The wordplanet comes from the Greekπλανήται (planḗtai)'wanderers'. Inantiquity, this word referred to theSun,Moon, and five points of light visible to the naked eye that moved across the background of the stars—namely, Mercury, Venus, Mars, Jupiter, and Saturn. Planets have historically had religious associations:multiple cultures identified celestial bodies with gods, and these connections with mythology andfolklore persist in the schemes for naming newly discovered Solar System bodies. Earth itself was recognized as a planet whenheliocentrism supplantedgeocentrism during the 16th and 17th centuries.
With the development of thetelescope, the meaning ofplanet broadened to include objects only visible with assistance: themoons of the planets beyond Earth; theice giants Uranus and Neptune;Ceres and other bodies later recognized to be part of theasteroid belt; andPluto, later found to be the largest member of the collection of icy bodies known as theKuiper belt. The discovery of other large objects in the Kuiper belt, particularlyEris, spurred debate about how exactly to define a planet. In 2006, theInternational Astronomical Union (IAU) adopted a definition of a planet in the Solar System, placing the four terrestrial planets and the four giant planets in the planet category; Ceres, Pluto, and Eris are in the category ofdwarf planet.[2][3][4] Manyplanetary scientists have nonetheless continued to apply the termplanet more broadly, including dwarf planets as well as rounded satellites like the Moon.[5]
Further advances in astronomy led to the discovery of over five thousand planets outside the Solar System, termedexoplanets. These often show unusual features that the Solar System planets do not show, such ashot Jupiters—giant planets that orbit close to their parent stars, like51 Pegasi b—and extremelyeccentric orbits, such asHD 20782 b. The discovery of brown dwarfs and planets larger than Jupiter also spurred debate on the definition, regarding where exactly to draw the line between a planet and a star. Multiple exoplanets have been found to orbit in thehabitable zones of their stars (where liquid water can potentially exist on aplanetary surface), but Earth remains the only planet known to supportlife.
It is not known with certainty how planets are formed. The prevailing theory is that they coalesce during the collapse of anebula into a thin disk of gas and dust. Aprotostar forms at the core, surrounded by a rotatingprotoplanetary disk. Throughaccretion (a process of sticky collision) dust particles in the disk steadily accumulatemass to form ever-larger bodies. Local concentrations of mass known asplanetesimals form, and these accelerate the accretion process by drawing in additional material by their gravitational attraction. These concentrations become increasingly dense until they collapse inward under gravity to formprotoplanets.[6] After a planet reaches a mass somewhat larger than Mars's mass, it begins to accumulate an extendedatmosphere,[7] greatly increasing the capture rate of the planetesimals by means ofatmospheric drag.[8][9] Depending on the accretion history of solids and gas, agiant planet, anice giant, or aterrestrial planet may result.[10][11][12] It is thought that theregular satellites of Jupiter, Saturn, and Uranus formed in a similar way;[13][14] however,Triton was likelycaptured by Neptune,[15] and Earth's Moon[16] and Pluto's Charon might have formed in collisions.[17]
When the protostar has grown such that it ignites to form a star, the surviving disk is removed from the inside outward byphotoevaporation, thesolar wind,Poynting–Robertson drag and other effects.[18][19] Thereafter there still may be many protoplanets orbiting the star or each other, but over time many will collide, either to form a larger, combined protoplanet or release material for other protoplanets to absorb.[20] Those objects that have become massive enough will capture most matter in their orbital neighbourhoods to become planets. Protoplanets that have avoided collisions may becomenatural satellites of planets through a process of gravitational capture, or remain in belts of other objects to become either dwarf planets orsmall bodies.[21][22]
The energetic impacts of the smaller planetesimals (as well asradioactive decay) will heat up the growing planet, causing it to at least partially melt. The interior of the planet begins to differentiate by density, with higher density materials sinking toward thecore.[23] Smaller terrestrial planets lose most of their atmospheres because of this accretion, but the lost gases can be replaced by outgassing from themantle and from the subsequent impact ofcomets[24] (smaller planets will lose any atmosphere they gain through variousescape mechanisms[25]).
With the discovery and observation ofplanetary systems around stars other than the Sun, it is becoming possible to elaborate, revise or even replace this account. The level ofmetallicity—an astronomical term describing the abundance ofchemical elements with anatomic number greater than 2 (helium)—appears to determine the likelihood that a star will have planets.[26][27] Hence, a metal-richpopulation I star is more likely to have a substantial planetary system than a metal-poor,population II star.[28]
According to theIAU definition, there are eight planets in the Solar System, which are (in increasing distance from the Sun):[2] Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Jupiter is the largest, at 318Earth masses, whereas Mercury is the smallest, at 0.055 Earth masses.[29]
The planets of the Solar System can be divided into categories based on their composition.Terrestrials are similar to Earth, with bodies largely composed ofrock and metal: Mercury, Venus, Earth, and Mars. Earth is the largest terrestrial planet.[30]Giant planets are significantly more massive than the terrestrials: Jupiter, Saturn, Uranus, and Neptune.[30] They differ from the terrestrial planets in composition. Thegas giants, Jupiter and Saturn, are primarily composed ofhydrogen and helium and are the most massive planets in the Solar System. Saturn is one third as massive as Jupiter, at 95 Earth masses.[31] Theice giants, Uranus and Neptune, are primarily composed of low-boiling-point materials such as water,methane, andammonia, with thick atmospheres of hydrogen and helium. They have a significantly lower mass than the gas giants (only 14 and 17 Earth masses).[31]
The Sun's, planets', dwarf planets' and moons' size to scale, labelled. Distance of objects is not to scale. The asteroid belt lies between the orbits of Mars and Jupiter, the Kuiper belt lies beyond Neptune's orbit.
Dwarf planets are gravitationally rounded, but have not cleared their orbits of otherbodies. In increasing order of average distance from the Sun, the ones generally agreed among astronomers areCeres,Orcus,Pluto,Haumea,Quaoar,Makemake,Gonggong,Eris, andSedna.[32][33] Ceres is the largest object in theasteroid belt, located between the orbits of Mars and Jupiter. The other eight all orbit beyond Neptune. Orcus, Pluto, Haumea, Quaoar, and Makemake orbit in theKuiper belt, which is a second belt of small Solar System bodies beyond the orbit of Neptune. Gonggong and Eris orbit in thescattered disc, which is somewhat further out and, unlike the Kuiper belt, is unstable towards interactions with Neptune. Sedna is the largest knowndetached object, a population that never comes close enough to the Sun to interact with any of the classical planets; the origins of their orbits are still being debated. All nine are similar to terrestrial planets in having a solid surface, but they are made of ice and rock rather than rock and metal. Moreover, all of them are smaller than Mercury, with Pluto being the largest known dwarf planet and Eris being the most massive.[34][35]
There are at least nineteenplanetary-mass moons or satellite planets—moons large enough to take on ellipsoidal shapes:[4]
The Moon, Io, and Europa have compositions similar to the terrestrial planets; the others are made of ice and rock like the dwarf planets, withTethys being made of almost pure ice. Europa is often considered an icy planet, though, because its surface ice layer makes it difficult to study its interior.[4][36] Ganymede and Titan are larger than Mercury by radius, and Callisto almost equals it, but all three are much less massive. Mimas is the smallest object generally agreed to be ageophysical planet, at about six millionths of Earth's mass, though there are many larger bodies that may not be geophysical planets (e.g.Salacia).[32]
In early 1992, radio astronomersAleksander Wolszczan andDale Frail announced the discovery of two planets orbiting thepulsarPSR 1257+12.[40] This discovery was confirmed and is generally considered to be the first definitive detection of exoplanets. Researchers suspect they formed from a disk remnant left over from thesupernova that produced the pulsar.[41]
In 2011, theKepler space telescope team reported the discovery of the first Earth-sized exoplanets orbiting aSun-like star,Kepler-20e andKepler-20f.[45][46][47] Since that time, more than 100 planets have been identified that are approximately thesame size as Earth, 20 of which orbit in thehabitable zone of their star—the range of orbits where a terrestrial planet could sustain liquid water on its surface, given enough atmospheric pressure.[48][49][50] One in five Sun-like stars is thought to have an Earth-sized planet in its habitable zone, which suggests that the nearest would be expected to be within 12 light-years distance from Earth.[a] The frequency of occurrence of such terrestrial planets is one of the variables in theDrake equation, which estimates the number ofintelligent, communicating civilizations that exist in the Milky Way.[53]
There are types of planets that do not exist in the Solar System:super-Earths andmini-Neptunes, which have masses between that of Earth and Neptune. Objects less than about twice the mass of Earth are expected to be rocky like Earth; beyond that, they become a mixture of volatiles and gas like Neptune.[54] The planetGliese 581c, with a mass 5.5–10.4 times the mass of Earth,[55] attracted attention upon its discovery for potentially being in the habitable zone,[56] though later studies concluded that it is actually too close to its star to be habitable.[57] Planets more massive than Jupiter are also known, extending seamlessly into the realm of brown dwarfs.[58]
Exoplanets have been found that are much closer to their parent star than any planet in the Solar System is to the Sun. Mercury, the closest planet to the Sun at 0.4 AU, takes 88 days for an orbit, butultra-short period planets can orbit in less than a day. TheKepler-11 system has five of its planets in shorter orbits than Mercury's, all of them much more massive than Mercury. There arehot Jupiters, such as 51 Pegasi b,[42] that orbit very close to their star and may evaporate to becomechthonian planets, which are the leftover cores. There are also exoplanets that are much farther from their star. Neptune is 30 AU from the Sun and takes 165 years to orbit, but there are exoplanets that are thousands of AU from their star and take more than a million years to orbit (e.g.COCONUTS-2b).[59]
Attributes
Although each planet has unique physical characteristics, a number of broad commonalities do exist among them. Some of these characteristics, such asrings or natural satellites, have only as yet been observed in planets in the Solar System, whereas others are commonly observed in exoplanets.[60]
The orbit of the planet Neptune compared to that ofPluto. Note the elongation of Pluto's orbit in relation to Neptune's (eccentricity), as well as its large angle to the ecliptic (inclination).
