called Phainon, the shiner; followed by Phaethon, "bright"; the red planet was known as Pyroeis, the "fiery"; the brightest was known as Phosphoros, the light bringer; and the fleeting final planet was called Stilbon, the gleamer. The term ancient Greece refers to the period of Greek history lasting from the Greek Dark Ages ca In Greek mythology the Sun was personified as Helios (ˈhiliˌɑs ( Ἥλιος Latinized as Helius) In Greek mythology, Selene (Σελήνη " Moon " English sɛˈliːniː was an archaic Lunar deity and the daughter of the Titans The Greeks also made each planet sacred to one of their pantheon of gods, the Olympians: Helios and Selene were the names of both planets and gods; Phainon was sacred to Kronos, the Titan who fathered the Olympians; Phaethon was sacred to Zeús, Kronos's son who deposed him as king; Pyroeis was given to Ares, son of Zeus and god of war; Phosphorus was ruled by Aphrodite, the goddess of love; and Hermes, messenger of the gods and god of learning and wit, ruled over Stilbon. The Twelve Olympians, also known as the Dodekatheon ( Greek: Δωδεκάθεον Cronus or Kronos, ( Ancient Greek Κρόνος Krónos) was the leader and the youngest of the first generation of Titans, divine descendants In Greek mythology, the Titans ( Greek: Tītā́n; plural Tītânes) were a race of powerful Deities that ruled during the legendary Zeus (zjuːs in Greek: nominative: Zeús /zdeús/ genitive: Diós; Modern Greek /'zefs/ in Greek mythology In Greek mythology, Ares ( Ancient Greek:, Μodern Greek Άρης) is the son of Zeus and Hera. Hermes ( Greek,, ˈhɝmiːz in Greek mythology, is the Olympian god of boundaries and of the travelers who cross them of Shepherds and ^[12]

The Greek practice of grafting of their gods' names onto the planets was almost certainly borrowed from the Babylonians. The Babylonians named Phosphorus after their goddess of love, Ishtar; Pyroeis after their god of war, Nergal, Stilbon after their god of wisdom Nabu, and Phaethon after their chief god, Marduk. Ishtar ( D IŠTAR 𒀭𒌋𒁯 is the Assyrian and Babylonian counterpart to the Sumerian Inanna and to The name Nergal (or Nirgal, Nirgali) refers to a Deity in Babylonia with the main seat Nabu is the Babylonian god of Wisdom and Writing, worshipped by Babylonians as the son of Marduk and his consort Sarpanitum, Marduk ( Sumerian spelling in Akkadian: AMARUTU 𒀫 𒌓 "solar calf" perhaps from MERI ^[42] There are too many concordances between Greek and Babylonian naming conventions for them to have arisen separately. ^[12] The translation was not perfect. For instance, the Babylonian Nergal was a god of war, and thus the Greeks identified him with Ares. However, unlike Ares, Nergal was also god of pestilence and the underworld. ^[43]

Today, most people in the western world know the planets by names derived from the Olympian pantheon of gods. The Twelve Olympians, also known as the Dodekatheon ( Greek: Δωδεκάθεον While modern Greeks still use their ancient names for the planets, other European languages, because of the influence of the Roman Empire and, later, the Catholic Church, use the Roman (or Latin) names rather than the Greek ones. The Roman Empire was the post-Republican phase of the ancient Roman civilization, characterised by an autocratic form of government and large territorial 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. The existence of similarities among the deities and religious practices of the Indo-European (IE peoples allows glimpses of a common Proto-Indo-European Roman mythology, or more appropriately Latin mythology, refers to the mythological beliefs of the Italic people inhabiting the region of Latium and its Greek mythology is the body of stories belonging to the ancient Greeks concerning their gods and Heroes the nature of the world and the origins and significance 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. The Roman Republic was the phase of the ancient Roman civilization characterized by a Republican form of government a period which began with the overthrow of the ^[44] When the Romans studied Greek astronomy, they gave the planets their own gods' names: Mercurius (for Hermes), Venus (Aphrodite), Mars (Ares), Iuppiter (Zeus) and Saturnus (Kronos). "Alipes" redirects here For the Centipede Genus, see Alipes (centipede. Venus was a major Roman Goddess principally associated with Love, Beauty and fertility, the equivalent of the Greek goddess Mars was the Roman Warrior god, the son of Juno and Jupiter, husband of Bellona, and the lover of Venus. In Roman mythology, Jupiter was the king of the gods and the god of Sky and Thunder. Saturn ( Latin: Saturnus) was a major Roman God of agriculture and harvest When subsequent planets were discovered in the 18th and 19th centuries, the naming practice was retained: Uranus (Ouranos) and Neptūnus (Poseidon). Uranus (ˈjʊərənəs jʊˈreɪnəs is the Latinized form of Ouranos () the Greek word for Sky. Neptune (Neptūnus is the god of water and the sea in Roman mythology, a brother of Jupiter and Pluto. In Greek mythology, Poseidon ( Greek:; Latin: Neptūnus) was the god of the Sea and as "Earth-Shaker"

Some Romans, following a belief possibly originating in Mesopotamia but developed in Hellenistic Egypt, believed that the seven gods after whom the planets were named took hourly shifts in looking after affairs on Earth. Ancient Rome was a Civilization that grew out of a small agricultural community founded on the Italian Peninsula as early as the 10th century BC Mesopotamia (from the Greek meaning "land between the rivers" is an area geographically located between the Tigris and Euphrates rivers largely corresponding Ptolemaic Egypt began when Ptolemy I Soter declared himself Pharaoh of Egypt in 305 BC and ended with the death of queen Cleopatra The order of shifts went Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon (from the farthest to the closest planet). ^[45] Therefore, the first day was started by Saturn (1st hour), second day by Sun (25th hour), followed by Moon (49th hour), Mars, Mercury, Jupiter and Venus. Since each day was named by the god that started it, this is also the order of the days of the week in the Roman calendar – and still preserved many modern languages. The names of the days of the week in various world languages can be classified as either numerical or planetary; however in either case the names of one or more days The Roman calendar changed its form several times in the time between the foundation of Rome and the fall of the Roman Empire. ^[46] 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), the Anglo-Saxon gods considered similar or equivalent to Mars, Mercury, Jupiter, and Venus respectively. Wōden is a god in Anglo-Saxon paganism, together with Norse Odin representing a development of a Proto-Germanic god * Wōdanaz Thor ( Old Norse: Þórr) is the red-haired and bearded God of Thunder in Germanic paganism and its subset Norse paganism Frīge ( Anglo-Saxon) Friia ( German) or Frea ( Langobard) was a love goddess in Germanic paganism, and the wife of Continental Germanic mythology is a subset of Germanic mythology, going back to South Germanic polytheism as practiced in parts of Central Europe before

