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Roman mythology is to thank for the monikers of most of the eight planets in the solar system. The Romans bestowed the names of gods and goddesses on the five planets that could be seen in the night sky with the naked eye. Jupiter, the solar system’s biggest planet, was named for the king of the Roman gods, while the reddish color of the planet Mars led the Romans to name it after their god of war. Mercury, which makes a complete trip around the Sun in just 88 Earth days, is named after the fast-moving messenger of the gods. Saturn, the solar system’s second-largest planet, takes 29 Earth years to make a full revolution of the Sun and is named for the god of agriculture. The Romans named the brightest planet, Venus, for their goddess of love and beauty.
Two other planets, Uranus and Neptune, were discovered after the telescope was invented in the early 1600s. Astronomer William Herschel, who is credited with discovering Uranus in 1781, wanted to call it “Georgium Sidus,” (George’s Star) for the British ruler at the time, King George III. Other astronomers were interested in dubbing the planet Herschel. It was German astronomer Johann Bode who recommended the name Uranus, a Latinized version of the Greek god of the sky, Ouranos; however, the name Uranus didn’t gain full acceptance until the mid-1800s. Neptune, the planet farthest from the Sun (it makes a solar revolution once every 165 years), was first seen by telescope in 1846 by German astronomer Johann Gottfried Galle, using the mathematical calculations of French astronomer Urbain Le Verrier and British astronomer John Couch Adams. There was some discussion of naming the planet after Le Verrier, but ultimately Neptune, which has a vivid blue color, got its name from the Roman god of the sea.
Pluto, which was classified as a planet in 1930 before being stripped of that celestial honor in 2006, was named after the Roman god of the underworld—thanks to the suggestion of an 11-year-old English schoolgirl named Venetia Burney. As for Earth, the planet that’s currently home to an estimated 7.3 billion people, its name comes not from Roman or Greek mythology but rather from Old English and Germanic words meaning “ground.”
Keeping Time: Origins of the Days of the Week
As the days pass, the cycle of the week shapes how we live our lives. Have you ever wondered, "Why is a week seven days long?" How about where the names of each weekday come from?
The seven-day week originates from the calendar of the Babylonians, which in turn is based on a Sumerian calendar dated to 21st-century B.C. Seven days corresponds to the time it takes for a moon to transition between each phase: full, waning half, new and waxing half. Because the moon cycle is 29.53 days long, the Babylonians would insert one or two days into the final week of each month.
Jewish tradition also observes a seven-day week. The book of Genesis (and hence the seven-day account of creation) was likely written around 500 B.C. during the Jewish exile to Babylon. Assyriologists such as Friedrich Delitzsch and Marcello Craveri have suggested that the Jews inherited the cycle of seven days from the Babylonian calendar.
The Romans also inherited this system from Babylonian tradition, though they didn&rsquot begin using it until the instatement of the Julian Calendar in the first-century B.C. Up until this point the Romans had used the &ldquonundinal cycle,&rdquo a system they inherited from the Etruscans. This was a market cycle of eight days labeled A-H. On market day, country folk would come to the city and city dwellers would buy eight days' worth of groceries. By the time the seven-day week was officially adopted by Constantine in A.D. 321, the nundinal cycle had fallen out of use.
The Romans named the days of the week after their gods and corresponded to the five known planets plus the sun and moon (which the Romans also considered planets). To this day, all Romance languages (most familiarly Spanish, French, and Italian) still bear the mark of Roman day names, the exception being Sunday, which now translates to &ldquoLord&rsquos Day&rdquo and Saturday, which translates to "Sabbath."
Born Gustavus Theodore von Holst on 21 September 1874 at 4 Pittville Terrace, Cheltenham, Holst was the elder of two children of Adolph von Holst, a professional musician, and his wife Clara Cox, nee Lediard, the daughter of a respected Cirencester solicitor. Holst came from a musical family – his father taught piano and was organist and choirmaster at All Saints’ Church, Cheltenham and his mother was a talented singer and pianist. His great-grandfather, Matthias Holst, born in Riga, Latvia, served as composer and harp-teacher to the Imperial Russian Court in St Petersburg.
When his mother, Clara, died from heart disease when he was only eight years old, Holst and his younger brother Emil (who later became known as Ernest Cossart a successful actor in the West End, New York and Hollywood) were looked after by their aunt Nina, who alongside their father, taught Holst how to play the piano and compose music. Between 1886 and 1891 he attended Cheltenham Grammar School (now Pate’s Grammar School) and, after having spent four months in Oxford studying counterpoint with George Frederick Sims, organist of Merton College, Holst gave held his first concert at the Montpellier Rotunda in Cheltenham at the age of 21.
In 1893 Holst left Cheltenham for London to study composition at the Royal College of Music. After finishing at the college, Holst discovered that ‘man could not live by composition alone’ and took various organist posts at London churches and became a professional musician, playing the trombone in theatre orchestras. In 1903 Holst decided to abandon orchestral playing to concentrate on composition, however his earnings as a composer were too little to live off and in 1905 he accepted the offer of a teaching post at James Allen’s Girls’ School, Dulwich which he held until 1921. This was followed by the teaching posts for which he is probably best well known – director of music as St Paul’s Girls’ School, Hammersmith, from 1905 until his death and director of music at Morley College from 1907 to 1924. Whilst working as a teacher Holst wrote many pieces of music, including The Planets.
The Planets suite
Disillusioned and depressed by his lack of success as a composer, Holst went on a walking holiday with friends to Spain in the spring of 1913. One member of the group was Clifford Bax, a writer with an interest in the esoteric. According to Bax in his book Ideas and People, it was Bax who first introduced Holst to astrology. As a result Holst became quite a devotee of the subject and became an enthusiastic creator of horoscopes, casting them for his friends for fun.
It was during this time that Holst first started thinking about a piece of music inspired by the planets. The concept of the work is astrological rather than astronomical, with each movement intended to convey the different ‘personalities’ of the planets and the ideas and emotions associated with the influence of the planets on the psyche not the Roman deities.
Originally, Holst composed The Planets on the piano, using a piano in his newly built sound-proofed room in the music wing at St Paul’s Girls School, as well as the piano at home in Thaxted. The piece was composed in a version for four hands, two pianos and was scored by two of his colleagues at St Paul’s, Vally Lasker and Nora Day, who acted as amanuenses. The help of colleagues was necessary due to neuritis pain which Holst frequently suffered in his right hand.
On this score Holst would mark indications of instrumentation in red ink and his amanuenses worked from this to produce the full orchestral score. The exception to this version was the last movement, Neptune, as Holst considered the piano to be unsuitable as it was not mysterious enough for the distant planet. Instead it was scored for a single organ, but the orchestral version has the addition of an offstage choir of women’s voices.
As it was being composed, Vally Lasker and Nora Day played the two piano version to Holst, movement by movement. They continued to play this version in concerts and rehearsals in various parts of the country, and also to visiting conductors, including Adrian Boult. The piece was originally entitled Seven Pieces for Large Orchestra, probably inspired by Schoenberg’s Five Orchestral Pieces, which Holst had attended a performance of in January 1914. However, by the time of the first public performance in 1919 it had been renamed The Planets.
When Holst started composing the suite in 1914, the movements appeared not quite in their final sequence. Begun in May 1914, shortly before the outbreak of World War I, Mars was the first movement to be written and is frequently seen as Holst’s critique of war. This was followed by Venus and Jupiter. Saturn, Uranus and Neptune were all composed in 1915 and Mercury was completed in 1916.
The final suite comprised seven movements, each named after a planet and its corresponding astrological character:
i. Mars, the Bringer of War (1914)
ii. Venus, the Bringer of Peace (1914)
iii. Mercury, the Winged Messenger (1916)
iv. Jupiter, the Bringer of Jollity (1914)
v. Saturn, the Bringer of Old Age (1915)
vi. Uranus, the Magician (1915)
vii. Neptune, the Mystic (1915)
The orchestral score was created after the two piano version by Holst, his amanuenses Vally Lasker, Nora Day and one of his St Paul’s pupils, Jane Joseph, who was the main amanuensis for the orchestral score of Neptune. It was in this form that that The Planets became enormously popular, yet the work was not heard in a complete public performance until some years after it was completed.