In the Solar System, all the planets orbit the Sun in the same direction as theSun rotates:counter-clockwise as seen from above the Sun's north pole. At least one exoplanet,WASP-17b, has been found to orbit in the opposite direction to its star's rotation.[61] The period of one revolution of a planet's orbit is known as itssidereal period oryear.[62] A planet's year depends on its distance from its star; the farther a planet is from its star, the longer the distance it must travel and the slower its speed, since it is less affected by its star'sgravity.
No planet's orbit is perfectly circular, and hence the distance of each from the host star varies over the course of its year. The closest approach to its star is called itsperiastron, orperihelion in the Solar System, whereas its farthest separation from the star is called itsapastron (aphelion). As a planet approaches periastron, its speed increases as it tradesgravitational potential energy forkinetic energy, just as a falling object on Earth accelerates as it falls. As the planet nears apastron, its speed decreases, just as an object thrown upwards on Earth slows down as it reaches the apex of itstrajectory.[63]
Each planet's orbit is delineated by a set of elements:
Theeccentricity of an orbit describes the elongation of a planet's elliptical (oval) orbit. Planets with low eccentricities have more circular orbits, whereas planets with high eccentricities have more elliptical orbits. The planets and large moons in the Solar System have relatively low eccentricities, and thus nearly circular orbits.[62] The comets and many Kuiper belt objects, as well as several exoplanets, have very high eccentricities, and thus exceedingly elliptical orbits.[64][65]
Thesemi-major axis gives the size of the orbit. It is the distance from the midpoint to the longest diameter of its elliptical orbit. This distance is not the same as its apastron, because no planet's orbit has its star at its exact centre.[62]
Theinclination of a planet tells how far above or below an established reference plane its orbit is tilted. In the Solar System, the reference plane is the plane of Earth's orbit, called theecliptic. For exoplanets, the plane, known as thesky plane orplane of the sky, is the plane perpendicular to the observer's line of sight from Earth.[66] The orbits of the eight major planets of the Solar System all lie very close to the ecliptic; however, some smaller objects like Pallas, Pluto, and Eris orbit at far more extreme angles to it, as do comets.[67] The large moons are generally not very inclined to their parent planets'equators, but Earth's Moon, Saturn's Iapetus, and Neptune's Triton are exceptions. Triton is unique among the large moons in that it orbitsretrograde, i.e. in the direction opposite to its parent planet's rotation.[68]
The points at which a planet crosses above and below its reference plane are called itsascending anddescending nodes.[62] Thelongitude of the ascending node is the angle between the reference plane's 0 longitude and the planet's ascending node. Theargument of periapsis (or perihelion in the Solar System) is the angle between a planet's ascending node and its closest approach to its star.[62]
Earth'saxial tilt is about 23.4°. It oscillates between 22.1° and 24.5° on a 41,000-year cycle and is currently decreasing.
Planets have varying degrees of axial tilt; they spin at an angle to theplane of their stars' equators. This causes the amount of light received by each hemisphere to vary over the course of its year; when theNorthern Hemisphere points away from its star, theSouthern Hemisphere points towards it, and vice versa. Each planet therefore hasseasons, resulting in changes to theclimate over the course of its year. The time at which each hemisphere points farthest or nearest from its star is known as itssolstice. Each planet has two in the course of its orbit; when one hemisphere has its summer solstice with its day being the 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 continually in sunlight or continually in darkness around the time ofits solstices.[69] In the Solar System, Mercury, Venus, Ceres, and Jupiter have very small tilts; Pallas, Uranus, and Pluto have extreme ones; and Earth, Mars, Vesta, Saturn, and Neptune have moderate ones.[70][71][72][73] Among exoplanets, axial tilts are not known for certain, though most hot Jupiters are believed to have a negligible axial tilt as a result of their proximity to their stars.[74] Similarly, the axial tilts of the planetary-mass moons are near zero,[75] with Earth's Moon at 6.687° as the biggest exception;[76] additionally, Callisto's axial tilt varies between 0 and about 2 degrees on timescales of thousands of years.[77]
The planets rotate around invisible axes through their centres. A planet'srotation period is known as astellar 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'snorth pole. The exceptions are Venus[78] and Uranus,[79] 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.[80] Regardless of which convention is used, Uranus has aretrograde rotation relative to its orbit.[79]
Comparison of the rotation period (sped up 10 000 times, negative values denoting retrograde), flattening and axial tilt of the planets and the Moon(SVG animation)
The rotation of a planet can be induced by several factors during formation. A netangular momentum can be induced by the individual angular momentum contributions of accreted objects. The accretion of gas by the giant planets contributes to the angular momentum. Finally, during the last stages of planet building, astochastic process of protoplanetary accretion can randomly alter the spin axis of the planet.[81] 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.[82] The rotational periods of exoplanets are not known, but forhot Jupiters, their proximity to their stars means that they aretidally locked (that is, 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.[83] Mercury and Venus, the closest planets to the Sun, similarly exhibit very slow rotation: Mercury is tidally locked into a 3:2 spin–orbit resonance (rotating three times for every two revolutions around the Sun),[84] and Venus's rotation may be in equilibrium betweentidal forces slowing it down andatmospheric tides created by solar heating speeding it up.[85][86]
All the large moons are tidally locked to their parent planets;[87] Pluto and Charon are tidally locked to each other,[88] as are Eris and Dysnomia,[89] and probablyOrcus and its moonVanth.[90] The other dwarf planets with known rotation periods rotate faster than Earth; Haumea rotates so fast that it has been distorted into atriaxial ellipsoid.[91] The exoplanetTau Boötis b and its parent starTau Boötis appear to be mutually tidally locked.[92][93]
The defining dynamic characteristic of a planet, according to the IAU definition, is that it hascleared its neighborhood. A planet that has cleared its neighborhood has accumulated enough mass to gather up or sweep away all theplanetesimals in its orbit. In effect, it orbits its star in isolation, as opposed to sharing its orbit with a multitude of similar-sized objects. As described above, this characteristic was mandated as part of theIAU's officialdefinition of a planet in August 2006.[2] 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 theircircumstellar discs.[94]
Gravity causes planets to be pulled into a roughly spherical shape, so a planet's size can be expressed roughly by an average radius (for example,Earth radius orJupiter radius). However, planets are not perfectly spherical; for example, theEarth's rotation causes it to be slightly flattened at the poles with abulge around the equator.[95] Therefore, a better approximation of Earth's shape is anoblate spheroid, whose equatorial diameter is 43 kilometers (27 mi) larger than thepole-to-pole diameter.[96] Generally, a planet's shape may be described by giving polar and equatorial radii of aspheroid or specifying areference ellipsoid. From such a specification, the planet's flattening, surface area, and volume can be calculated; itsnormal gravity can be computed knowing its size, shape, rotation rate, and mass.[97]
A planet's defining physical characteristic is that it is massive enough for the force of its own gravity to dominate over theelectromagnetic forces binding its physical structure, leading to a state ofhydrostatic 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.[98]