Since Earth was only generally accepted as a planet in the 17th century,^[18] there is no tradition of naming it after a god (the same is true, in English at least, of the Sun and the Moon, though they are no longer considered planets). The name originates from the 8th century Anglo-Saxon word erda, which means ground or soil and was first used in writing as the name of the sphere of the Earth perhaps around 1300. ^[47][48] It is the only planet whose name in English is not derived from greco-roman mythology. Greek mythology is the body of stories belonging to the ancient Greeks concerning their gods and Heroes the nature of the world and the origins and significance Roman mythology, or more appropriately Latin mythology, refers to the mythological beliefs of the Italic people inhabiting the region of Latium and its Many of the Romance languages retain the old Roman word terra (or some variation of it) that was used with the meaning of "dry land" (as opposed to "sea"). The Romance languages (sometimes referred to as Romanic languages, or Neolatin languages) are a branch of the Indo-European language family comprising all Terra Mater or Tellus was a Goddess personifying the Earth in Roman mythology. ^[49] However, the non-Romance languages use their own respective native words. 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,"^[48] as can be seen in the English Earth, the German Erde, the Dutch Aarde, and the Scandinavian Jorde. Gaia (ˈgeɪə or /ˈgaɪə/ (" land " or " Earth " from the Ancient Greek Γαîα also Gæa or Gea The Germanic languages are a group of related languages that constitute a branch of the Indo-European (IE Language family.

Non-European cultures use other planetary naming systems. India uses a naming system based on the Navagraha, which incorporates the seven traditional planets (Surya for the Sun, Chandra for the Moon, and Budha, Shukra, Mangala, Bṛhaspati and Shani for the traditional planets Mercury, Venus, Mars, Jupiter and Saturn) and the ascending and descending lunar nodes Rahu and Ketu. India, officially the Republic of India (भारत गणराज्य inc-Latn Bhārat Gaṇarājya; see also other Indian languages) is a country Graha (from Sanskrit ग्रह gráha -- seizing laying hold of holding is a 'cosmic influencer' on the living beings of mother Bhumidevi ( Earth) In Hinduism, Surya ( Devanagari: सूर्य sūrya, lit "the Supreme Light" Malay: Suria; Thai: In Hinduism, Chandra (lit "shining is a Lunar deity and a Graha. In Hindu mythology, Budha (Sanskrit बुध not to be confused with Buddha) is the name for the planet Mercury, a son of Chandra (the Shukra (शुक्र the Sanskrit for "clear pure" or "brightness clearness" is the name the son of Bhrgu and Urjaswathi In Jyotish Astrology, Mangala ( Devanagari: मंगल is the name for Mars, the red planet Brihaspati (or Brahmanaspati) is the name of a Vedic deity, personification of piety and religion the chief offerer of prayers and sacrifices represented as Shani ( Sanskrit Śani शनि is one of the Navagraha which are the nine primary celestial beings in Hindu astrology or The lunar nodes are the Orbital nodes of the Moon, that is the points where the orbit of the Moon crosses the Ecliptic (which is the apparent In Hindu mythology, Rahu is a Snake that swallows the Sun or the Moon causing Eclipses He is depicted in art as a dragon with no body Ketu ( Sanskrit: केतु, IAST: ketú is the descending lunar node China and the countries of eastern Asia influenced by it (such as Japan, Korea and Vietnam) use a naming system based on the five Chinese elements: water (Mercury), metal (Venus), fire (Mars), wood (Jupiter) and earth (Saturn). China ( Wade-Giles ( Mandarin) Chung¹kuo² is a cultural region, an ancient Civilization, and depending on perspective a National For a topic outline on this subject see List of basic Japan topics. Korea is a geographic area composed of two sovereign countries a civilization and a former state situated on the Korean Peninsula in East Asia. Vietnam (ˌviːɛtˈnɑːm Việt Nam) officially In traditional Chinese philosophy, natural phenomena can be classified into the Wu Xing ( or the Five Phases, usually translated as five elements, Water has been important to all peoples of the earth and it is rich in spiritual tradition In traditional Chinese philosophy, Metal is classified as one of the Wu xing ( or the Five Elements, also translated as five phases Fire has been an important part of many cultures and religions from pre-history to modern day and was vital to the development of civilization In traditional Chinese philosophy, Wood is classified as one of the Wu xing ( or the Five Elements, also translated as five phases Earth, home and origin of humanity has often been worshipped in its own right with its own unique spiritual tradition ^[46]

Formation

Main article: Nebular hypothesis

It is not known with certainty how planets are formed. In Cosmogony, the nebular hypothesis is the most widely accepted model explaining the Formation and evolution of the Solar System. The prevailing theory is that they are formed during the collapse of a nebula into a thin disk of gas and dust. A nebula (from Latin: "mist" pl nebulae or nebulæ, with ligature or nebulas) is an Interstellar cloud of A protostar forms at the core, surrounded by a rotating protoplanetary disk. A protostar is a large object that forms by contraction out of the gas of a Giant molecular cloud in the Interstellar medium. A protoplanetary disk (or proplyd) is a rotating Circumstellar disk of dense gas surrounding a young newly formed star a T Tauri star or Herbig star Through accretion (a process of sticky collision) dust particles in the disk steadily accumulate mass to form ever-larger bodies. In Astrophysics, the term accretion is used for at least two distinct processes Local concentrations of mass known as planetesimals form, and these accelerate the accretion process by drawing in additional material by their gravitational attraction. Planetesimals are solid objects thought to exist in Protoplanetary disks and in Debris disks A widely accepted theory of planet formation the so-called planetesimal These concentrations become ever denser until they collapse inward under gravity to form protoplanets. Protoplanets are moon-sized planets or larger embryos within Protoplanetary discs They are believed to form out of kilometer-sized Planetesimals that attract each ^[50] After a planet reaches a diameter larger than the Earth's moon, it begins to accumulate an extended atmosphere, greatly increasing the capture rate of the planetesimals by means of atmospheric drag. In Fluid dynamics, drag (sometimes called fluid resistance) is the force that resists the movement of a Solid object through a Fluid (a ^[51]