First World War
Holst had tried to enlist at the outbreak of the First World War, but was rejected as unfit for military service due to his health problems. However, we wanted to contribute to the war effort and so volunteered to teach music to the troops under the direction of the YMCA. In 1918, just as the war neared its end, he was posted to Salonika in Northern Greece to assume the post of Musical Organiser, helping to organise music activities in military training camps and hospitals. Morley College and St Paul’s Girls’ School both offered him a year’s leave of absence, but one obstacle remained – his name. The YMCA felt that his surname ‘von Holst’ was far too Germanic to be acceptable in such a role, so a prerequisite of him taking up the post was a change of name. He formally changed it by deed poll to the less inflammatory ‘Holst’.
Holst was given a spectacular send-off when his friend, fellow composer, Balfour Gardiner, organised a private performance of The Planets as a farewell gift.
The orchestral premiere of The Planets suite was held at short notice on 29 September 1918 during the last few weeks of World War I, in the Queen’s Hall, London. Conducted at Holst’s request by Adrian Boult, it was a private performance by the Queen’s Hall Orchestra, organised by Balfour Gardiner as a farewell to Holst, before he left England for Salonika to teach music to the troops as part of the war effort. It was hastily rehearsed – it is said that the musicians only saw the complicated music two hours before the performance and the choir for Neptune was recruited from pupils at St Paul’s Girls’ School, where Holst taught. A comparatively intimate affair, it was attended by around 250 invited associates, including conductor Sir Henry Wood and most of the professional musicians in London. Nevertheless, Holst regarded this very first performance as the public premiere, inscribing Boult’s copy of the score, ‘This copy is the property of Adrian Boult who first caused the planets to shine in public and thereby earned the gratitude of Gustav Holst’.
Introducing The Planets to the public
Although there were three further performances between February 1919 and October 1920, they were all incomplete. At a public concert in London on 27 February 1919 under the auspices of the Royal Philharmonic Society and conducted by Boult, five of the seven movements were played. The decision not to play all the movements was made by Boult, who felt that introducing something so new to the public would be more than they could take in. Holst then conducted Venus, Mercury and Jupiter at a Queen’s Hall symphony concert on 22 November 1919 and there was another incomplete performance in Birmingham on 10 October 1920, where five movements were played. The first complete public performance was finally given in London at the Queen’s Hall by the London Symphony Orchestra, conducted by Albert Coates – this was the first time that Neptune had been heard in a public performance, as all the other movements had been given earlier public hearings.
After The Planets
Holst returned to England in June 1919 and resumed his teaching and composing. In addition to his existing wok he accepted a lectureship in composition at the University of Reading and joined his great friend Vaughan Williams in teaching composition at their alma mater the Royal College of Music. Boosted by the international popularity of The Planets, Holst, by now in his forties, suddenly found himself in demand and becoming increasingly famous. The strain caused by the demand on him became too great, and in 1924 he cancelled all professional engagements on doctor’s orders and retreated to Thaxted. In 1925 he resumed his work at St Paul’s, where he continued to pioneer music education for women, but he did not return to any of his other posts.
Holst continued to write and teach music his productivity as a composer almost immediately benefiting once he was released from his other work. His works from this period include the Choral Symphony to words by Keats, a short Shakespearean opera At the Boar’s Head, an orchestral piece called Egdon Heath inspired by Thomas Hardy’s Wessex and Choral Fantasia. However, Holst’s health deteriorated in the last years of his life and he died in 1934 at the age of 59 of heart failure, following an operation on a duodenal ulcer. Vaughan Williams conducted music by Holst and himself at the funeral and Holst’s ashes were then interred at Chichester Cathedral.
The Storied History of the Word 'Planet'
The word"planet" has meant many different things over the millennia and evenstill its definition is evolving.
The word istypically traced back to the ancient Greeks, who believed the Earth wasstationary at the center of the universe while objects in the sky revolvedaround it. The Greek term asters planetai mean "wanderingstars" and described the tiny lights that moved across the sky moredramatically than stars when compared over weeks and months. These wanderingstars, back then, amounted to Mercury, Venus, Mars, Jupiter and Saturn.
Some thinkthe Greeks and Romans of ancient times considered the sun and Earth's moon asplanets. An Elizabethan-era stage play and comedy published in 1597, called"The Woman in the Moon," depicted thesolar system with seven planets, including Saturn, Jupiter, Mars, Mercury,Venus, Sol (the sun) and Luna (the moon).
NicolausCopernicus, in 1543, published his mathematical evidence of a heliocentricuniverse where the six planets revolved around the sun.
Only sixplanets, including Earth, were known until the 18th Century. In 1781, SirWilliam Herschel discovered Uranus in that he determined the point of light wasa planet and not another star as it had been considered until then.
As planetaryscientists and astronomers probe the solar system and beyond, with loads of newdiscoveries, this idea of a planet has changed and along with it celestialbodies either get thrown onto or off the planet list.
For instance,when Plutowas discovered by Clyde Tombaugh, the icy world seemed to be the king ofits neighborhood with no other similar-sized objects in sight. That all changedin 1992 when the first Kuiper Belt Object was found, with currently more than1,000 such icy bodies spotted in a disk-shaped region beyond the orbit ofNeptune, including some around the same size as Pluto. The discovery bringscontext to Pluto, leading some astronomers to contend Pluto looked more like aKuiper Belt Object than a planet.
In 2006, theInternational Astronomical Union (IAU) issued a formal definition of planet,one that led to Pluto's boot from planethood.
The IAUprovided three criteria an object must meet to reachplanet status:
A planet is acelestial body that
1. It orbitsaround the sun.
2. It has?sufficient mass for its self-gravity to overcome rigid body forces so that itassumes a hydrostatic equilibrium (nearly round) shape, and
3. It hascleared the neighborhood around its orbit.
Severalproblems with this definition immediately pop up for astronomers.
Under the IAUcriteria, the more than 300 extrasolar planets identified to date would not beconsidered planets.
"Thereis no acceptable planet definition for exoplanets," said Sara Seager, anastrophysicist at MIT. The current IAU planet definition necessitates a planetmust orbit the sun. Well, an exoplanet, has its own host star and it's not thesun.
Seager joinedother astronomers and planetary scientists last week at the Johns HopkinsUniversity Applied Physics Laboratory (APL) in Laurel, Md., for "The GreatPlanet Debate: Science as Process" conference. The argued about Pluto'sstatus and also discussed worlds beyond our solar system.
The problems,it turns out, are small and big.
Severalobjects not currently called exoplanetssit along the upper-limit mass cutoff of 13 Jupiter masses, beyond whichobjects are typically thought to be a class of failed star called brown dwarf. Butthese borderline objects could go either way, and Seager said a definition mustaccount for them.
The"cleared the neighborhood around its orbit" criterion is also asticky issue. That's because the farther away a planetary object is from itsstar the longer it takes to complete its orbit. So depending on the age of thesystem, that object may not have completed many orbits and thus If Earth werepositioned at a distance of 100 astronomical units (100 times farther than itis now), ?our homebase would not fit the IAU definition of a planet, argueHal Levison of the Southwest Research Institute in Boulder, Colo., and others.
Planetdefinition still evolving
The differentplanet definitions put forth at last week's meeting could leave the solarsystem with as few as eight planets or as manyas 13, with the possibility of many more lurking out there yet to bediscovered.
Two flavorsof definitions include the so-called dynamical definition and the geophysicalone. For the dynamical one, a planet is a planet if it has cleared out itsorbit of rocky litter either by eating up that material, and becoming fatter inthe process, or kicking the junk into other orbits. But that's just asimplistic view. What about Jupiter, which has a slew of captured asteroidsthat orbit the sun in lockstep with the giant planet?