Mass is the prime attribute by which planets are distinguished from stars. No objects between the masses of the Sun and Jupiter exist in the Solar System, but there are exoplanets of this size. The lowerstellar mass limit is estimated to be around 75 to 80 times that of Jupiter (MJ). Some authors advocate that this be used as the upper limit for planethood, on the grounds that the internal physics of objects does not change between approximately one Saturn mass (beginning of significant self-compression) and the onset of hydrogen burning and becoming ared dwarf star.[54] Beyond roughly 13MJ (at least for objects with solar-typeisotopic abundance), an object achieves conditions suitable fornuclear fusion ofdeuterium: this has sometimes been advocated as a boundary,[99] even though deuterium burning does not last very long and most brown dwarfs have long since finished burning their deuterium.[58] This is not universally agreed upon: theexoplanets Encyclopaedia includes objects up to 60MJ,[100] and theExoplanet Data Explorer up to 24MJ.[101]
The smallest known exoplanet with an accurately known mass isPSR B1257+12A, one of the first exoplanets discovered, which was found in 1992 in orbit around apulsar. Its mass is roughly half that of the planet Mercury.[102] Even smaller isWD 1145+017 b, orbiting a white dwarf; its mass is roughly that of the dwarf planet Haumea, and it is typically termed a minor planet.[103] The smallest known planet orbiting a main-sequence star other than the Sun isKepler-37b, with a mass (and radius) that is probably slightly higher than that of the Moon.[44] The smallest object in the Solar System generally agreed to be a geophysical planet is Saturn's moon Mimas, with a radius about 3.1% of Earth's and a mass about 0.00063% of Earth's.[104] Saturn's smaller moonPhoebe, currently an irregular body of 1.7% Earth's radius[105] and 0.00014% Earth's mass,[104] is thought to have attained hydrostatic equilibrium and differentiation early in its history before being battered out of shape by impacts.[106] Some asteroids may be fragments ofprotoplanets that began to accrete and differentiate, but suffered catastrophic collisions, leaving only a metallic or rocky core today,[107][108][109] or a reaccumulation of the resulting debris.[110]
Illustration of the interior of Jupiter, with a rocky core overlaid by a deep layer of metallic hydrogen
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 adifferentiated interior consisting of a denseplanetary core surrounded by amantle that either is or was afluid. The terrestrial planets' mantles are sealed within hardcrusts,[111] but in the giant planets the mantle simply blends into the upper cloud layers. The terrestrial planets have cores of elements such asiron andnickel and mantles ofsilicates. Jupiter and Saturn are believed to have cores of rock and metal surrounded by mantles ofmetallic hydrogen.[112] Uranus and Neptune, which are smaller, have rocky cores surrounded by mantles of water,ammonia,methane, and otherices.[113] The fluid action within these planets' cores creates ageodynamo that generates amagnetic field.[111] Similar differentiation processes are believed to have occurred on some of the large moons and dwarf planets,[32] though the process may not always have been completed: Ceres, Callisto, and Titan appear to be incompletely differentiated.[114][115] The asteroid Vesta, though not a dwarf planet because it was battered by impacts out of roundness, has a differentiated interior[116] similar to that of Venus, Earth, and Mars.[109]
All of the Solar System planetsexcept Mercury[117] have substantialatmospheres because their gravity is strong enough to keep gases close to the surface. Saturn's largest moonTitan also has a substantial atmosphere thicker than that of Earth;[118] Neptune's largest moonTriton[119] and the dwarf planetPluto have more tenuous atmospheres.[120] 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 intospace.[121] Analysis of exoplanets suggests that the threshold for being able to hold on to these light gases occurs at about2.0+0.7 −0.6M🜨, so that Earth and Venus are near the maximum size for rocky planets.[54]
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 molecularoxygen.[122] The atmospheres of Mars and Venus are both dominated bycarbon dioxide, but differ drastically in density: the average surface pressure ofMars's atmosphere is less than 1% that of Earth's (too low to allow liquid water to exist),[123] while the average surface pressure ofVenus's atmosphere is about 92 times that of Earth's.[124] It is likely that Venus's atmosphere was the result of arunaway greenhouse effect in its history, which today makes it the hottest planet by surface temperature, hotter even than Mercury.[125] Despite hostile surface conditions, temperature, and pressure at about 50–55 km altitude in Venus's atmosphere are close to Earthlike conditions (the only place in the Solar System beyond Earth where this is so), and this region has been suggested as a plausible base for futurehuman exploration.[126] Titan has the onlynitrogen-rich planetary atmosphere in the Solar System other than Earth's. Just as Earth's conditions are close to thetriple point of water, allowing it to exist in all three states on the planet's surface, so Titan's are to the triple point ofmethane.[127]
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.[132][133] These planets may have vast differences in temperature between their day and night sides that produce supersonic winds,[134] although multiple factors are involved and the details of the atmospheric dynamics that affect the day-night temperature difference are complex.[135][136]
One important characteristic of the planets is their intrinsicmagnetic 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 ofelectrically 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 theionosphere with the solar wind, which cannot effectively protect the planet.[137]
Of the eight planets in the Solar System, only Venus and Mars lack such a magnetic field.[137] Of the magnetized planets, the magnetic field of Mercury is the weakest and is barely able to deflect thesolar wind. Jupiter's moonGanymede has a magnetic field several times stronger, and Jupiter's is the strongest in the Solar System (so intense in fact that it poses a serious health risk to future crewed missions to all its moons inward of Callisto[138]). The magnetic fields of the other giant planets, measured at their surfaces, 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 to the planets' rotationalaxes and displaced from the planets' centres.[137]
In 2003, a team of astronomers in Hawaii observing the starHD 179949 detected a bright spot on its surface, apparently created by the magnetosphere of an orbiting hot Jupiter.[139][140]
Several planets or dwarf planets in the Solar System (such as Neptune and Pluto) have orbital periods that are inresonance with each other or with smaller bodies. This is common in satellite systems (e.g. the resonance between Io,Europa, and Ganymede around Jupiter, or between Enceladus and Dione around Saturn). All except Mercury and Venus havenatural satellites, often called "moons". Earth has one, Mars has two, and the giant planets have numerous moons in complex planetary-type systems. Except for Ceres and Sedna, all the consensus dwarf planets are known to have at least one moon as well. 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 and Enceladus).[141][142][143][144][145]
The four giant planets are orbited byplanetary 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 are not precisely known, they are believed to be the result of natural satellites that fell below their parent planets'Roche limits and were torn apart bytidal forces.[146][147] The dwarf planets Haumea[148] and Quaoar also have rings.[149]
No secondary characteristics have been observed around exoplanets. Thesub-brown dwarfCha 110913−773444, which has been described as arogue planet, is believed to be orbited by a tinyprotoplanetary disc,[150] and the sub-brown dwarfOTS 44 was shown to be surrounded by a substantial protoplanetary disk of at least 10 Earth masses.[151]
The idea of planets has evolved over the history of astronomy, 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 multitudes of other extrasolar systems. The consensus as to what counts as a planet, as opposed to other objects, has changed several times. It previously encompassedasteroids,moons, anddwarf planets likePluto,[152][153][154] and there continues to be some disagreement today.[154]
Ancient civilizations and classical planets
The motion of 'lights' moving across the sky is the basis of the classical definition of planets: wandering stars.