An artist's impression of protoplanetary disk

When the protostar has grown such that it ignites to form a star, the surviving disk is removed from the inside outward by photoevaporation, the solar wind, Poynting-Robertson drag and other effects. A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth The solar wind is a Stream of charged particles&mdasha plasma &mdashthat are ejected from the upper atmosphere of the Sun. The Poynting-Robertson effect, also known as Poynting-Robertson drag, named after John Henry Poynting and Howard Percy Robertson, is a process by which ^[52][53] Thereafter there still may be many protoplanets orbiting the star or each other, but over time many will collide, either to form a single larger planet or release material for other larger protoplanets or planets to absorb. ^[54] Those objects that have become massive enough will capture most matter in their orbital neighbourhoods to become planets. Meanwhile, protoplanets that have avoided collisions may become natural satellites of planets through a process of gravitational capture, or remain in belts of other objects to become either dwarf planets or small Solar System bodies. A natural satellite or moon is a Celestial body that Orbits a Planet or smaller body which is called the primary. Small Solar System Body ( SSSB) is a term defined in 2006 by the International Astronomical Union to describe objects in the Solar System that

The energetic impacts of the smaller planetesimals (as well as radioactive decay) will heat up the growing planet, causing it to at least partially melt. Radioactive decay is the process in which an unstable Atomic nucleus loses energy by emitting ionizing particles and Radiation. The interior of the planet begins to differentiate by mass, developing a denser core. ^[55] Smaller terrestrial planets lose most of their atmospheres because of this accretion, but the lost gases can be replaced by outgassing from the mantle and from the subsequent impact of comets. A comet is a small Solar System body that orbits the Sun and when close enough to the Sun exhibits a visible coma (atmosphere or a tail — ^[56] (Smaller planets will lose any atmosphere they gain through various escape mechanisms. There are several different processes that can lead to the escape of a planetary atmosphere. )

With the discovery and observation of planetary systems around stars other than our own, it is becoming possible to elaborate, revise or even replace this account. The level of metallicity – an astronomical term describing the abundance of chemical elements with an atomic number greater than 2 (helium) – is now believed to determine the likelihood that a star will have planets. In Astronomy and Physical cosmology, the metallicity of an object is the proportion of its matter made up of Chemical elements other than Hydrogen A chemical element is a type of Atom that is distinguished by its Atomic number; that is by the number of Protons in its nucleus. See also List of elements by atomic number In Chemistry and Physics, the atomic number (also known as the proton Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical ^[57] Hence it is thought less likely that a metal-poor, population II star will possess a more substantial planetary system than a metal-rich population I star. In Astronomy and Physical cosmology, the metallicity of an object is the proportion of its matter made up of Chemical elements other than Hydrogen In Astronomy and Physical cosmology, the metallicity of an object is the proportion of its matter made up of Chemical elements other than Hydrogen

Solar System

The terrestrial planets: Mercury, Venus, Earth, Mars (Sizes to scale)

The four gas giants against the Sun: Jupiter, Saturn, Uranus, Neptune (Sizes to scale)

Main article: Table of planets and dwarf planets in the Solar System

According to the IAU's current definitions, there are eight planets in the Solar System. The Solar System consists of the Sun and those celestial objects bound to it by Gravity. In increasing distance from the Sun, they are:

1. Mercury
2. Venus
3. Earth
4. Mars
5. Jupiter
6. Saturn
7. Uranus
8. Neptune

Jupiter is the largest, at 318 Earth masses, while Mercury is smallest, at 0. The Sun (Sol is the Star at the center of the Solar System. The VENUS ( V ictoria E xperimental N etwork U nder the S ea project is a cabled sea floor observatory operated by the University EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 Neptune ( English|AmE] ] is the eighth and farthest Planet from the Sun in the Solar System. 055 Earth masses.

The planets of the Solar System can be divided into categories based on their composition:

• Terrestrials: Planets that are similar to Earth, with bodies largely composed of rock: Mercury, Venus, Earth and Mars. A terrestrial planet, telluric planet or rocky planet is a Planet that is primarily composed of Silicate rocks Within our In Geology, rock is a naturally occurring aggregate of Minerals and/or Mineraloids The Earth's outer solid layer the ‘ Lithosphere
• Gas giants: Planets with a composition largely made up of gaseous material and are significantly more massive than terrestrials: Jupiter, Saturn, Uranus, Neptune. A gas giant (sometimes also known as a Jovian planet after the planet Jupiter, or giant planet) is a large Planet that is not primarily This page is about the physical properties of gas as a state of matter Ice giants, comprising Uranus and Neptune, are a sub-class of gas giants, distinguished from gas giants by their significantly lower mass, and by depletion in hydrogen and helium in their atmospheres together with a significantly higher proportion of rock and ice. A gas giant (sometimes also known as a Jovian planet after the planet Jupiter, or giant planet) is a large Planet that is not primarily

Dwarf planets

Main article: Dwarf planet

Before the August 2006 decision, several objects were proposed by astronomers, including at one stage by the IAU, as planets. A dwarf planet, as defined by the International Astronomical Union (IAU is a Celestial body Orbiting the Sun that is massive enough to be rounded However in 2006 several of these objects were reclassified as dwarf planets, objects distinct from planets. Currently three dwarf planets in the Solar System are recognized by the IAU: Ceres, Pluto and Eris. Several other objects in both the Asteroid belt and the Kuiper belt are under consideration, with as many as 50 that could eventually qualify. The asteroid belt is the region of the Solar System located roughly between the orbits of the Planets Mars and Jupiter. The Kuiper belt (ˈkaɪpɚ to rhyme with "viper" sometimes called the Edgeworth-Kuiper belt, is a region of the Solar System beyond the planets extending There may be as many as 200 that could be discovered once the Kuiper belt has been fully explored. Dwarf planets share many of the same characteristics as planets, although notable differences remain – namely that they are not dominant in their orbits. Their attributes are:

By definition, all dwarf planets are members of larger populations. In Biology a population is the collection of inter-breeding organisms of a particular Species; in Sociology Ceres is the largest body in the asteroid belt, while Pluto is a member of the Kuiper belt and Eris is a member of the scattered disc. The asteroid belt is the region of the Solar System located roughly between the orbits of the Planets Mars and Jupiter. The scattered disc (or scattered disk) is a distant region of the Solar System that is sparsely populated by icy Minor planets known as scattered Scientists such as Mike Brown believe that there may soon be over forty trans-Neptunian objects that qualify as dwarf planets under the IAU's recent definition. Education Brown is a Huntsville Alabama native and graduated from Virgil Grissom High School in 1983 A trans-Neptunian object (TNO is any object in the Solar system that Orbits the sun at a greater distance on average than Neptune. ^[58]

Extrasolar planets

Main article: Extrasolar planet

False-color infrared image of the brown dwarf 2M1207 (blue) and its planetary companion 2M1207b (red), as viewed by the Very Large Telescope (obtained by direct imaging)

The first confirmed discovery of an extrasolar planet orbiting an ordinary main-sequence star occurred on 6 October 1995, when Michel Mayor and Didier Queloz of the University of Geneva announced the detection of an exoplanet around 51 Pegasi. An extrasolar planet, or exoplanet, is a Planet beyond the Solar System, orbiting around other Stars As of September 2008 312 2M1207, 2M1207A or 2MASSW J1207334−393254 is a Brown dwarf located in the Constellation Centaurus; a companion object 2M1207b 2M1207b is a Planetary mass object orbiting the Brown dwarf 2M1207, in the constellation Centaurus, approximately 170 Light-years The Very Large Telescope ( VLT) is a system of four separate Optical telescopes (the Antu telescope, the Kueyen telescope, the Melipal Any Planet is an extremely faint light source compared to its parent Star. Events 105 BC - Battle of Arausio: The Cimbri inflict the heaviest defeat on the Roman army of Gnaeus Mallius Maximus Year 1995 ( MCMXCV) was a Common year starting on Sunday. Events of 1995 Michel G E Mayor (born 12 January, 1942) is a Swiss professor in the Department of Astronomy at the University of Geneva. Didier Queloz (born February 23 1966) is a Geneva -based Astronomer with a prolific record in finding Extrasolar planets He is understudy The University of Geneva (Université de Genève is a university in Geneva, Switzerland. 51 Pegasi is a Sun -like star (although somewhat older than the Sun located 15 Of the 270 extrasolar planets discovered by January 2008, most have masses which are comparable to or larger than Jupiter's, though masses ranging from just below that of Mercury to many times Jupiter's mass. ^[59] The smallest extrasolar planets found to date have been discovered orbiting burned-out star remnants called pulsars, such as PSR B1257+12. Pulsars are highly magnetized rotating Neutron stars that emit a beam of Electromagnetic radiation in the form of radio waves PSR B1257+12, sometimes abbreviated as PSR 1257+12, is a Pulsar located 980 Light-years from the Sun. ^[60] There have been roughly a dozen extrasolar planets found of between 10 and 20 Earth masses,^[59] such as those orbiting the stars Mu Arae, 55 Cancri and GJ 436. Mu Arae (μ Ara / μ Arae often cataloged as HD 160691, is a Yellow subgiant Star approximately 50 Light-years away in the 55 Cancri (ˈkæŋkraɪ or /ˈkæŋkriː/ also catalogued Rho1 Cancri or abbreviated 55 Cnc, is a Binary star approximately 41 Light-years Gliese 436 is a Red dwarf star approximately 33 Light-years away in the Constellation of Leo. ^[61] These planets have been nicknamed "Neptunes" because they roughly approximate that planet's mass (17 Earths). ^[62] Another new category are the so-called "super-Earths", possibly terrestrial planets far larger than Earth but smaller than Neptune or Uranus. A Super-Earth is an extrasolar Terrestrial planet more massive than the Earth A terrestrial planet, telluric planet or rocky planet is a Planet that is primarily composed of Silicate rocks Within our To date, five possible super-Earths have been found: Gliese 876 d, which is roughly six times Earth's mass,^[63] OGLE-2005-BLG-390Lb and MOA-2007-BLG-192Lb, frigid icy worlds discovered through gravitational microlensing,^[64][65] and two planets orbiting the nearby red dwarf Gliese 581. Gliese 876 d, also cataloged as IL Aquarii d, is an Extrasolar planet approximately 15 Light-years away in the Constellation of Aquarius Gravitational microlensing is an astronomical phenomenon due to the Gravitational lens effect According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool Star, of the Main sequence, either late K Gliese 581 (ˈgliːzə is a Red dwarf Star with spectral type M3V located 20 Gliese 581 d is roughly 7. Gliese 581 d (ˈgliːzə is an Extrasolar planet approximately 20 Light-years away in the Constellation of Libra. 7 times Earth's mass,^[66] while Gliese 581 c is five times Earth's mass and the first terrestrial planet found within a star's habitable zone. Gliese 581 c (ˈgliːzə is a " Super-earth " In Astronomy a habitable zone ( HZ) is a region of space where conditions are favorable for Life as it may be found on Earth. ^[67]

It is far from clear if the newly discovered large planets would resemble the gas giants in the Solar System or if they are of an entirely different type as yet unknown, like ammonia giants or carbon planets. A carbon planet, also referred to as a diamond planet or carbide planet, is a theoretical type of planet proposed by Marc Kuchner that could form In particular, some of the newly-discovered planets, known as hot Jupiters, orbit extremely close to their parent stars, in nearly circular orbits. Hot Jupiters (also called roasters, epistellar jovians, pegasids or pegasean planets) are a class of Extrasolar planets whose They therefore receive much more stellar radiation than the gas giants in the Solar System, which makes it questionable whether they are the same type of planet at all. There may also exist a class of hot Jupiters, called Chthonian planets, that orbit so close to their star that their atmospheres have been blown away completely by stellar radiation. A Chthonian planet (ˈkθoʊniən sometimes misspelled Cthonian) is a hypothetical class of celestial objects resulting from the stripping away of a Gas giant 's While many hot Jupiters have been found in the process of losing their atmospheres, as of 2008, no genuine cthonian planets have been discovered. ^[68]