Thegeophysical definition would include as planets objects massive enough for gravityto make them about spherical but not so massive that internal nuclear fusionexists, as is the case with stars.
"You gothrough and look at how the definition [of planet] has evolved over time andthey all have one thing in common. The basic characteristic of a planet is theygo around the sun, historically," Levison said. "This is a dynamicaldefinition. So to say you can't use dynamics, that somehow it's wrong to usedynamics, in order to characterize a planet is historically inaccurate. That'sthe way we've always defined planets."
Mark Sykes,directory of the Planetary Science Institute in Tucson, Ariz., supports ageophysical definition of round objects that orbit a star. The key here is thatonce an object gets that big, important geophysical processes begin. Such anobject is large and round enough that heat can build up in its core to triggergeophysical processes ? akin to volcanicactivity and tectonic movement on Earth ? and a process known asdifferentiation in which the less dense material sinks to the center and thevolatiles float toward the surface.
It's alsoroughly the mass at which atmospheres can form, as gases are gravitationallytrapped around the object's surface. Internal or surface oceans also becomepossible, as the volatiles condense toward the object's surface.
Thegeophysical definition leaves open the planet window for some satellites,including Jupiter'smajor moons: Io, Europa, Ganymede and Callisto. While Io is the mostvolcanically active body in the solar system, Callisto is the solar system'sthird largest satellite and Europa likely has an iron core, mantle and surfaceocean similar to Earth's structure.
"Theseare massive worlds," said William McKinnon of Washington University inSaint Louis. "They are planets in all but name. They just happen to begoing around Jupiter."
But does itmatter what an object is called, at the end of the day?
"There'san implicit hierarchy. If you're a planet, you are first-class, A-list, you getinside the rope to the club and, otherwise you're nothing," McKinnon said."There's got to be some way to communicate that these are worlds in theirown right, as worthy of study as Mars."
Who named the planets? - HISTORY
How do planets and their moons get their names?
The official names of planets and their moons are governed by an organization called the International Astronomical Union (IAU). The IAU was established in 1919. Its mission is "to promote and safeguard the science of astronomy in all its aspects through international cooperation". Its individual members are professional astronomers from all over the World. The IAU is the internationally recognized authority for assigning names to celestial bodies and any surface features on them.
The IAU recognizes that astronomy is an old science and many of its names come from long-standing traditions and/or are founded in history. For many of the names of the objects in the solar system, this is especially so. Most of the objects in our solar system received names long ago based on Greek or Roman mythology. The IAU has therefore adopted this tradition in its rules for naming certain types of objects in the solar system.
With the exception of Earth, all of the planets in our solar system have names from Greek or Roman mythology. This tradition was continued when Uranus, Neptune, and Pluto were discovered in more modern times.
- Mercury is the god of commerce, travel and thievery in Roman mythology. The planet probably received this name because it moves so quickly across the sky.
- Venus is the Roman goddess of love and beauty. The planet is aptly named since it makes a beautiful sight in the sky, with only the Sun and the Moon being brighter.
- Earth is the only planet whose English name does not derive from Greek/Roman mythology. The name derives from Old English and Germanic. There are, of course, many other names for our planet in other languages.
- Mars is the Roman god of War. The planet probably got this name due to its red color.
- Jupiter was the King of the Gods in Roman mythology, making the name a good choice for what is by far the largest planet in our solar system.
- Saturn is the Roman god of agriculture.
- Uranus is the ancient Greek deity of the Heavens, the earliest supreme god.
- Neptune , was the Roman god of the Sea. Given the beautiful blue color of this planet, the name is an excellent choice!
- Pluto is the Roman god of the underworld in Roman mythology. Perhaps the planet received this name because it's so far from the Sun that it is in perpetual darkness.
For those moons have been known for a long time (such as the Galilean moons of Jupiter), the names were assigned from mythological characters. For example, the moons of Jupiter were named for characters who had roles in the life of Zeus (the Greek mythology counterpart of the Roman God Jupiter).
For recently discovered natural satellites of the planets, they are first given a "provisional" or temporary name while additional observations are made to confirm their existence. This temporary name (usually consisting of the year of discovery and some number indicating the order of discovery in that year) is assigned by an organization called the Central Bureau for Astronomical Telegrams (CBAT). For example, when Voyager 2 found a bunch of new moons in its 1989 Neptune encounter, they were named S/1989 N 1, S/1989 N 2, etc. When the existence of the object is confirmed (and its orbit determined), it is given a final name. The name is suggested by the discoverer(s), but following tradition is strongly encouraged.
Note that the moons of Uranus are a special case in our solar system. They are named after literary characters (from works by William Shakespeare and Alexander Pope) rather then characters from mythology.
Landscape features on planets and natural satellites follow a set of complicated conventions set by the IAU Nomenclature Committee. The rules set restrictions on allowable names such as: a planetary feature may not bear the name of a living person or of a political or religious figure from the last 200 years.
Why there are 7 days in a week?
Before Babylonians, Romans used to have a eight day week for market days, named A to H. The Babylonians first started the use of a seven day week in 6th century BC. Since then, it has been the standard time period for most part of the world. Every culture follows the same week method which recycles after seven days. The number seven had a mysterious significance to the Babylonians. They have named the seven days after the seven heavenly planets. They held the seventh day for religious purpose.
Who Named Planet Earth?
Jupiter was the mighty chief of the gods in ancient Rome's pre-Christian religion. It was also said that Rome's legendary founders, Romulus and Remus, were the children of the warlike god Mars, himself Jupiter's son.
Astronomy has always been popular with those who study the capital "C" Classics. Seven out of the eight planets in our solar system were named after Greek or Roman deities. You're living on the only exception to that rule.
The word "earth" has roots in the Old English term "eorþe." Eorþe had multiple meanings like "soil," "dirt," "ground," "dry land" and "country."
Yet the story didn't begin there. Old English is the earliest known phase of what became our modern English tongue. Used until about 1150 C.E., it evolved from a parent language that scholars call "Proto-Germanic."
The German that's spoken today is part of the same linguistic family. "Earth" and "eorþe" are therefore related to the modern German word "Erde." Not only is this the German language's name for our home planet, but it can also be used to refer to dirt and soil.
Our dear Earth has relatives in some other languages, too. For example, there's the Old Saxon "ertha," the Old Frisian "erthe" and the Dutch word "aarde." All these likely descend from a Proto-Germanic term that was never recorded. (As far as we know.)
Nevertheless, linguists have been able to go back and reconstruct this mystery word. Spelled "ertho" in scholarly texts, it's always preceded by an asterisk. This asterisk acknowledges the lack of written confirmation that the word was really used.
Nobody knows when people started using words like "Earth" or "Erde" to refer to the planet as a whole and not just the ground they walked on.
Back in 1783, German astronomer Johann Elert Bode named the seventh planet from our sun "Uranus" (after a Greek god). And though Pluto is no longer considered a planet, we know that 11-year-old Venetia Burney named it in 1930.
But if a single person gave planet Earth its English name — which is unlikely to say the least — his or her identity has been lost to the sands of time.
Still, it's clear that while Mercury, Venus, Mars, Jupiter, Saturn, Uranus and Neptune all started out as the proper names of ancient gods, "Earth" did not. That's why our planet is sometimes called "the earth" with a lowercase "e."
However, according to the University of Oxford Style Guide, the word "Earth" should be capitalized when one is "referring to the name of the planet but not when referring to the ground/soil etc."
What a capital idea!
Urban legend says Venetia Burney named Pluto after the dog from Walt Disney's Mickey Mouse cartoons. Pop culture historians have set the record straight: That cartoon pooch went by "Rover" until 1931, when his name was switched to "Pluto." By then, Burney had already suggested the name "Pluto" for the faraway dwarf planet.
The idea of planets has evolved over its history, from the divine lights of antiquity to the earthly objects of the scientific age. The concept has expanded to include worlds not only in the Solar System, but in hundreds of other extrasolar systems. The ambiguities inherent in defining planets have led to much scientific controversy.