The fiveclassical planets of theSolar System, being visible to the naked eye, have been known since ancient times and have had a significant impact onmythology,religious cosmology, and ancientastronomy. 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.[155] Ancient Greeks called these lightsπλάνητεςἀστέρες (planētes asteres)'wandering stars' or simplyπλανῆται (planētai)'wanderers'[156] from which today's word "planet" was derived.[157][158][159] Inancient Greece,China,Babylon, and indeed all pre-modern civilizations,[160][161] it was almost universally believed that Earth was thecenter 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[162] and the apparentlycommon-sense perceptions that Earth was solid and stable and that it was not moving but at rest.[163]
The first civilization known to have a functional theory of the planets were theBabylonians, who lived inMesopotamia in the first and second millennia BC. The oldest surviving planetary astronomical text is the BabylonianVenus 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.[164] TheMUL.APIN is a pair ofcuneiform tablets dating from the 7th century BC that lays out the motions of the Sun, Moon, and planets over the course of the year.[165] Late Babylonian astronomy is the origin of Western astronomy and indeed all Western efforts in theexact sciences.[166] TheEnuma anu enlil, written during theNeo-Assyrian period in the 7th century BC,[167] comprises a list ofomens and their relationships with various celestial phenomena including the motions of the planets.[168][169] Theinferior planetsVenus andMercury and the superior planetsMars,Jupiter, andSaturn were all identified byBabylonian astronomers. These would remain the only known planets until the invention of thetelescope in early modern times.[170]
Theancient Greeks initially did not attach as much significance to the planets as the Babylonians. In the 6th and 5th centuries BC, thePythagoreans appear to have developedtheir 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 orParmenides 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 LatinVenus),[171] though this had long been known in Mesopotamia.[172][173] In the 3rd century BC,Aristarchus of Samos proposed aheliocentric system, according to which Earth and the planets revolved around the Sun. The geocentric system remained dominant until theScientific Revolution.[163]
By the 1st century BC, during theHellenistic 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 theAlmagest written byPtolemy 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.[164][174] To the Greeks and Romans, there were seven known planets, each presumed to becircling 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.[159][174][175]
1660 illustration of Claudius Ptolemy's geocentric model
After thefall of the Western Roman Empire, astronomy developed further in India and the medieval Islamic world. In 499 CE, the Indian astronomerAryabhata propounded a planetary model that explicitly incorporatedEarth'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 theorized that the orbits of planets wereelliptical.[176] Aryabhata's followers were particularly strong inSouth India, where his principles of the diurnal rotation of Earth, among others, were followed and a number of secondary works were based on them.[177]
True-scale Solar System poster made byEmanuel Bowen in 1747. At that time, Uranus, Neptune, and the asteroid belts had all not yet been discovered.
With the advent of theScientific Revolution and theheliocentric model ofCopernicus,Galileo, andKepler, use of the term "planet" changed from something that moved around the sky relative to thefixed star to a body that orbited the Sun, directly (a primary planet) or indirectly (a secondary or satellite planet). Thus the Earth was added to the roster of planets,[181] and the Sun was removed. The Copernican count of primary planets stood until 1781, whenWilliam Herschel discoveredUranus.[182]
When four satellites of Jupiter (theGalilean moons) and five of Saturn were discovered in the 17th century, they joined Earth's Moon in the category of "satellite planets" or "secondary planets" orbiting the primary planets, though in the following decades they would come to be called simply "satellites" for short. Scientists generally considered planetary satellites to also be planets until about the 1920s, although this usage was not common among non-scientists.[154]
In the first decade of the 19th century, four new 'planets' were discovered:Ceres (in 1801),Pallas (in 1802),Juno (in 1804), andVesta (in 1807). It soon became apparent that they were rather different from previously known planets: they shared the same general region of space, between Mars and Jupiter (theasteroid belt), with sometimes overlapping orbits. This was an area where only one planet had been expected, and they were much smaller than all other planets; indeed, it was suspected that they might be shards of a larger planet that had broken up. Herschel called themasteroids (from the Greek for "starlike") because even in the largest telescopes they resembled stars, without a resolvable disk.[153][183]
The situation was stable for four decades, but in the 1840s several additional asteroids were discovered (Astraea in 1845;Hebe,Iris, andFlora in 1847;Metis in 1848; andHygiea in 1849). New "planets" were discovered every year; as a result, astronomers began tabulating the asteroids (minor planets) separately from the major planets and assigning them numbers instead of abstractplanetary symbols,[153] although they continued to be considered as small planets.[184]
Neptune was discovered in 1846, its position having been predicted thanks to its gravitational influence upon Uranus. Because the orbit of Mercury appeared to be affected in a similar way, it was believed in the late 19th century that there might beanother planet even closer to the Sun. However, the discrepancy between Mercury's orbit and the predictions of Newtonian gravity was instead explained by an improved theory of gravity, Einstein'sgeneral relativity.[185][186]
Pluto was discovered in 1930. After initial observations led to the belief that it was larger than Earth,[187] the object was immediately accepted as the ninth major planet. Further monitoring found the body was actually much smaller: in 1936,Ray Lyttleton suggested that Pluto may be an escaped satellite ofNeptune,[188] andFred Whipple suggested in 1964 that Pluto may be a comet.[189] The discovery of its large moonCharon in 1978 showed that Pluto was only 0.2% the mass of Earth.[190] As this was still substantially more massive than any known asteroid, and because no othertrans-Neptunian objects had been discovered at that time, Pluto kept its planetary status, only officially losing it in 2006.[191][192]
In the 1950s,Gerard Kuiper published papers on the origin of the asteroids. He recognized that asteroids were typically not spherical, as had previously been thought, and that theasteroid families were remnants of collisions. Thus he differentiated between the largest asteroids as "true planets" versus the smaller ones as collisional fragments. From the 1960s onwards, the term "minor planet" was mostly displaced by the term "asteroid", and references to the asteroids as planets in the literature became scarce, except for the geologically evolved largest three: Ceres, and less often Pallas and Vesta.[184]
The beginning of Solar System exploration by space probes in the 1960s spurred a renewed interest in planetary science. A split in definitions regarding satellites occurred around then: planetary scientists began to reconsider the large moons as also being planets, but astronomers who were not planetary scientists generally did not.[154] (This is not exactly the same as the definition used in the previous century, which classedall satellites as secondary planets, even non-round ones like Saturn'sHyperion or Mars'sPhobos andDeimos.)[193][194] All the eight major planets and their planetary-mass moons have since been explored by spacecraft, as have many asteroids and the dwarf planets Ceres and Pluto; however, so far the only planetary-mass body beyond Earth that has been explored by humans is the Moon.[b]
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 (theKuiper belt) during the 1990s and early 2000s. Pluto was found to be just one "small" body in a population of thousands.[195] They often referred to the demotion of the asteroids as a precedent, although that had been done based on their geophysical differences from planets rather than their being in a belt.[154] Some of the largertrans-Neptunian objects, such asQuaoar,Sedna,Eris, andHaumea,[196] were heralded in the popular press as thetenth planet.