More detailed observation of extrasolar planets will require a new generation of instruments, including space telescopes. A space observatory is any instrument in Outer space which is used for observation of distant planets galaxies and other outer space objects Currently the COROT spacecraft is searching for stellar luminosity variations due to transiting planets. COROT ( CO nvection RO tation and planetary T ransits is a space mission led by the French Space Agency (CNES in conjunction with the European The term transit or astronomical transit has three meanings in astronomy A transit is the astronomical event that occurs when one Several projects have also been proposed to create an array of space telescopes to search for extrasolar planets with masses comparable to the Earth. A space observatory is any instrument in Outer space which is used for observation of distant planets galaxies and other outer space objects These include the proposed NASA's Kepler Mission, Terrestrial Planet Finder, and Space Interferometry Mission programs, the ESA's Darwin, and the CNES' PEGASE. The Kepler Mission is a Space Photometer being developed by NASA. The Terrestrial Planet Finder ( TPF) was a plan by the National Aeronautics and Space Administration of the United States for a Telescope The Space Interferometry Mission, also known as SIM PlanetQuest, is a planned space telescope being developed by the U The European Space Agency ( ESA) established in 1975 is an intergovernmental organisation dedicated to the exploration of space, currently with 17 member The Darwin Mission is a proposed European Space Agency (ESA program designed to directly detect Earth -like planets orbiting nearby stars, and search for PEGASE is a proposed space mission to build a double-aperture Interferometer composed of three free-flying Satellites The goal of the mission is the study of ^[69] The New Worlds Mission is an occulting device that may work in conjunction with the James Webb Space Telescope. The New Worlds Mission is a project funded by NASA Institute for Advanced Concepts (NIAC headed by Dr The James Webb Space Telescope ( JWST) is a planned space Infrared observatory the successor to the aging Hubble Space Telescope. However, funding for some of these projects remains uncertain. The first spectra of extrasolar planets were reported in February 2007 (HD 209458 b and HD 189733 b). HD 209458 b, also unofficially named Osiris, is an Extrasolar planet that orbits the Solar twin Star HD 209458 in the constellation HD 189733 b is an Extrasolar planet approximately 63 Light-years away in the Constellation of Vulpecula (the Fox) ^[70][71] The frequency of occurrence of such terrestrial planets is one of the variables in the Drake equation which estimates the number of intelligent, communicating civilizations that exist in our galaxy. The Drake equation (also sometimes called the "Green Bank equation" the "Green Bank Formula" or often erroneously labeled the " Sagan equation" Extraterrestrial life is Life originating outside of the Earth. ^[72]

Interstellar "planets"

Main article: Rogue planet

Several computer simulations of stellar and planetary system formation have suggested that some objects of planetary mass would be ejected into interstellar space. A rogue planet is an object which has equivalent mass to a Planet and is not gravitationally bound to any Star, and that therefore moves through space as A computer simulation, a computer model or a computational model is a Computer program, or network of computers that attempts to simulate an A planemo is a celestial object with mass greater than that of an irregularly shaped asteroid yet smaller than a nuclear reactive Brown dwarf or Star. Space is the extent within which Matter is physically extended and objects and Events have positions relative to one another ^[73] Some scientists have argued that such objects found roaming in deep space should be classed as "planets". However, others have suggested that they could be low-mass stars. ^[74] The IAU's working definition on extrasolar planets takes no position on the issue.

In 2005, astronomers announced the discovery of Cha 110913-773444, the smallest brown dwarf found to date, at only seven times Jupiter's mass. Cha 110913-773444 (sometimes abbreviated Cha 110913) is an astronomical object surrounded by what appears to be a Protoplanetary disk. Since it was not found in orbit around a fusing star, it is a sub-brown dwarf according to the IAU's working definition. Sub-brown dwarf is a Planetary mass object whose mass is smaller than the low-mass cut-off for Brown dwarfs (around 13 times the mass of Jupiter) However, some astronomers believe it should be referred to as a planet. ^[74] For a brief time in 2006, astronomers believed they had found a binary system of such objects, Oph 162225-240515, which the discoverers described as "planemos", or "planetary mass objects". Oph 162225-240515, often abbreviated Oph1622, is a pair of Brown dwarfs that have been reported as orbiting each other A planemo is a celestial object with mass greater than that of an irregularly shaped asteroid yet smaller than a nuclear reactive Brown dwarf or Star. However, recent analysis of the objects has determined that their masses are probably each greater than 13 Jupiter-masses, making the pair brown dwarfs. Brown dwarfs are sub- stellar objects with a mass below that necessary to maintain Hydrogen -burning Nuclear fusion reactions in their cores as do stars ^[75][76][77]

Attributes

Although each planet has unique physical characteristics, a number of broad commonalities do exist between them. Some of these characteristics, such as rings or natural satellites, have only as yet been observed in planets in the Solar System, whilst others are also common to extrasolar planets.

Dynamic characteristics

See also: Kepler's laws of planetary motion

Orbit

The orbit of the planet Neptune compared to that of Pluto. In Astronomy, Kepler's Laws of Planetary Motion are three mathematical laws that describe the motion of Planets in the Solar System. Note the elongation of Pluto's orbit in relation to Neptune's (eccentricity), as well as its large angle to the ecliptic (inclination). In Astrodynamics, under standard assumptions, any Orbit must be of Conic section shape Inclination in general is the Angle between a Reference plane and another plane or axis of direction

All planets revolve around stars. In the Solar System, all the planets orbit in the same direction as the Sun rotates. It is not yet known whether all extrasolar planets follow this pattern. The period of one revolution of a planet's orbit is known as its sidereal period or year. The orbital period is the time taken for a given object to make one complete Orbit about another object A year (from Old English gēr) is the time between two recurrences of an event related to the Orbit of the Earth around the Sun ^[78] A planet's year depends on its distance from its star; the farther a planet is from its star, not only the longer the distance it must travel, but also the slower its speed, as it is less affected by the star's gravity. Gravitation is a natural Phenomenon by which objects with Mass attract one another Because no planet's orbit is perfectly circular, the distance of each varies over the course of its year. The closest approach to its star is called its periastron (perihelion in the Solar System), while its farthest separation from the star is called its apastron (aphelion). In Celestial mechanics, an apsis, plural apsides (ˈæpsɨdɪːz is the point of greatest or least distance of the Elliptical orbit of an object from In Celestial mechanics, an apsis, plural apsides (ˈæpsɨdɪːz is the point of greatest or least distance of the Elliptical orbit of an object from In Celestial mechanics, an apsis, plural apsides (ˈæpsɨdɪːz is the point of greatest or least distance of the Elliptical orbit of an object from In Celestial mechanics, an apsis, plural apsides (ˈæpsɨdɪːz is the point of greatest or least distance of the Elliptical orbit of an object from As a planet approaches periastron, its speed increases as the pull of its star's gravity strengthens; as it reaches apastron, its speed decreases. ^[79]