The five classical planets of the Solar System, being visible to the naked eye, have been known since ancient times and have had a significant impact on mythology, religious cosmology, and ancient astronomy. In ancient times, astronomers noted how certain lights moved across the sky, as opposed to the "fixed stars", which maintained a constant relative position in the sky.  Ancient Greeks called these lights πλάνητες ἀστέρες (planētes asteres, "wandering stars") or simply πλανῆται (planētai, "wanderers"),  from which today's word "planet" was derived.    In ancient Greece, China, Babylon, and indeed all pre-modern civilizations,   it was almost universally believed that Earth was the center of the Universe and that all the "planets" circled Earth. The reasons for this perception were that stars and planets appeared to revolve around Earth each day  and the apparently common-sense perceptions that Earth was solid and stable and that it was not moving but at rest.
The first civilization known to have a functional theory of the planets were the Babylonians, who lived in Mesopotamia in the first and second millennia BC. The oldest surviving planetary astronomical text 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 MUL.APIN is a pair of cuneiform tablets dating from the 7th century BC that lays out the motions of the Sun, Moon, and planets over the course of the year.  The Babylonian astrologers also laid the foundations of what would eventually become Western astrology.  The Enuma anu enlil, written during the Neo-Assyrian period in the 7th century BC,  comprises a list of omens and their relationships with various celestial phenomena including the motions of the planets.   Venus, Mercury, and the outer planets Mars, Jupiter, and Saturn were all identified by Babylonian astronomers. These would remain the only known planets until the invention of the telescope in early modern times. 
The ancient Greeks initially did not attach as much significance to the planets as the Babylonians. The Pythagoreans, in the 6th and 5th centuries BC appear to have developed their own independent planetary theory, which consisted of the Earth, Sun, Moon, and planets revolving around a "Central Fire" at the center of the Universe. Pythagoras or Parmenides is said to have been the first to identify the evening star (Hesperos) and morning star (Phosphoros) as one and the same (Aphrodite, Greek corresponding to Latin Venus),  though this had long been known by the Babylonians. In the 3rd century BC, Aristarchus of Samos proposed a heliocentric system, according to which Earth and the planets revolved around the Sun. The geocentric system remained dominant until the Scientific Revolution.
By the 1st century BC, during the Hellenistic period, 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 most 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 CE. 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.   To the Greeks and Romans there were seven known planets, each presumed to be circling Earth according to the complex laws laid out by Ptolemy. They were, in increasing order from Earth (in Ptolemy's order and using modern names): the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn.   
Cicero, in his De Natura Deorum, enumerated the planets known during the 1st century BCE using the names for them in use at the time: 
"But there is most matter for wonder in the movements of the five stars which are falsely called wandering falsely, because nothing wanders which through all eternity preserves its forward and retrograde courses, and its other movements, constant and unaltered. . For instance, the star which is farthest from the earth, which is known as the star of Saturn, and is called by the Greeks Φαίνων (Phainon), accomplishes its course in about thirty years, and though in that course it does much that is wonderful, first preceding the sun, and then falling off in speed, becoming invisible at the hour of evening, and returning to view in the morning, it never through the unending ages of time makes any variation, but performs the same movements at the same times. Beneath it, and nearer to the earth, moves the planet of Jupiter, which is called in Greek Φαέθων (Phaethon) it completes the same round of the twelve signs in twelve years, and performs in its course the same variations as the planet of Saturn. The circle next below it is held by Πυρόεις (Pyroeis), which is called the planet of Mars, and traverses the same round as the two planets above it in four and twenty months, all but, I think, six days. Beneath this is the planet of Mercury, which is called by the Greeks Στίλβων (Stilbon) it traverses the round of the zodiac in about the time of the year's revolution, and never withdraws more than one sign's distance from the sun, moving at one time in advance of it, and at another in its rear. The lowest of the five wandering stars, and the one nearest the earth, is the planet of Venus, which is called Φωσϕόρος (Phosphoros) in Greek, and Lucifer in Latin, when it is preceding the sun, but Ἕσπερος (Hesperos) when it is following it it completes its course in a year, traversing the zodiac both latitudinally and longitudinally, as is also done by the planets above it, and on whichever side of the sun it is, it never departs more than two signs' distance from it."
In 499 CE, the Indian astronomer Aryabhata propounded a planetary model that explicitly incorporated Earth's rotation about its axis, which he explains as the cause of what appears to be an apparent westward motion of the stars. He also believed that the orbits of planets are elliptical.  Aryabhata's followers were particularly strong in South India, where his principles of the diurnal rotation of Earth, among others, were followed and a number of secondary works were based on them. 
In 1500, Nilakantha Somayaji of the Kerala school of astronomy and mathematics, in his Tantrasangraha, revised Aryabhata's model.  In his Aryabhatiyabhasya, a commentary on Aryabhata's Aryabhatiya, he developed a planetary model where Mercury, Venus, Mars, Jupiter and Saturn orbit the Sun, which in turn orbits Earth, similar to the Tychonic system later proposed by Tycho Brahe in the late 16th century. Most astronomers of the Kerala school who followed him accepted his planetary model.  
Medieval Muslim astronomy
In the 11th century, the transit of Venus was observed by Avicenna, who established that Venus was, at least sometimes, below the Sun.  In the 12th century, Ibn Bajjah observed "two planets as black spots on the face of the Sun", which was later identified as a transit of Mercury and Venus by the Maragha astronomer Qotb al-Din Shirazi in the 13th century.  Ibn Bajjah could not have observed a transit of Venus, because none occurred in his lifetime. 
With the advent of the Scientific Revolution, use of the term "planet" changed from something that moved across the sky (in relation to the star field) to a body that orbited Earth (or that was believed to do so at the time) and by the 18th century to something that directly orbited the Sun when the heliocentric model of Copernicus, Galileo and Kepler gained sway.
Thus, Earth became included in the list of planets,  whereas the Sun and Moon were excluded. At first, when the first satellites of Jupiter and Saturn were discovered in the 17th century, the terms "planet" and "satellite" were used interchangeably – although the latter would gradually become more prevalent in the following century.  Until the mid-19th century, the number of "planets" rose rapidly because 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, Juno, and Vesta) were very different from the traditional ones. 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". In the absence of any formal definition, a "planet" came to be understood as any "large" body that orbited the Sun. Because 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. 
In the 20th century, Pluto was discovered. After initial observations led to the belief that it was larger than Earth,  the object was immediately accepted as the ninth planet. Further monitoring found the body was actually much smaller: in 1936, Ray Lyttleton suggested that Pluto may be an escaped satellite of Neptune,  and Fred Whipple suggested in 1964 that Pluto may be a comet.  As it was still larger than all known asteroids and the population of dwarf planets & other trans-Neptunian objects was not well observed,  it kept its status until 2006.
In 1992, astronomers Aleksander Wolszczan and Dale Frail announced the discovery of planets around a pulsar, PSR B1257+12.  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 Geneva Observatory announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi). 
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 generally considered stars due to their ability to fuse deuterium, a heavier isotope of hydrogen. Although objects more massive than 75 times that of Jupiter fuse hydrogen, objects of only 13 Jupiter masses can fuse deuterium. Deuterium is quite rare, and most brown dwarfs would have ceased fusing deuterium long before their discovery, making them effectively indistinguishable from supermassive planets. 
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 were particular disagreements 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.
A growing number of astronomers argued for Pluto to be declassified as a planet, because many similar objects approaching its size had been found in the same region of the Solar System (the Kuiper belt) during the 1990s and early 2000s. Pluto was found to be just one small body in a population of thousands.
Some of them, such as Quaoar, Sedna, and Eris, were heralded in the popular press as the tenth planet, failing to receive widespread scientific recognition. The announcement of Eris in 2005, an object then thought of as 27% more massive than Pluto, created the necessity and public desire for an official definition of a planet.
Acknowledging the problem, the IAU set about creating the definition of planet, and produced one in August 2006. The number of planets dropped to the eight significantly larger bodies that had cleared their orbit (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), and a new class of dwarf planets was created, initially containing three objects (Ceres, Pluto and Eris). 