The announcement of Eris in 2005, an object 27% more massive than Pluto, created the impetus for an official definition of a planet,[195] as considering Pluto a planet would logically have demanded that Eris be considered a planet as well. Since different procedures were in place for naming planets versus non-planets, this created an urgent situation because under the rules Eris could not be named without defining what a planet was.[154] At the time, it was also thought that the size required for a trans-Neptunian object to become round was about the same as that required for the moons of the giant planets (about 400 km diameter), a figure that would have suggested about 200 round objects in the Kuiper belt and thousands more beyond.[197][198] Many astronomers argued that the public would not accept a definition creating a large number of planets.[154]
To acknowledge the problem, theInternational Astronomical Union (IAU) set about creating thedefinition of planet and produced one in August 2006. Under this definition, the Solar System is considered to have eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune). Bodies that fulfill the first two conditions but not the third are classified asdwarf planets, provided they are notnatural satellites of other planets. Originally an IAU committee had proposed a definition that would have included a larger number of planets as it did not include (c) as a criterion.[199] After much discussion, it was decided via a vote that those bodies should instead be classified as dwarf planets.[192][200]
The planetary-mass moons to scale, compared with Mercury, Venus, Earth, Mars, and Pluto. Sub-planetaryProteus andNereid (about the same size as Mimas) have been included for comparison. UnimagedDysnomia (intermediate in size between Tethys and Enceladus) is not shown; it is in any case probably not a solid body.[90]
The IAU definition has not been universally used or accepted. Inplanetary geology, celestial objects aredefined as planets by geophysical characteristics. A celestial body may acquire a dynamic (planetary) geology at approximately the mass required for its mantle to become plastic under its own weight. This leads to a state ofhydrostatic equilibrium where the body acquires a stable, round shape, which is adopted as the hallmark of planethood by geophysical definitions. For example:[201]
a substellar-mass body that has never undergone nuclear fusion and has enough gravitation to be round due to hydrostatic equilibrium, regardless of its orbital parameters.[202]
In the Solar System, this mass is generally less than the mass required for a body to clear its orbit; thus, some objects that are considered "planets" under geophysical definitions are not considered as such under the IAU definition, such as Ceres and Pluto.[4] (In practice, the requirement for hydrostatic equilibrium is universally relaxed to a requirement for rounding and compaction under self-gravity; Mercury is not actually in hydrostatic equilibrium,[203] but is universally included as a planet regardless.)[204] Proponents of such definitions often argue that location should not matter and that planethood should be defined by the intrinsic properties of an object.[4] Dwarf planets had been proposed as a category of small planet (as opposed toplanetoids as sub-planetary objects) and planetary geologists continue to treat them as planets despite the IAU definition.[32]
The number of dwarf planets even among known objects is not certain. In 2019, Grundy et al. argued based on the low densities of some mid-sized trans-Neptunian objects that the limiting size required for a trans-Neptunian object to reach equilibrium was in fact much larger than it is for the icy moons of the giant planets, being about 900–1000 km diameter.[32] There is general consensus on Ceres in the asteroid belt[205] and on the eight trans-Neptunians that probably cross this threshold—Orcus,Pluto,Haumea,Quaoar,Makemake,Gonggong,Eris, andSedna.[206][33]
Planetary geologists may include the nineteen knownplanetary-mass moons as "satellite planets", including Earth's Moon and Pluto'sCharon, like the early modern astronomers.[4][207] Some go even further and include as planets relatively large, geologically evolved bodies that are nonetheless not very round today, such as Pallas and Vesta;[4] rounded bodies that were completely disrupted by impacts and re-accreted like Hygiea;[208][209][110] or even everything at least the diameter of Saturn's moonMimas, the smallest planetary-mass moon. (This may even include objects that are not round but happen to be larger than Mimas, like Neptune's moonProteus.)[4]
AstronomerJean-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.[210] The formula produces a value calledπ that is greater than 1 for planets.[c] The eight known planets and all known exoplanets haveπ values above 100, while Ceres, Pluto, and Eris haveπ values of 0.1, or less. Objects withπ values of 1 or more are expected to be approximately spherical, so that objects that fulfill the orbital-zone clearance requirement around Sun-like stars will also fulfill the roundness requirement[211] – though this may not be the case around very low-mass stars.[212] In 2024, Margot and collaborators proposed a revised version of the criterion with a uniform clearing timescale of 10 billion years (the approximate main-sequence lifetime of the Sun) or 13.8 billion years (theage of the Universe) to accommodate planets orbiting brown dwarfs.[212]
Even before the discovery ofexoplanets, there were particular disagreements over whether an object should be considered a planet if it was part of a distinct population such as abelt, or if it was large enough to generate energy by thethermonuclear fusion ofdeuterium.[195] Complicating the matter even further, bodies too small to generate energy by fusing deuterium can form bygas-cloud collapse just like stars and brown dwarfs, even down to the mass of Jupiter:[213] there was thus disagreement about whether how a body formed should be taken into account.[195]
The discovery of exoplanets led to another ambiguity in defining a planet: the point at which a planet becomes a star. Many known exoplanets are many times the mass of Jupiter, approaching that of stellar objects known asbrown dwarfs. Brown dwarfs are generally considered stars due to their theoretical ability to fusedeuterium, a heavier isotope ofhydrogen. Although objects more massive than 75 times that of Jupiter fuse simple hydrogen, objects of 13 Jupiter masses can fuse deuterium. Deuterium is quite rare, constituting less than 0.0026% of the hydrogen in the galaxy, and most brown dwarfs would have ceased fusing deuterium long before their discovery, making them effectively indistinguishable from supermassive planets.[215]
IAU working definition of exoplanets
The 2006 IAU definition presents some challenges for exoplanets because the language is specific to the Solar System and the criteria of roundness and orbital zone clearance are not presently observable for exoplanets.[1] In 2018, this definition was reassessed and updated as knowledge of exoplanets increased.[216] The current official working definition of an exoplanet is as follows:[99]
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 theL4/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.
Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed nor where they are located.
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).[99]
The IAU noted that this definition could be expected to evolve as knowledge improves.[99] A 2022 review article discussing the history and rationale of this definition suggested that the words "in young star clusters" should be deleted in clause 3, as such objects have now been found elsewhere, and that the term "sub-brown dwarfs" should be replaced by the more current "free-floating planetary mass objects". The term "planetary mass object" has also been used to refer to ambiguous situations concerning exoplanets, such as objects with mass typical for a planet that are free-floating or orbit a brown dwarf instead of a star.[216] Free-floating objects of planetary mass have sometimes been called planets anyway, specificallyrogue planets.[217]
The limit of 13 Jupiter masses is not universally accepted. Objects below this mass limit can sometimes burn deuterium, and the amount of deuterium that is burned depends on an object's composition.[218][219] Furthermore, deuterium is quite scarce, so the stage of deuterium burning does not actually last very long; unlike hydrogen burning in a star, deuterium burning does not significantly affect the future evolution of an object.[58] The relationship between mass and radius (or density) show no special feature at this limit, according to which brown dwarfs have the same physics and internal structure as lighter Jovian planets, and would more naturally be considered planets.[58][54]
Thus, many catalogues of exoplanets include objects heavier than 13 Jupiter masses, sometimes going up to 60 Jupiter masses.[220][100][101][221] (The limit for hydrogen burning and becoming ared dwarf star is about 80 Jupiter masses.)[58] The situation of main-sequence stars has been used to argue for such an inclusive definition of "planet" as well, as they also differ greatly along the two orders of magnitude that they cover, in their structure, atmospheres, temperature, spectral features, and probably formation mechanisms; yet they are all considered as one class, being all hydrostatic-equilibrium objects undergoing nuclear burning.[58]
The naming of planets differs between planets of theSolar System andexoplanets (planets of otherplanetary systems). Exoplanets are commonly named after their parent star and their order of discovery within its planetary system, such asProxima Centauri b. (The lettering starts at b, with a considered to represent the parent star.)