Each planet's orbit is delineated by a set of elements:

• The eccentricity of an orbit describes how elongated a planet's orbit is. The elements of an orbit are the parameters needed to specify that Orbit uniquely given a model of two point-masses obeying the Newtonian laws of motion and the In Astrodynamics, under standard assumptions, any Orbit must be of Conic section shape Planets with low eccentricities have more circular orbits, while planets with high eccentricities have more elliptical orbits. The planets in our Solar System have very low eccentricities, and thus nearly circular orbits. ^[78] Comets and Kuiper belt objects (as well as several extrasolar planets) have very high eccentricities, and thus exceedingly elliptical orbits. ^[80][81]

Illustration of the semi-major axis

• The semi-major axis is the distance from a planet to the half-way point along the longest diameter of its elliptical orbit (see image). In Geometry, the semi-major axis (also semimajor axis) is used to describe the dimensions of ellipses and hyperbolae This distance is not the same as its apasteron, as no planet's orbit has its star at its exact centre. ^[78]

• The inclination of a planet tells how far above or below an established reference plane its orbit lies. Inclination in general is the Angle between a Reference plane and another plane or axis of direction In our Solar System, the reference plane is the plane of Earth's orbit, called the ecliptic. The ecliptic is the apparent path that the Sun traces out in the sky during the year For extrasolar planets, the plane, known as the sky plane or plane of the sky, is the plane of the observer's line of sight from Earth. ^[82] The eight planets of our Solar System all lie very close to the ecliptic; comets and Kuiper belt objects like Pluto are at far more extreme angles to it. The Kuiper belt (ˈkaɪpɚ to rhyme with "viper" sometimes called the Edgeworth-Kuiper belt, is a region of the Solar System beyond the planets extending ^[83] The points at which a planet crosses above and below its reference plane are called its ascending and descending nodes. An orbital node is one of the two points where an Orbit crosses a Plane of reference which it is inclined to An orbital node is one of the two points where an Orbit crosses a Plane of reference which it is inclined to ^[78] The longitude of the ascending node is the angle between the reference plane's 0 longitude and the planet's ascending node. The longitude of the ascending node (☊ or Ω is one of the Orbital elements used to specify the Orbit of an object in space The argument of periapsis (or perihelion in our Solar System) is the angle between a planet's ascending node and its closest approach to its star. The argument of periapsis (or argument of perifocus) ( ω) is the Orbital element describing the Angle of an Orbiting body's periapsis ^[78]

Earth's axial tilt is about 23°.

Axial tilt

Planets also have varying degrees of axial tilt; they lie at an angle to the plane of their stars' equators. In Astronomy, axial tilt is the Inclination angle of a planet's rotational axis in relation to its orbital plane. A term used in Celestial mechanics, the plane of reference is the plane by means of which Orbital elements (positions are defined Inclination in general is the Angle between a Reference plane and another plane or axis of direction This causes the amount of light received by each hemisphere to vary over the course of its year; when the northern hemisphere points away from its star, the southern hemisphere points towards it, and vice versa. Each planet therefore possesses seasons; changes to the climate over the course of its year. A season is one of the major divisions of the Year, generally based on yearly periodic changes in Weather. The point at which each hemisphere is farthest or nearest from its star is known as its solstice. Solstices occur twice a year when the tilt of the Earth's axis is most oriented toward or away from the Sun, causing the Sun to reach its northernmost and southernmost extremes Each planet has two in the course of its orbit; when one hemisphere has its summer solstice, when its day is longest, the other has its winter solstice, when its day is shortest. Jupiter's axial tilt is very small, so its seasonal variation is minimal; Uranus, on the other hand, has an axial tilt so extreme it is virtually on its side, which means that its hemispheres are either perpetually in sunlight or perpetually in darkness around the time of its solstices. ^[84] Among extrasolar planets, axial tilts are not known for certain, though most hot Jupiters are believed to possess negligible to no axial tilt, as a result of their proximity to their stars. ^[85]

Rotation

The planets also rotate around invisible axes through their centres. A planet's rotation period is known as its day. The rotation period of an astronomical object is the time it takes to complete one revolution around its Axis of rotation relative to the background stars A day (symbol d is a unit of Time equivalent to 24 Hours and the duration of a single Rotation of planet Earth with respect to the All planets in the Solar System rotate in a counter-clockwise direction, except for Venus, which rotates clockwise^[86] (Uranus is generally said to be rotating clockwise as well^[87] though because of its extreme axial tilt, it can be said to be rotating either clockwise or anti-clockwise, depending on whether one states it to be inclined 82° from the ecliptic in one direction, or 98° in the opposite direction). Direct motion is the motion of a Planetary body in a direction similar to that of other bodies within its system and is sometimes called prograde motion. ^[88] There is great variation in the length of day between the planets, with Venus taking 243 Earth days to rotate, and the gas giants only a few hours. A day (symbol d is a unit of Time equivalent to 24 Hours and the duration of a single Rotation of planet Earth with respect to the ^[89] The rotational periods of extrasolar planets are not known; however their proximity to their stars means that hot Jupiters are tidaly locked (their orbits are in sync with their rotations). A separate article treats the phenomenon of Tidal resonance in Oceanography. This means they only ever show one face to their stars, with one side in perpetual day, the other in perpetual night. ^[90]

Orbital clearance

The defining dynamic characteristic of a planet is that it has cleared its neighborhood. A planet that has cleared its neighborhood has accumulated enough mass to gather up or sweep away all the planetesimals in its orbit. Planetesimals are solid objects thought to exist in Protoplanetary disks and in Debris disks A widely accepted theory of planet formation the so-called planetesimal In effect, it orbits its star in isolation, as opposed to sharing its orbit with a multitude of similar-sized objects. This characteristic was mandated as part of the IAU's official definition of a planet in August, 2006. This criterion excludes such planetary bodies as Pluto, Eris and Ceres from full-fledged planethood, making them instead dwarf planets. Ceres (ˈsɪəriːz A dwarf planet, as defined by the International Astronomical Union (IAU is a Celestial body Orbiting the Sun that is massive enough to be rounded ^[1] Although to date this criterion only applies to our Solar System, a number of young extrasolar systems have been found in which evidence suggests orbital clearing is taking place within their circumstellar discs. ^[91]