There is no official definition of extrasolar planets. In 2003, the International Astronomical Union (IAU) Working Group on Extrasolar Planets issued a position statement, but this position statement was never proposed as an official IAU resolution and was never voted on by IAU members. The positions statement incorporates the following guidelines, mostly focused upon the boundary between planets and brown dwarfs: 
- 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  ) 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 the Solar System.
- 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.
- 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).
This working definition was amended by the IAU's Commission F2: Exoplanets and the Solar System in August 2018.  The official working definition of an exoplanet is now as follows:
- Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars, brown dwarfs or stellar remnants and that have a mass ratio with the central object below the L4/L5 instability (M/Mcentral < 2/(25+ √ 621 ) are "planets" (no matter how they formed).
- The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System.
The IAU noted that this definition could be expected to evolve as knowledge improves.
One definition of a sub-brown dwarf is a planet-mass object that formed through cloud collapse rather than accretion. This formation distinction between a sub-brown dwarf and a planet is not universally agreed upon astronomers are divided into two camps as whether to consider the formation process of a planet as part of its division in classification.  One reason for the dissent is that often it may not be possible to determine the formation process. For example, a planet formed by accretion around a star may get ejected from the system to become free-floating, and likewise a sub-brown dwarf that formed on its own in a star cluster through cloud collapse may get captured into orbit around a star.
One study suggests that objects above 10 M Jup formed through gravitational instability and should not be thought of as planets. 
The 13 Jupiter-mass cutoff represents an average mass rather than a precise threshold value. Large objects will fuse most of their deuterium and smaller ones will fuse only a little, and the 13 M J value is somewhere in between. In fact, calculations show that an object fuses 50% of its initial deuterium content when the total mass ranges between 12 and 14 M J.  The amount of deuterium fused depends not only on mass but also on the composition of the object, on the amount of helium and deuterium present.  As of 2011 the Extrasolar Planets Encyclopaedia included objects up to 25 Jupiter masses, saying, "The fact that there is no special feature around 13 M Jup in the observed mass spectrum reinforces the choice to forget this mass limit".  As of 2016 this limit was increased to 60 Jupiter masses  based on a study of mass–density relationships.  The Exoplanet Data Explorer includes objects up to 24 Jupiter masses with the advisory: "The 13 Jupiter-mass distinction by the IAU Working Group is physically unmotivated for planets with rocky cores, and observationally problematic due to the sin i ambiguity."  The NASA Exoplanet Archive includes objects with a mass (or minimum mass) equal to or less than 30 Jupiter masses. 
Another criterion for separating planets and brown dwarfs, rather than deuterium fusion, formation process or location, is whether the core pressure is dominated by coulomb pressure or electron degeneracy pressure.  
2006 IAU definition of planet
The matter of the lower limit was addressed during the 2006 meeting of the IAU's General Assembly. After much debate and one failed proposal, a large majority of those remaining at the meeting voted to pass a resolution. The 2006 resolution defines planets within the Solar System as follows: 
Under this definition, the Solar System is considered to have eight planets. Bodies that fulfill the first two conditions but not the third (such as Ceres, Pluto, and Eris) are classified as dwarf planets, provided they are not also natural satellites of other planets. 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.  After much discussion, it was decided via a vote that those bodies should instead be classified as dwarf planets. 
This definition is based in theories of planetary formation, in which planetary embryos initially clear their orbital neighborhood of other smaller objects. As described by astronomer Steven Soter:
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. Minor planets and comets, including KBOs [Kuiper belt objects], differ from planets in that they can collide with each other and with planets. 
The 2006 IAU definition presents some challenges for exoplanets because the language is specific to the Solar System and because the criteria of roundness and orbital zone clearance are not presently observable.
Astronomer Jean-Luc Margot proposed a mathematical criterion that determines whether an object can clear its orbit during the lifetime of its host star, based on the mass of the planet, its semimajor axis, and the mass of its host star.   The formula produces a value [f] called π that is greater than 1 for planets. The eight known planets and all known exoplanets have π values above 100, while Ceres, Pluto, and Eris have π values of 0.1, or less. Objects with π values of 1 or more are also expected to be approximately spherical, so that objects that fulfill the orbital zone clearance requirement automatically fulfill the roundness requirement. 
Objects formerly considered planets
The table below lists Solar System bodies once considered to be planets but no longer considered as such by the IAU, as well as whether they would be considered planets under alternative definitions, such as Soter's 2006 definition  that favors dynamical dominance or Stern's 2002  and 2017 definitions  that favor hydrostatic equilibrium.
Ceres was subsequently classified by the IAU as a dwarf planet in 2006.
The reporting of newly discovered large Kuiper belt objects as planets – particularly Eris – triggered the August 2006 IAU decision on what a planet is.
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. In ancient Greece, the two great luminaries the Sun and the Moon were called Helios and Selene, two ancient Titanic deities the slowest planet (Saturn) was called Phainon, the shiner followed by Phaethon (Jupiter), "bright" the red planet (Mars) was known as Pyroeis, the "fiery" the brightest (Venus) was known as Phosphoros, the light bringer and the fleeting final planet (Mercury) was called Stilbon, the gleamer. The Greeks also assigned each planet to one among their pantheon of gods, the Olympians and the earlier Titans:
- and Selene were the names of both planets and gods, both of them Titans (later supplanted by OlympiansApollo and Artemis)
- Phainon was sacred to Cronus, the Titan who fathered the Olympians
- Phaethon was sacred to Zeus, Cronus's son who deposed him as king
- Pyroeis was given to Ares, son of Zeus and god of war
- Phosphoros was ruled by Aphrodite, the goddess of love and
- Stilbon with its speedy motion, was ruled over by Hermes, messenger of the gods and god of learning and wit. 
The Greek practice of grafting their gods' names onto the planets was almost certainly borrowed from the Babylonians. The Babylonians named Phosphoros [Venus] after their goddess of love, Ishtar Pyroeis [Mars] after their god of war, Nergal, Stilbon [Saturn] after their god of wisdom Nabu, and Phaethon [Jupiter] after their chief god, Marduk.  There are too many concordances between Greek and Babylonian naming conventions for them to have arisen separately.  The translation was not perfect. For instance, the Babylonian Nergal was a god of war, and thus the Greeks identified him with Ares. Unlike Ares, Nergal was also god of pestilence and the underworld. 
Today, most people in the western world know the planets by names derived from the Olympian pantheon of gods. Although 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 (Latin) names rather than the Greek ones. 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: Mercurius (for Hermes), Venus (Aphrodite), Mars (Ares), Iuppiter (Zeus) and Saturnus (Cronus). When subsequent planets were discovered in the 18th and 19th centuries, the naming practice was retained with Neptūnus (Poseidon). Uranus is unique in that it is named for a Greek deity rather than his Roman counterpart.
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. The order of shifts went Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon (from the farthest to the closest planet).  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. Because 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 after the Nundinal cycle was rejected – and still preserved in many modern languages.  In English, Saturday, Sunday, and Monday are straightforward translations of these Roman names. The other days were renamed after Tīw (Tuesday), Wōden (Wednesday), Þunor (Thursday), and Frīġ (Friday), the Anglo-Saxon gods considered similar or equivalent to Mars, Mercury, Jupiter, and Venus, respectively.
Earth is the only planet whose name in English is not derived from Greco-Roman mythology. Because it was only generally accepted as a planet in the 17th century,  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 generally considered planets.) The name originates from the Old English word eorþe, which was the word for "ground" and "dirt" as well as the Earth itself.  As with its equivalents in the other Germanic languages, it derives ultimately from the Proto-Germanic word erþō, as can be seen in the English earth, the German Erde, the Dutch aarde, and the Scandinavian jord. 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 non-Romance languages use their own native words. The Greeks retain their original name, Γή (Ge).