The names for the planets of theSolar System (other thanEarth) in theEnglish language are derived from naming practices developed consecutively by theBabylonians,Greeks, andRomans ofantiquity. The practice of grafting the names of gods onto the planets was almost certainly borrowed from the Babylonians by the ancient Greeks, and thereafter from the Greeks by the Romans. The Babylonians named Venus after theSumerian goddess of love with theAkkadian nameIshtar; Mars after their god of war,Nergal; Mercury after their god of wisdomNabu; Jupiter after their chief god,Marduk; and Saturn after their god of farming,Ninurta.[222] There are too many concordances between Greek and Babylonian naming conventions for them to have arisen separately.[164] Given the differences in mythology, the correspondence 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 a god of pestilence and ruler of the underworld.[223][224][225]
In ancient Greece, the two great luminaries, the Sun and the Moon, were calledHelios andSelene, two ancientTitanic deities; the slowest planet, Saturn, was calledPhainon, the shiner; followed byPhaethon, Jupiter, "bright"; the red planet, Mars was known asPyroeis, the "fiery"; the brightest, Venus, was known asPhosphoros, the light bringer; and the fleeting final planet, Mercury, was calledStilbon, the gleamer. The Greeks assigned each planet to one among their pantheon of gods, theOlympians and the earlier Titans:[164]
Helios and Selene were the names of both planets and gods, both of them Titans (later supplanted by OlympiansApollo andArtemis);
Phainon was sacred toCronus, the Titan who fathered the Olympians, associated with the harvest;
Phaethon was sacred toZeus, Cronus's son who deposed him as king;
Pyroeis was given toAres, son of Zeus and god of war;
Phosphoros was ruled byAphrodite, the goddess of love; and
Stilbon with its speedy motion, was ruled over byHermes, messenger of the gods and god of learning and wit.[164]
The Greek gods ofOlympus, after whom theSolar System's Roman names of the planets are derived
Although modern Greeks still use their ancient names for the planets, other European languages, because of the influence of theRoman Empire and, later, theCatholic Church, use the Roman (Latin) names rather than the Greek ones. The Romans inheritedProto-Indo-European mythology as the Greeks did and shared with them acommon pantheon under different names, but the Romans lacked the rich narrative traditions that Greek poetic culture had giventheir gods. During the later period of theRoman Republic, Roman writers borrowed much of the Greek narratives and applied them to their own pantheon, to the point where they became virtually indistinguishable.[226] When the Romans studied Greek astronomy, they gave the planets their own gods' names:Mercurius (for Hermes),Venus (Aphrodite),Mars (Ares),Iuppiter (Zeus), andSaturnus (Cronus). However, there was not much agreement on which god a particular planet was associated with; according toPliny the Elder, while Phainon and Phaethon's associations with Saturn and Jupiter respectively were widely agreed upon, Pyroeis was also associated with the demi-godHercules, Stilbon was also associated withApollo, god of music, healing, and prophecy; Phosphoros was also associated with prominent goddessesJuno andIsis.[227] Some Romans, following a belief possibly originating inMesopotamia but developed inHellenistic 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).[228] 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 became the order of thedays of the week in theRoman calendar.[229] In English,Saturday,Sunday, andMonday are straightforward translations of these Roman names. The other days were renamed afterTīw (Tuesday),Wōden (Wednesday),Þunor (Thursday), andFrīġ (Friday), theAnglo-Saxon gods considered similar or equivalent to Mars, Mercury, Jupiter, and Venus, respectively.[230]
Earth's name in English is not derived from Greco-Roman mythology. Because it was only generally accepted as a planet in the 17th century,[181] 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 theOld English wordeorþe, which was the word for "ground" and "dirt" as well as the world itself.[231] As with its equivalents in the otherGermanic languages, it derives ultimately from theProto-Germanic worderþō, as can be seen in the Englishearth, the GermanErde, the Dutchaarde, and the Scandinavianjord. Many of theRomance languages retain the old Roman wordterra (or some variation of it) that was used with the meaning of "dry land" as opposed to "sea".[232] The non-Romance languages use their own native words. The Greeks retain their original name,Γή(Ge).[233]
Non-European cultures use other planetary-naming systems.India uses a system based on theNavagraha, which incorporates the seven traditional planets and the ascending and descendinglunar nodesRahu andKetu. The planets areSurya 'Sun',Chandra 'Moon',Budha for Mercury,Shukra ('bright') for Venus,Mangala (the god of war) for Mars,Bṛhaspati (councilor of the gods) for Jupiter, andShani (symbolic of time) for Saturn.[234]
The nativePersian names of most of the planets are based on identifications of the Mesopotamian gods with Iranian gods, analogous to the Greek and Latin names. Mercury isTir (Persian:تیر) for the western Iranian god Tīriya (patron of scribes), analogous to Nabu; Venus isNāhid (ناهید) forAnahita; Mars isBahrām (بهرام) forVerethragna; and Jupiter isHormoz (هرمز) forAhura Mazda. The Persian name for Saturn,Keyvān (کیوان), is a borrowing fromAkkadiankajamānu, meaning "the permanent, steady".[235]
In traditionalHebrew astronomy, the seven traditional planets have (for the most part) descriptive names—the Sun is חמהḤammah or "the hot one", the Moon is לבנהLevanah or "the white one", Venus is כוכב נוגהKokhav Nogah or "the bright planet", Mercury is כוכבKokhav or "the planet" (given its lack of distinguishing features), Mars is מאדיםMa'adim or "the red one", and Saturn is שבתאיShabbatai or "the resting one" (in reference to its slow movement compared to the other visible planets).[236] The odd one out is Jupiter, called צדקTzedeq or "justice".[236] These names, first attested in theBabylonian Talmud, are not the original Hebrew names of the planets. In 377Epiphanius of Salamis recorded another set of names that seem to have pagan orCanaanite associations: those names, since replaced for religious reasons, were probably the historical Semitic names, and may have much earlier roots going back to Babylonian astronomy.[236] The etymologies for the Arabic names of the planets are less well understood. Mostly agreed among scholars are Venus (Arabic:الزهرة,az-Zuhara, "the bright one"[237]), Earth (الأرض,al-ʾArḍ, from the same root aseretz), and Saturn (زُحَل,Zuḥal, "withdrawer"[238]). Multiple suggested etymologies exist for Mercury (عُطَارِد,ʿUṭārid), Mars (اَلْمِرِّيخ,al-Mirrīkh), and Jupiter (المشتري,al-Muštarī), but there is no agreement among scholars.[239][240][241][242]
When subsequent planets were discovered in the 18th and 19th centuries, Uranus was named for aGreek deity and Neptune for aRoman one (the counterpart ofPoseidon). The asteroids were initially named from mythology as well—Ceres,Juno, andVesta are major Roman goddesses, and Pallas is an epithet of the major Greek goddessAthena—but as more and more were discovered, they first started being named after more minor goddesses, and the mythological restriction was dropped starting from the twentieth asteroidMassalia in 1852.[243] Pluto (named after theGreek god of the underworld) was given a classical name, as it was considered a major planet when it was discovered.