Physical characteristics

Mass

A planet's defining physical characteristic is that it is massive enough for the force of its own gravity to dominate over the electromagnetic forces binding its physical structure, leading to a state of hydrostatic equilibrium. Electromagnetism is the Physics of the Electromagnetic field: a field which exerts a Force on particles that possess the property of Hydrostatic equilibrium occurs when compression due to Gravity is balanced by a Pressure gradient which creates a Pressure gradient force in the opposite This effectively means that all planets are spherical or spheroidal. Up to a certain mass, an object can be irregular in shape, but beyond that point, which varies depending on the chemical makeup of the object, gravity begins to pull an object towards its own centre of mass until the object collapses into a sphere. ^[92]

Mass is also the prime attribute by which planets are distinguished from stars. A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth The upper mass limit for planethood is roughly 13 times Jupiter's mass, beyond which it achieves conditions suitable for nuclear fusion. In Physics and Nuclear chemistry, nuclear fusion is the process by which multiple- like charged atomic nuclei join together to form a heavier nucleus Other than the Sun, no objects of such mass exist in our Solar System; however a number of extrasolar planets lie at that threshold. The Extrasolar Planets Encyclopedia lists several planets that are close to this limit: HD 38529c, AB Pictorisb, HD 162020b, and HD 13189b. HD 38529 (also known as HR 1988) is a Binary star approximately 138 Light-years away in the Constellation of Orion. AB Pictoris, also catalogued as HD 44627, is a K-type Star, approximately 148 Light-years away in the Constellation of Pictor HD 162020 is an Orange dwarf Star in Scorpius constellation with a Brown dwarf companion HD 13189 is a 8th magnitude Star in Triangulum constellation. A number of objects of higher mass are also listed, but since they lie above the fusion threshold, they would be better described as brown dwarfs. Brown dwarfs are sub- stellar objects with a mass below that necessary to maintain Hydrogen -burning Nuclear fusion reactions in their cores as do stars ^[59]

The smallest known planet, excluding dwarf planets and satellites, is PSR B1257+12 a, one of the first extrasolar planets discovered, which was found in 1992 in orbit around a pulsar. PSR B1257+12, sometimes abbreviated as PSR 1257+12, is a Pulsar located 980 Light-years from the Sun. Pulsars are highly magnetized rotating Neutron stars that emit a beam of Electromagnetic radiation in the form of radio waves Its mass is roughly half that of the planet Mercury. ^[59]

Illustration the interior of Jupiter, with a rocky core overlaid by a deep layer of metallic hydrogen

Internal differentiation

Every planet began its existence in an entirely fluid state; in early formation, the denser, heavier materials sank to the centre, leaving the lighter materials near the surface. Each therefore has a differentiated interior consisting of a dense planetary core surrounded by a mantle which either is or was a fluid. In Planetary science, planetary differentiation is the process of separating out different constituents of a planetary body as a consequence of their physical or chemical The planetary core consists of the innermost layer(s of a Planet. The mantle is a part of an Astronomical object. The interior of the Earth, similar to the other Terrestrial planets, is Chemically divided FLUID ( F ast L ight '''U'''ser '''I'''nterface D esigner is a graphical editor that is used to produce FLTK Source code The terrestrial planets are sealed within hard crusts,^[93] but in the gas giants the mantle simply dissolves into the upper cloud layers. In Geology, a crust is the outermost solid shell of a planet or moon The terrestrial planets possess cores of magnetic elements such as iron and nickel, and mantles of silicates. Iron (ˈаɪɚn is a Chemical element with the symbol Fe (ferrum and Atomic number 26 Nickel (ˈnɪkəl is a metallic Chemical element with the symbol Ni and Atomic number 28 For the Artificial intelligence Androids of the 1990s Science fiction series Space Above and Beyond, see Silicate (AI Jupiter and Saturn are believed to possess cores of rock and metal surrounded by mantles of metallic hydrogen. Metallic hydrogen results when Hydrogen is sufficiently Compressed and undergoes a phase change it is an example of Degenerate matter. ^[94] Uranus and Neptune, which are smaller, possess rocky cores surrounded by mantles of water, ammonia, methane and other ices. Neptune ( English|AmE] ] is the eighth and farthest Planet from the Sun in the Solar System. Water is a common Chemical substance that is essential for the survival of all known forms of Life. Ammonia is a compound with the formula N[[hydrogen H3]] It is normally encountered as a Gas with a characteristic pungent Odor Methane is a Chemical compound with the molecular formula. It is the simplest Alkane, and the principal component of Natural gas. Ice is a Solid phase, usually crystalline, of a Non-metalic substance that is liquid or gas at Room temperature, such as Ammonia ^[95] The fluid action within these planets' cores creates a geodynamo that generates a magnetic field. The dynamo theory proposes a mechanism by which a celestial body such as the Earth generates a Magnetic field. In Physics, a magnetic field is a Vector field that permeates space and which can exert a magnetic force on moving Electric charges ^[93]

Atmosphere

See also: Extraterrestrial atmospheres

Earth's atmosphere

All of the Solar System planets have atmospheres as their large masses mean gravity is strong enough to keep gaseous particles close to the surface. The study of extraterrestrial atmospheres is an active field of research both as an aspect of astronomy and to gain insight into Earth's atmosphere An atmosphere (from Greek ατμός - atmos, " Vapor " + σφαίρα - sphaira, " Sphere " The larger gas giants are massive enough to keep large amounts of the light gases hydrogen and helium close by, while the smaller planets lose these gases into space. Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical Space is the extent within which Matter is physically extended and objects and Events have positions relative to one another ^[96] The composition of the Earth's atmosphere is different from the other planets because the various life processes that have transpired on the planet have introduced free molecular oxygen. Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the ^[97] The only solar planet without a true atmosphere is Mercury which had it mostly, although not entirely, blasted away by the solar wind. The solar wind is a Stream of charged particles&mdasha plasma &mdashthat are ejected from the upper atmosphere of the Sun. ^[98]