Non-European cultures use other planetary-naming systems. India uses a system based on the Navagraha, which incorporates the seven traditional planets (Surya for the Sun, Chandra for the Moon, Budha for Mercury, Shukra for Venus, Mangala for Mars, Bṛhaspati for Jupiter, and Shani for Saturn) and the ascending and descending lunar nodes Rahu and Ketu.
China and the countries of eastern Asia historically subject to Chinese cultural influence (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). 
In traditional Hebrew astronomy, the seven traditional planets have (for the most part) descriptive names – the Sun is חמה Ḥammah or "the hot one," the Moon is לבנה Levanah or "the white one," Venus is כוכב נוגה Kokhav Nogah or "the bright planet," Mercury is כוכב Kokhav or "the planet" (given its lack of distinguishing features), Mars is מאדים Ma'adim or "the red one," and Saturn is שבתאי Shabbatai or "the resting one" (in reference to its slow movement compared to the other visible planets).  The odd one out is Jupiter, called צדק Tzedeq or "justice". Steiglitz suggests that this may be a euphemism for the original name of כוכב בעל Kokhav Ba'al or "Baal's planet", seen as idolatrous and euphemized in a similar manner to Ishbosheth from II Samuel. 
In Arabic, Mercury is عُطَارِد (ʿUṭārid, cognate with Ishtar / Astarte), Venus is الزهرة (az-Zuhara, "the bright one",  an epithet of the goddess Al-'Uzzá  ), Earth is الأرض (al-ʾArḍ, from the same root as eretz), Mars is اَلْمِرِّيخ (al-Mirrīkh, meaning "featherless arrow" due to its retrograde motion  ), Jupiter is المشتري (al-Muštarī, "the reliable one", from Akkadian  ) and Saturn is زُحَل (Zuḥal, "withdrawer"  ).  
It is not known with certainty how planets are formed. The prevailing theory is that they are formed during the collapse of a nebula into a thin disk of gas and dust. A protostar forms at the core, surrounded by a rotating protoplanetary disk. Through accretion (a process of sticky collision) dust particles in the disk steadily accumulate mass to form ever-larger bodies. Local concentrations of mass known as planetesimals form, and these accelerate the accretion process by drawing in additional material by their gravitational attraction. These concentrations become ever denser until they collapse inward under gravity to form protoplanets.  After a planet reaches a mass somewhat larger than Mars' mass, it begins to accumulate an extended atmosphere,  greatly increasing the capture rate of the planetesimals by means of atmospheric drag.   Depending on the accretion history of solids and gas, a giant planet, an ice giant, or a terrestrial planet may result.   
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.   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.  Those objects that have become massive enough will capture most matter in their orbital neighbourhoods to become planets. 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 bodies.
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. The interior of the planet begins to differentiate by mass, developing a denser core.  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.  (Smaller planets will lose any atmosphere they gain through various escape mechanisms.)
With the discovery and observation of planetary systems around stars other than the Sun, 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 thought to determine the likelihood that a star will have planets.  Hence, it is thought that a metal-rich population I star will likely have a more substantial planetary system than a metal-poor, population II star.
According to the IAU definition, there are eight planets in the Solar System, which are in increasing distance from the Sun:
Jupiter is the largest, at 318 Earth masses, whereas Mercury is the smallest, at 0.055 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. At 0.055 Earth masses, Mercury is the smallest terrestrial planet (and smallest planet) in the Solar System. Earth is the largest terrestrial planet.
- Giant planets (Jovians): Massive planets significantly more massive than the terrestrials: Jupiter, Saturn, Uranus, Neptune.
- Gas giants, Jupiter and Saturn, are giant planets primarily composed of hydrogen and helium and are the most massive planets in the Solar System. Jupiter, at 318 Earth masses, is the largest planet in the Solar System, and Saturn is one third as massive, at 95 Earth masses.
- Ice giants, Uranus and Neptune, are primarily composed of low-boiling-point materials such as water, methane, and ammonia, with thick atmospheres of hydrogen and helium. They have a significantly lower mass than the gas giants (only 14 and 17 Earth masses).
The number of geophysical planets in the Solar System is unknown - previously considered to be potentially in the hundreds, but now only estimated at only the low double digits. 
(a) Find absolute values in article Earth
An exoplanet (extrasolar planet) is a planet outside the Solar System. As of 22 June 2021, there are 4,768 confirmed exoplanets in 3,527 planetary systems, with 783 systems having more than one planet.    
In early 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12.  This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of giant planets that survived the supernova and then decayed into their current orbits.
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. From then until the Kepler mission most known extrasolar planets were gas giants comparable in mass to Jupiter or larger as they were more easily detected. The catalog of Kepler candidate planets consists mostly of planets the size of Neptune and smaller, down to smaller than Mercury.
There are types of planets that do not exist in the Solar System: super-Earths and mini-Neptunes, which could be rocky like Earth or a mixture of volatiles and gas like Neptune—a radius of 1.75 times that of Earth is a possible dividing line between the two types of planet.  There are hot Jupiters that orbit very close to their star and may evaporate to become chthonian planets, which are the leftover cores. Another possible type of planet is carbon planets, which form in systems with a higher proportion of carbon than in the Solar System.
A 2012 study, analyzing gravitational microlensing data, estimates an average of at least 1.6 bound planets for every star in the Milky Way. 
Around 1 in 5 Sun-like stars have an "Earth-sized" [d] planet in the habitable [e] zone, so the nearest would be expected to be within 12 light-years distance from Earth.   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 the Milky Way. 
There are exoplanets that are much closer to their parent star than any planet in the Solar System is to the Sun, and there are also exoplanets that are much farther from their star. Mercury, the closest planet to the Sun at 0.4 AU, takes 88 days for an orbit, but the shortest known orbits for exoplanets take only a few hours, see Ultra-short period planet. The Kepler-11 system has five of its planets in shorter orbits than Mercury's, all of them much more massive than Mercury. Neptune is 30 AU from the Sun and takes 165 years to orbit, but there are exoplanets that are hundreds of AU from their star and take more than a thousand years to orbit, e.g. 1RXS1609 b.
A planetary-mass object (PMO), planemo,  or planetary body is a celestial object with a mass that falls within the range of the definition of a planet: massive enough to achieve hydrostatic equilibrium (to be rounded under its own gravity), but not enough to sustain core fusion like a star.   By definition, all planets are planetary-mass objects, but the purpose of this term is to refer to objects that do not conform to typical expectations for a planet. These include dwarf planets, which are rounded by their own gravity but not massive enough to clear their own orbit, planetary-mass moons, and free-floating planemos, which may have been ejected from a system (rogue planets) or formed through cloud-collapse rather than accretion (sometimes called sub-brown dwarfs).
A dwarf planet is a planetary-mass object that is neither a true planet nor a natural satellite it is in direct orbit of a star, and is massive enough for its gravity to compress it into a hydrostatically equilibrious shape (usually a spheroid), but has not cleared the neighborhood of other material around its orbit. Planetary scientist and New Horizons principal investigator Alan Stern, who proposed the term 'dwarf planet', has argued that location should not matter and that only geophysical attributes should be taken into account, and that dwarf planets are thus a subtype of planet. The IAU accepted the term (rather than the more neutral 'planetoid') but decided to classify dwarf planets as a separate category of object. 
Several computer simulations of stellar and planetary system formation have suggested that some objects of planetary mass would be ejected into interstellar space.  Such objects are typically called rogue planets.
Stars form via the gravitational collapse of gas clouds, but smaller objects can also form via cloud-collapse. Planetary-mass objects formed this way are sometimes called sub-brown dwarfs. Sub-brown dwarfs may be free-floating such as Cha 110913-773444  and OTS 44,  or orbiting a larger object such as 2MASS J04414489+2301513.
Binary systems of sub-brown dwarfs are theoretically possible Oph 162225-240515 was initially thought to be a binary system of a brown dwarf of 14 Jupiter masses and a sub-brown dwarf of 7 Jupiter masses, but further observations revised the estimated masses upwards to greater than 13 Jupiter masses, making them brown dwarfs according to the IAU working definitions.   