The names of Uranus (天王星 "sky king star"), Neptune (海王星 "sea king star"), and Pluto (冥王星 "underworld king star") in Chinese, Korean, and Japanese arecalques based on the roles of those gods in Roman and Greek mythology.[244][245][d] In the 19th century,Alexander Wylie andLi Shanlan calqued the names of the first 117 asteroids into Chinese, and many of their names are still used today, e.g. Ceres (穀神星 "grain goddess star"), Pallas (智神星 "wisdom goddess star"), Juno (婚神星 "marriage goddess star"), Vesta (灶神星 "hearth goddess star"), and Hygiea (健神星 "health goddess star").[247] Such translations were extended to some later minor planets, including some of the dwarf planets discovered in the 21st century, e.g. Haumea (妊神星 "pregnancy goddess star"), Makemake (鳥神星 "bird goddess star"), and Eris (鬩神星 "quarrel goddess star"). However, except for the better-known asteroids and dwarf planets, many of them are rare outside Chinese astronomical dictionaries.[244]
Hebrew names were chosen for Uranus (אורוןOron, "small light") and Neptune (רהבRahab, a Biblical sea monster) in 2009;[248] prior to that the names "Uranus" and "Neptune" had simply been borrowed.[249]
After more objects were discovered beyond Neptune, naming conventions depending on their orbits were put in place: those in the 2:3 resonance with Neptune (theplutinos) are given names from underworld myths, while others are given names from creation myths. Most of the trans-Neptunian planetoids are named after gods and goddesses from other cultures (e.g.Quaoar is named after aTongva god). There are a few exceptions which continue the Roman and Greek scheme, notably including Eris as it had initially been considered a tenth planet.[250][251]
The moons (including the planetary-mass ones) are generally given names with some association with their parent planet. The planetary-mass moons of Jupiter are named after four of Zeus' lovers (or other sexual partners); those of Saturn are named after Cronus' brothers and sisters, the Titans; those of Uranus are named after characters fromShakespeare andPope (originally specifically from fairy mythology,[252] but that ended with the naming ofMiranda). Neptune's planetary-mass moon Triton is named afterthe god's son; Pluto's planetary-mass moon Charon is named after theferryman of the dead, who carries the souls of the newly deceased to the underworld (Pluto's domain).[253]
The written symbols for Mercury, Venus, Jupiter, Saturn, and possibly Mars have been traced to forms found in late Greek papyrus texts.[254] The symbols for Jupiter and Saturn are identified asmonograms of the corresponding Greek names, and the symbol for Mercury is a stylizedcaduceus.[254]
According toAnnie Scott Dill Maunder, antecedents of the planetary symbols were used in art to represent the gods associated with the classical planets.Bianchini'splanisphere, discovered by Francesco Bianchini in the 18th century but produced in the 2nd century,[255] shows Greek personifications of planetary gods charged with early versions of the planetary symbols. Mercury has acaduceus; Venus has, attached to her necklace, a cord connected to another necklace; Mars, a spear; Jupiter, a staff; Saturn, a scythe; theSun, acirclet with rays radiating from it; and the Moon, a headdress with a crescent attached.[256] The modern shapes with the cross-marks first appeared around the 16th century. According to Maunder, the addition of crosses appears to be "an attempt to give a savour of Christianity to the symbols of the old pagan gods."[256] Earth itself was not considered a classical planet; its symbol descends from a pre-heliocentric symbol for thefour corners of the world.[257]
When further planets were discovered orbiting the Sun, symbols were invented for them. The most common astronomical symbol for Uranus, ⛢,[258] was invented byJohann Gottfried Köhler, and was intended to represent the newly discovered metalplatinum.[259][260] An alternative symbol, ♅, was invented byJérôme Lalande, and represents a globe with a H on top, for Uranus's discoverer Herschel.[261] Today, ⛢ is mostly used by astronomers and ♅ byastrologers, though it is possible to find each symbol in the other context.[258] The first few asteroids were considered to be planets when they were discovered, and were likewise given abstract symbols, e.g. Ceres' sickle (⚳), Pallas' spear (⚴), Juno's sceptre (⚵), and Vesta's hearth (⚶). However, as their number rose further and further, this practice stopped in favour of numbering them instead. (Massalia, the first asteroid not named from mythology, is also the first asteroid that was not assigned a symbol by its discoverer.) The symbols for the first four asteroids, Ceres through Vesta, remained in use for longer than the others,[153] and even in the modern dayNASA has used the Ceres symbol—Ceres being the only asteroid that is also a dwarf planet.[262] Neptune's symbol (♆) representsthe god's trident.[260] The astronomical symbol for Pluto is a P-L monogram (♇),[263] though it has become less common since the IAU definition reclassified Pluto.[262] Since Pluto's reclassification, NASA has used the traditional astrological symbol of Pluto (⯓), a planetary orb over Pluto'sbident.[262]
Some rarer planetary symbols in Unicode
Earth
Vesta
Juno
Ceres
Pallas
Hygiea
Orcus
Pluto or
Charon
Haumea
Quaoar
Makemake
Gonggong
Eris
Sedna
The IAU discourages the use of planetary symbols in modern journal articles in favour of one-letter or (to disambiguate Mercury and Mars) two-letter abbreviations for the major planets. The symbols for the Sun and Earth are nonetheless common, assolar mass,Earth mass, and similar units are common in astronomy.[264] Other planetary symbols today are mostly encountered in astrology. Astrologers have resurrected the old astronomical symbols for the first few asteroids and continue to invent symbols for other objects.[262] This includes relatively standard astrological symbols for the dwarf planets discovered in the 21st century, which were not given symbols by astronomers because planetary symbols had mostly fallen out of use in astronomy by the time they were discovered. Many astrological symbols are included inUnicode, and a few of these new inventions (the symbols of Haumea, Makemake, and Eris) have since been used by NASA in astronomy.[262] The Eris symbol is a traditional one fromDiscordianism, a religion worshipping the goddess Eris. The other dwarf-planet symbols are mostly initialisms (except Haumea) in the native scripts of the cultures they come from; they also represent something associated with the corresponding deity or culture, e.g. Makemake's face or Gonggong's snake-tail.[262][265] Moskowitz also devised symbols for the planetary-mass moons; most of them are initialisms combined with a feature of their parent planet. The exception is Charon, which combines the high orb of Pluto's bident symbol with a crescent, suggesting both Charon as a moon and the mythological Charon's boat crossing the riverStyx.[266]
See also
Double planet – Binary system where two planetary-mass objects share an orbital axis external to both
^Here, "Earth-sized" means 1–2 Earth radii, and "habitable zone" means the region with 0.25 to 4 times Earth's stellar flux (corresponding to 0.5–2 AU for the Sun). Data forG-type stars like the Sun is not available. This statistic is an extrapolation from data onK-type stars.[51][52]
^In Korean, these names are more often written inHangul rather than Chinese characters, e.g. 명왕성 for Pluto. In Vietnamese,calques are more common than directly reading these names asSino-Vietnamese, e.g.sao Thuỷ rather thanThuỷ tinh for Mercury. Pluto is notsao Minh Vương butsao Diêm Vương "Yama star".[246]
^Taylor, G. Jeffrey (31 December 1998)."Origin of the Earth and Moon".Planetary Science Research Discoveries. Hawai'i Institute of Geophysics and Planetology.Archived from the original on 10 June 2010. Retrieved7 April 2010.
^abLewis, John S. (2004).Physics and Chemistry of the Solar System (2nd ed.). Academic Press. p. 59.ISBN978-0-12-446744-6.
^abMarley, Mark (2 April 2019)."Not a Heart of Ice".planetary.org. The Planetary Society.Archived from the original on 12 August 2019. Retrieved5 May 2022.
^abEmery, J. P.; Wong, I.; Brunetto, R.; Cook, J. C.; Pinilla-Alonso, N.; Stansberry, J. A.; Holler, B. J.; Grundy, W. M.; Protopapa, S.; Souza-Feliciano, A. C.; Fernández-Valenzuela, E.; Lunine, J. I.; Hines, D. C. (2024). "A Tale of 3 Dwarf Planets: Ices and Organics on Sedna, Gonggong, and Quaoar from JWST Spectroscopy".Icarus.414.arXiv:2309.15230.Bibcode:2024Icar..41416017E.doi:10.1016/j.icarus.2024.116017.
^Chen, Rick (23 October 2018)."Top Science Results from the Kepler Mission".NASA. Archived fromthe original on 11 July 2022. Retrieved11 July 2022.The most common size of planet Kepler found doesn't exist in our solar system—a world between the size of Earth and Neptune—and we have much to learn about these planets.
^"The Habitable Exoplanets Catalog".Planetary Habitability Laboratory. University of Puerto Rico at Arecibo.Archived from the original on 20 October 2011. Retrieved12 July 2022.
^Drake, Frank (29 September 2003)."The Drake Equation Revisited". Astrobiology Magazine. Archived from the original on 28 June 2011. Retrieved23 August 2008.
^Tatum, J. B. (2007)."17. Visual binary stars".Celestial Mechanics. Personal web page.Archived from the original on 6 July 2007. Retrieved2 February 2008.
^abBelton, M. J. S.; Terrile, R. J. (1984). Bergstralh, J. T. (ed.).Rotational properties of Uranus and Neptune. Voyager "Uranus-Neptune" Workshop Pasadena 6–8 February 1984. pp. 327–347.Bibcode:1984NASCP2330..327B.
^Borgia, Michael P. (2006).The Outer Worlds; Uranus, Neptune, Pluto, and Beyond. Springer New York. pp. 195–206.
^"Planet Compare".Solar System Exploration. NASA.Archived from the original on 9 March 2018. Retrieved12 July 2022.
^Zarka, Philippe; Treumann, Rudolf A.; Ryabov, Boris P.; Ryabov, Vladimir B. (2001). "Magnetically-Driven Planetary Radio Emissions and Application to Extrasolar Planets".Astrophysics and Space Science.277 (1/2):293–300.Bibcode:2001Ap&SS.277..293Z.doi:10.1023/A:1012221527425.S2CID16842429.