Planetary atmospheres are affected by the varying degrees of energy received from either the Sun or their interiors, leading to the formation of dynamic weather systems such as hurricanes, (on Earth), planet-wide dust storms (on Mars), an Earth-sized anticyclone on Jupiter (called the Great Red Spot), and holes in the atmosphere (on Neptune). The weather is a set of all the phenomena occurring in a given Atmosphere at a given Time. A tropical cyclone is a storm system characterized by a low pressure center and numerous Thunderstorms that produce strong winds and Flooding An anticyclonic storm is a weather Storm where winds around the storm flow contrary to the direction dictated by the Coriolis effect about a region of low pressure ^[84] At least one extrasolar planet, HD 189733 b, has been claimed to possess such a weather system, similar to the Great Red Spot but twice as large. HD 189733 b is an Extrasolar planet approximately 63 Light-years away in the Constellation of Vulpecula (the Fox) ^[99]

Hot Jupiters have been shown to be losing their atmospheres into space due to stellar radiation, much like the tails of comets. ^[100][101] These planets may have vast differences in temperature between their day and night sides which produce supersonic winds,^[102] although the day and night sides of HD 189733b appear to have very similar temperatures, indicating that that planet's atmosphere effectively redistributes the star's energy around the planet. ^[99]

Magnetosphere

Schematic of Earth's magnetosphere

One important characteristic of the planets is their intrinsic magnetic moments which in turn give rise to magnetospheres. In Physics, Astronomy, Chemistry, and Electrical engineering, the term magnetic moment of a system (such as a loop of Electric current A magnetosphere' is a highly magnetized region around and possessed by an Astronomical object. The presence of a magnetic field indicates that the planet is still geologically alive. In other words, magnetized planets have flows of electrically conducting material in their interiors, which generate their magnetic fields. Electrical conductivity or specific conductivity is a measure of a material's ability to conduct an Electric current. These fields significantly change the interaction of the planet and solar wind. A magnetized planet creates a cavity around itself called magnetosphere, which the solar wind can not penetrate. A magnetosphere' is a highly magnetized region around and possessed by an Astronomical object. The size of the magnetosphere can be much larger than that of the planet itself. In contrast, non-magnetized planets have only small magnetospheres induced by interaction of the ionosphere with the solar wind, which can not effectively protect the planet. The ionosphere is the uppermost part of the atmosphere, distinguished because it is Ionized by solar radiation ^[103]

Of the eight planets in our Solar System, only Venus and Mars lack such a magnetic field. ^[103] In addition, the moon of Jupiter Ganymede also has one. TemplateInfobox Planet.--> Ganymede (ˈgænɨmiːd, or as Greek Of the magnetized planets Mercury has the weakest magnetic field, and is barely enough to deflect the solar wind. The solar wind is a Stream of charged particles&mdasha plasma &mdashthat are ejected from the upper atmosphere of the Sun. Ganymede's magnetic field is several times larger, and Jupiter's is the strongest in the Solar System. The magnetic fields of other giant planets are roughly similar in strength of that of Earth but their magnetic moments are significantly larger than the Earth's magnetic moment. The magnetic fields of Uranus and Neptune are strongly tilted relative the rotational axis and displaced from the centre of the planet. A rotation is a movement of an object in a circular motion A two- Dimensional object rotates around a center (or point) of rotation ^[103]

In 2004, a team of astronomers in Hawaii observed an extrasolar planet around the star HD 179949, which appeared to be creating a sunspot on the surface of its parent star. HD 179949 is a 6th magnitude Star in the Constellation of Sagittarius. The team hypothesised that the planet's magnetosphere was transferring energy onto the star's surface increasing its already high 14,000 degree surface temperature by an additional 750 degrees. ^[104]

The rings of Saturn

Secondary characteristics

Planets in our Solar System possess orbital resonances in their own right. All except Mercury and Venus have natural satellites, often called "moons. A natural satellite or moon is a Celestial body that Orbits a Planet or smaller body which is called the primary. " Earth has one, and Mars has two, and the gas giants have numerous moons in complex planetary systems. A gas giant (sometimes also known as a Jovian planet after the planet Jupiter, or giant planet) is a large Planet that is not primarily Many gas giant moons have similar features to the terrestrial planets and dwarf planets, and some have been studied for signs of life (especially Europa). TemplateInfobox Planet.--> Europa (jʊˈroʊpə; or as ^{[105][106][107]}

The four gas giants are also orbited by planetary rings of varying size and complexity. A planetary ring is a ring of Cosmic dust and other small particles Orbiting around a Planet in a flat disc-shaped region The rings are composed primarily of dust or particulate matter, but can host tiny 'moonlets' whose gravity shapes and maintains their structure. Although the origins of planetary rings is not precisely known, they are believed to be the result of natural satellites that fell below their parent planet's Roche limit and were torn apart by tidal forces. Determining the Roche limit The Roche limit depends on the rigidity of the satellite The tidal force is a secondary effect of the Force of Gravity and is responsible for the Tides It arises because the gravitational acceleration experienced ^[108][109]

No secondary characteristics have been observed around extrasolar planets. However the sub-brown dwarf Cha 110913-773444, which has been described as a rogue planet, is believed to be orbited by a tiny protoplanetary disc. Sub-brown dwarf is a Planetary mass object whose mass is smaller than the low-mass cut-off for Brown dwarfs (around 13 times the mass of Jupiter) Cha 110913-773444 (sometimes abbreviated Cha 110913) is an astronomical object surrounded by what appears to be a Protoplanetary disk. A rogue planet is an object which has equivalent mass to a Planet and is not gravitationally bound to any Star, and that therefore moves through space as A protoplanetary disk (or proplyd) is a rotating Circumstellar disk of dense gas surrounding a young newly formed star a T Tauri star or Herbig star ^[74]

See also

Notes

References

External links

• International Astronomical Union website
• Pictures of the Solar System
• NASA Planet Quest - Exoplanet Exploration
• Illustration comparing the sizes of the planets with each other, the Sun, and other stars
• IAU Press Releases since 1999 "The status of Pluto: A Clarification"
• "Regarding the criteria for planethood and proposed planetary classification schemes." article by Stern and Levinson