In close binary star systems one of the stars can lose mass to a heavier companion. Accretion-powered pulsars may drive mass loss. The shrinking star can then become a planetary-mass object. An example is a Jupiter-mass object orbiting the pulsar PSR J1719-1438.  These shrunken white dwarfs may become a helium planet or carbon planet.
Some large satellites (moons) are of similar size or larger than the planet Mercury, e.g. Jupiter's Galilean moons and Titan. Proponents of the geophysical definition of planets argue that location should not matter and that only geophysical attributes should be taken into account in the definition of a planet. Alan Stern proposes the term satellite planet for a planet-sized satellite. 
Rogue planets in stellar clusters have similar velocities to the stars and so can be recaptured. They are typically captured into wide orbits between 100 and 10 5 AU. The capture efficiency decreases with increasing cluster volume, and for a given cluster size it increases with the host/primary mass. It is almost independent of the planetary mass. Single and multiple planets could be captured into arbitrary unaligned orbits, non-coplanar with each other or with the stellar host spin, or pre-existing planetary system. 
Although each planet has unique physical characteristics, a number of broad commonalities do exist among them. Some of these characteristics, such as rings or natural satellites, have only as yet been observed in planets in the Solar System, whereas others are also commonly observed in extrasolar planets.
According to current definitions, all planets must revolve around stars thus, any potential "rogue planets" are excluded. In the Solar System, all the planets orbit the Sun in the same direction as the Sun rotates (counter-clockwise as seen from above the Sun's north pole). At least one extrasolar planet, WASP-17b, has been found to orbit in the opposite direction to its star's rotation.  The period of one revolution of a planet's orbit is known as its sidereal period or year.  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, because it is less affected by its star's gravity. No planet's orbit is perfectly circular, and hence 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), whereas its farthest separation from the star is called its apastron (aphelion). As a planet approaches periastron, its speed increases as it trades gravitational potential energy for kinetic energy, just as a falling object on Earth accelerates as it falls as the planet reaches apastron, its speed decreases, just as an object thrown upwards on Earth slows down as it reaches the apex of its trajectory. 
Each planet's orbit is delineated by a set of elements:
- The eccentricity of an orbit describes how elongated a planet's orbit is. Planets with low eccentricities have more circular orbits, whereas planets with high eccentricities have more elliptical orbits. The planets in the Solar System have very low eccentricities, and thus nearly circular orbits.  Comets and Kuiper belt objects (as well as several extrasolar planets) have very high eccentricities, and thus exceedingly elliptical orbits. 
Planets also have varying degrees of axial tilt they lie at an angle to the plane of their stars' equators. 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 has seasons, changes to the climate over the course of its year. The time at which each hemisphere points farthest or nearest from its star is known as its solstice. 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. The varying amount of light and heat received by each hemisphere creates annual changes in weather patterns for each half of the planet. 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.  Among extrasolar planets, axial tilts are not known for certain, though most hot Jupiters are believed to have negligible to no axial tilt as a result of their proximity to their stars. 
The planets rotate around invisible axes through their centres. A planet's rotation period is known as a stellar day. Most of the planets in the Solar System rotate in the same direction as they orbit the Sun, which is counter-clockwise as seen from above the Sun's north pole, the exceptions being Venus  and Uranus,  which rotate clockwise, though Uranus's extreme axial tilt means there are differing conventions on which of its poles is "north", and therefore whether it is rotating clockwise or anti-clockwise.  Regardless of which convention is used, Uranus has a retrograde rotation relative to its orbit.
The rotation of a planet can be induced by several factors during formation. A net angular momentum can be induced by the individual angular momentum contributions of accreted objects. The accretion of gas by the giant planets can also contribute to the angular momentum. Finally, during the last stages of planet building, a stochastic process of protoplanetary accretion can randomly alter the spin axis of the planet.  There is great variation in the length of day between the planets, with Venus taking 243 days to rotate, and the giant planets only a few hours.  The rotational periods of extrasolar planets are not known. However, for "hot" Jupiters, their proximity to their stars means that they are tidally locked (i.e., their orbits are in sync with their rotations). This means, they always show one face to their stars, with one side in perpetual day, the other in perpetual night. 
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. 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.  Although to date this criterion only applies to the Solar System, a number of young extrasolar systems have been found in which evidence suggests orbital clearing is taking place within their circumstellar discs. 
Size and shape
A planet's size is defined at least by an average radius (e.g., Earth radius, Jupiter radius, etc.) polar and equatorial radii of a spheroid or more general triaxial ellipsoidal shapes are often estimated (e.g., reference ellipsoid). Derived quantities include the flattening, surface area, and volume. Knowing further the rotation rate and mass, allows the calculation of normal gravity.
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. 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. 
Mass is also the prime attribute by which planets are distinguished from stars. The upper mass limit for planethood is roughly 13 times Jupiter's mass for objects with solar-type isotopic abundance, beyond which it achieves conditions suitable for nuclear fusion. Other than the Sun, no objects of such mass exist in the Solar System but there are exoplanets of this size. The 13-Jupiter-mass limit is not universally agreed upon and the Extrasolar Planets Encyclopaedia includes objects up to 60 Jupiter masses,  and the Exoplanet Data Explorer up to 24 Jupiter masses. 
The smallest known planet is PSR B1257+12A, one of the first extrasolar planets discovered, which was found in 1992 in orbit around a pulsar. Its mass is roughly half that of the planet Mercury.  The smallest known planet orbiting a main-sequence star other than the Sun is Kepler-37b, with a mass (and radius) slightly higher than that of the Moon.
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 that either is or was a fluid. The terrestrial planets are sealed within hard crusts,  but in the giant planets the mantle simply blends into the upper cloud layers. The terrestrial planets have cores of elements such as iron and nickel, and mantles of silicates. Jupiter and Saturn are believed to have cores of rock and metal surrounded by mantles of metallic hydrogen.  Uranus and Neptune, which are smaller, have rocky cores surrounded by mantles of water, ammonia, methane and other ices.  The fluid action within these planets' cores creates a geodynamo that generates a magnetic field. 
All of the Solar System planets except Mercury  have substantial atmospheres because their gravity is strong enough to keep gases close to the surface. The larger giant planets are massive enough to keep large amounts of the light gases hydrogen and helium, whereas the smaller planets lose these gases into space.  The composition of 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. 
Planetary atmospheres are affected by the varying insolation or internal energy, leading to the formation of dynamic weather systems such as hurricanes, (on Earth), planet-wide dust storms (on Mars), a greater-than-Earth-sized anticyclone on Jupiter (called the Great Red Spot), and holes in the atmosphere (on Neptune).  At least one extrasolar planet, HD 189733 b, has been claimed to have such a weather system, similar to the Great Red Spot but twice as large. 
Hot Jupiters, due to their extreme proximities to their host stars, have been shown to be losing their atmospheres into space due to stellar radiation, much like the tails of comets.   These planets may have vast differences in temperature between their day and night sides that produce supersonic winds,  although the day and night sides of HD 189733 b appear to have very similar temperatures, indicating that that planet's atmosphere effectively redistributes the star's energy around the planet. 
One important characteristic of the planets is their intrinsic magnetic moments, which in turn give rise to magnetospheres. 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. These fields significantly change the interaction of the planet and solar wind. A magnetized planet creates a cavity in the solar wind around itself called the magnetosphere, which the wind cannot penetrate. The magnetosphere can be much larger than the planet itself. In contrast, non-magnetized planets have only small magnetospheres induced by interaction of the ionosphere with the solar wind, which cannot effectively protect the planet. 
Of the eight planets in the Solar System, only Venus and Mars lack such a magnetic field.  In addition, the moon of Jupiter Ganymede also has one. Of the magnetized planets the magnetic field of Mercury is the weakest, and is barely able to deflect the solar wind. Ganymede's magnetic field is several times larger, and Jupiter's is the strongest in the Solar System (so strong in fact that it poses a serious health risk to future manned missions to its moons). The magnetic fields of the other giant planets are roughly similar in strength to that of Earth, but their magnetic moments are significantly larger. The magnetic fields of Uranus and Neptune are strongly tilted relative the rotational axis and displaced from the centre of the planet. 
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. The team hypothesized that the planet's magnetosphere was transferring energy onto the star's surface, increasing its already high 7,760 °C temperature by an additional 400 °C. 
Several planets or dwarf planets in the Solar System (such as Neptune and Pluto) have orbital periods that are in resonance with each other or with smaller bodies (this is also common in satellite systems). All except Mercury and Venus have natural satellites, often called "moons". Earth has one, Mars has two, and the giant planets have numerous moons in complex planetary-type systems. Many moons of the giant planets have features similar to those on the terrestrial planets and dwarf planets, and some have been studied as possible abodes of life (especially Europa).   
The four giant planets are also orbited by planetary rings of varying size and complexity. 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.  
No secondary characteristics have been observed around extrasolar planets. 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  and the sub-brown dwarf OTS 44 was shown to be surrounded by a substantial protoplanetary disk of at least 10 Earth masses. 
- – A binary system where two planetary-mass objects share an orbital axis external to both – Two planetary mass objects orbiting each other – A list of lists of planets sorted by diverse attributes – A celestial body smaller than Mercury but larger than Ceres – astronomical object in direct orbit around the Sun that isn't a planet or a comet – A celestial body smaller than a planet – Extent to which a planet is suitable for life as we know it – A phrase used to remember the names of the planets – Science of astronomical objects apparently in orbit around one or more stellar objects within a few light years – The scientific study of planets – Planet that only appears in works of fiction
- ^ According to the IAU definition of planet.
- ^ This definition is drawn from two separate IAU declarations a formal definition agreed by the IAU in 2006 (IAU Resolution 5A), and an informal working definition proposed in a position statement by an IAU Working Group in 2001/2003 for objects outside of the Solar System (no corresponding IAU resolution). The official 2006 definition applies only to the Solar System, whereas the 2003 working definition applies to planets around other stars. The extrasolar planet issue was deemed too complex to resolve at the 2006 IAU conference.
- ^ Data for G-type stars like the Sun is not available. This statistic is an extrapolation from data on K-type stars.
- ^ ab For the purpose of this 1 in 5 statistic, Earth-sized means 1–2 Earth radii
- ^ ab For the purpose of this 1 in 5 statistic, "habitable zone" means the region with 0.25 to 4 times Earth's stellar flux (corresponding to 0.5–2 AU for the Sun).
- ^Margot's parameter  is not to be confused with the famous mathematical constant π ≈3.14159265 . .
- ^ The Sun is excluded from Soter's planet definition because it is formed by core accretion from an interstellar cloud, not by secondary accretion from a disk.
- ^ The Sun is in hydrostatic equilibrium, but is excluded from Stern's planet definition because it generates energy in its interior with a self-sustaining nuclear fusion chain reaction.
- ^ Referred to by Huygens as a Planetes novus ("new planet") in his Systema Saturnium
- ^ ab Both labelled nouvelles planètes (new planets) by Cassini in his Découverte de deux nouvelles planetes autour de Saturne
- ^ ab Both once referred to as "planets" by Cassini in his An Extract of the Journal Des Scavans.. The term "satellite" had already begun to be used to distinguish such bodies from those around which they orbited ("primary planets").
- ^ Both Titania, Oberon were labelled "secondary planets" by Herschel in his 1787 account of their discovery. 
- ^ ab Measured relative to Earth.
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How the PlanetsGot Their Names
All of the planets, except for Earth, were named after Roman gods and goddesses.
Jupiter, Saturn, Mars, Venus and Mercury were given their names thousands of years ago. Those were the planets that the ancient Romans could see in the sky without a telescope.
The other planets in our solar system were not discovered until much later, when telescopes were invented. Even then, the tradition of naming the planets after Roman gods and goddesses continued.
Most of the moons and some asteroids are also named after the critters and creatures and gods and goddesses found in Roman mythology. Some of the constellations in our solar system are named after Roman gods as well.
MERCURY Roman Winged Messenger, winged god of travel because he moves so fast
VENUS Roman Goddess of Love, beautiful
JUPITER Chief Roman God (Jupiter is King of the Gods, an elected position)
SATURN Former Roman God of Agriculture, retired. Replaced by his daughter, Ceres
URANUS Former Roman God of the Sky, retired. Replaced by his grandson, Jupiter.
NEPTUNE Roman Lord of the Sea
PLUTO Roman Lord of the Underworld (Pluto is no longer considered a planet.)
Remembering the 8 planets:
My Very Excellent Mother Just Served Us Nachos
Once upon a time, remembering the 9 planets:
My Very Excellent Mother Just Served Us Nine Pizzas
Name of Planet Earth
Before exploring the origin of the name “earth,” it is crucial to take of the fact that every language has a name for planet earth. In Portuguese, the earth is known as “terra,” the Germans call it “erde,” "aarde" by the Dutch, and “dünya” in Turkey. For all the languages with different names, there is a history that explains the reason why the name was picked. Interestingly, all the names that earth has in the different languages all seem to be pointing towards the ground or the soil.
Dear Science: How did the planets get their names?
For as long as there have been lights in the night sky, humans have been coming up with names for them. Sumerian astronomers named the sun, moon and five visible planets (Mercury, Venus, Mars, Jupiter and Saturn) after their great gods. In ancient China, planetary nomenclature was based on things in nature — water, fire, wood. The English names for planets mostly come from the Romans, who borrowed their designations from gods and goddesses: Mercury was named for the messenger god because it appears to move so swiftly across the sky, Jupiter shares a title with the king of the gods because it's the solar system's giant, and so on.
Fainter and more distant celestial bodies, which can't be seen with a naked eye, generally got their titles from the people who found them. In keeping with historic trends, those scientists typically opted for names of ancient Greek and Roman gods. The English astronomer William Herschel supposedly wanted to name Uranus “Georgian Sidus” after King George III but was unsuccessful. Pluto was christened by 11-year-old Venetia Burney, a schoolgirl who suggested the name to her well-connected grandfather, who then got it approved by researchers at the Arizona observatory where the (now dwarf) planet was discovered.
We take these names for granted today, because English has become the international language of science. Most scientific journals are published in English. The International Astronomical Union (IAU), the organization responsible for assigning designations to celestial bodies, is based in France but does its business in English. When it was established, the IAU pretty much adopted all of the English designations for objects in our solar system.
These days the IAU has rules for naming new celestial bodies, which allow for a bit more creativity than the parameters of the major planetary designations. After all, there's a limited number of Greek and Roman gods, and we're finding more and more stuff in space every day.
Broadly, planetary nomenclature reflects the identity of the planet in question: features on Venus (named for the Roman goddess of love) are all named after women features on the Martian moon Deimos (which is itself named for the Greek god of terror) get their designations from authors who wrote about Mars. Some of the naming schemes are whimsical: craters on the asteroid Gaspra are named after spas of the world. Others are nerdy: clusters of hills or knobs on the Saturnian moon Titan are named after residents of Middle Earth.
These days, researchers on a specific mission — say, one of the Mars rovers — will compose lists of possible names that they can pull from as they discover new mountains, craters, ridges, etc. These informal names are used for initial exploration and research, then submitted to the IAU for ultimate approval. For classes of features that don't have an IAU naming scheme, scientists are free to indulge their wackiest impulses. When scientists on the Spirit Mars rover mission had to come up with classification system for soil types, they used flavors of ice cream now the Martian landscape is littered with rocks called “Cookies and Cream” and “Mudpie." (It's worth noting that the IAU does not approve those kinds of names — they're just for informal use).
If you are hoping to get something in space — perhaps a nice asteroid, or a crater on a minor moon — named after yourself, you are out of luck. It's extremely uncommon to name celestial bodies after people, and that honor is typically only given to prominent scientists who have passed away.
Watch the video: Who discovered all the planets? (August 2022).