^Young, Leslie A. (1997)."The Once and Future Pluto".Southwest Research Institute, Boulder, Colorado.Archived from the original on 30 March 2004. Retrieved26 March 2007.
^Rabinowitz, D. L.; Barkume, Kristina; Brown, Michael E.; Roe, Henry; Schwartz, Michael; Tourtellotte, Suzanne; Trujillo, Chad (2006). "Photometric Observations Constraining the Size, Shape, and Albedo of 2003 EL61, a Rapidly Rotating, Pluto-Sized Object in the Kuiper Belt".Astrophysical Journal.639 (2):1238–1251.arXiv:astro-ph/0509401.Bibcode:2006ApJ...639.1238R.doi:10.1086/499575.S2CID11484750.
^Gaffey, Michael (1984). "Rotational spectral variations of asteroid (8) Flora: Implications for the nature of the S-type asteroids and for the parent bodies of the ordinary chondrites".Icarus.60 (1):83–114.Bibcode:1984Icar...60...83G.doi:10.1016/0019-1035(84)90140-4.
^Hardersen, Paul S.; Gaffey, Michael J. & Abell, Paul A. (2005). "Near-IR spectral evidence for the presence of iron-poor orthopyroxenes on the surfaces of six M-type asteroid".Icarus.175 (1): 141.Bibcode:2005Icar..175..141H.doi:10.1016/j.icarus.2004.10.017.
^abAsphaug, E.; Reufer, A. (2014). "Mercury and other iron-rich planetary bodies as relics of inefficient accretion".Nature Geoscience.7 (8):564–568.Bibcode:2014NatGe...7..564A.doi:10.1038/NGEO2189.
^abYang, B.; Hanuš, J.; Carry, B.; Vernazza, P.; Brož, M.; Vachier, F.; Rambaux, N.; Marsset, M.; Chrenko, O.; Ševeček, P.; Viikinkoski, M.; Jehin, E.; Ferrais, M.; Podlewska-Gaca, E.; Drouard, A.; Marchis, F.; Birlan, M.; Benkhaldoun, Z.; Berthier, J.; Bartczak, P.; Dumas, C.; Dudziński, G.; Ďurech, J.; Castillo-Rogez, J.; Cipriani, F.; Colas, F.; Fetick, R.; Fusco, T.; Grice, J.; et al. (2020), "Binary asteroid (31) Euphrosyne: Ice-rich and nearly spherical",Astronomy & Astrophysics,641: A80,arXiv:2007.08059,Bibcode:2020A&A...641A..80Y,doi:10.1051/0004-6361/202038372,S2CID220546126
^Zeilik, Michael A.; Gregory, Stephan A. (1998).Introductory Astronomy & Astrophysics (4th ed.). Saunders College Publishing. p. 67.ISBN978-0-03-006228-5.
^Haberle, R. M. (2015), "Solar System/Sun, Atmospheres, Evolution of Atmospheres | Planetary Atmospheres: Mars", in North, Gerald R.; Pyle, John; Zhang, Fuqing (eds.),Encyclopedia of Atmospheric Sciences (2nd ed.), Academic Press, pp. 168–177,doi:10.1016/b978-0-12-382225-3.00312-1,ISBN978-0123822253
^abcKivelson, Margaret Galland; Bagenal, Fran (2007)."Planetary Magnetospheres". In Lucy-Ann McFadden; Paul Weissman; Torrence Johnson (eds.).Encyclopedia of the Solar System. Academic Press. p. 519.ISBN978-0-12-088589-3.
^Molnar, L. A.; Dunn, D. E. (1996). "On the Formation of Planetary Rings".Bulletin of the American Astronomical Society.28:77–115.Bibcode:1996DPS....28.1815M.
^ab"planet, n".Oxford English Dictionary. 2007.Archived from the original on 3 July 2012. Retrieved7 February 2008.Note: select the Etymology tab
^Neugebauer, Otto E. (1945). "The History of Ancient Astronomy Problems and Methods".Journal of Near Eastern Studies.4 (1):1–38.doi:10.1086/370729.S2CID162347339.
^Ronan, Colin (1996). "Astronomy Before the Telescope". In Walker, Christopher (ed.).Astronomy in China, Korea and Japan. British Museum Press. pp. 264–265.Bibcode:1996abt..conf..245R.
^Rochberg, Francesca (2000). "Astronomy and Calendars in Ancient Mesopotamia". In Jack Sasson (ed.).Civilizations of the Ancient Near East. Vol. III. p. 1930.
^Aaboe, Asger (1991), "The culture of Babylonia: Babylonian mathematics, astrology, and astronomy", inBoardman, John;Edwards, I. E. S.;Hammond, N. G. L.; Sollberger, E.; Walker, C. B. F (eds.),The Assyrian and Babylonian Empires and other States of the Near East, from the Eighth to the Sixth Centuries B.C., The Cambridge Ancient History, vol. 3, Cambridge: Cambridge University Press, pp. 276–292,ISBN978-0521227179
^Hermann Hunger, ed. (1992).Astrological reports to Assyrian kings. State Archives of Assyria. Vol. 8. Helsinki University Press.ISBN978-951-570-130-5.
^Lambert, W. G.; Reiner, Erica (1987). "Babylonian Planetary Omens. Part One. Enuma Anu Enlil, Tablet 63: The Venus Tablet of Ammisaduqa".Journal of the American Oriental Society.107 (1):93–96.doi:10.2307/602955.JSTOR602955.
^Cooley, Jeffrey L. (2008)."Inana and Šukaletuda: A Sumerian Astral Myth".KASKAL.5:161–172.ISSN1971-8608.Archived from the original on 24 December 2019. Retrieved26 November 2022.The Greeks, for example, originally identified the morning and evening stars with two separate deities, Phosphoros and Hesporos respectively. In Mesopotamia, it seems that this was recognized prehistorically. Assuming its authenticity, a cylinder seal from the Erlenmeyer collection attests to this knowledge in southern Iraq as early as the Late Uruk / Jemdet Nasr Period, as do the archaic texts of the period. [...] Whether or not one accepts the seal as authentic, the fact that there is no epithetical distinction between the morning and evening appearances of Venus in any later Mesopotamian literature attests to a very, very early recognition of the phenomenon.
^Sarma, K. V. (1997). "Astronomy in India". InSelin, Helaine (ed.).Encyclopaedia of the History of Science, Technology, and Medicine in Non-Western Cultures. Kluwer Academic Publishers. p. 116.ISBN0-7923-4066-3.
^Bausani, Alessandro (1973). "Cosmology and Religion in Islam".Scientia/Rivista di Scienza.108 (67): 762.
^Hunter, Robert; Williams, John A.; Heritage, S. J., eds. (1897).The American Encyclopædic Dictionary. Vol. 8. Chicago and New York: R. S. Peale and J. A. Hill. pp. 3553–3554.Archived from the original on 30 October 2023. Retrieved25 October 2023.
^Stern, S. Alan; Levison, Harold F. (2002), Rickman, H. (ed.), "Regarding the criteria for planethood and proposed planetary classification schemes",Highlights of Astronomy,12, San Francisco: Astronomical Society of the Pacific:205–213,Bibcode:2002HiA....12..205S,doi:10.1017/S1539299600013289,ISBN978-1-58381-086-6 See p. 208.
^Sean Solomon, Larry Nittler & Brian Anderson, eds. (2018)Mercury: The View after MESSENGER. Cambridge Planetary Science series no. 21, Cambridge University Press, pp. 72–73.
^Bodenheimer, Peter; D'Angelo, Gennaro; Lissauer, Jack J.; Fortney, Jonathan J.; Saumon, Didier (2013). "Deuterium Burning in Massive Giant Planets and Low-mass Brown Dwarfs Formed by Core-nucleated Accretion".The Astrophysical Journal.770 (2): 120.arXiv:1305.0980.Bibcode:2013ApJ...770..120B.doi:10.1088/0004-637X/770/2/120.S2CID118553341.
^Wiggermann, Frans A. M. (1998)."Nergal A. Philological".Reallexikon der Assyriologie. Bavarian Academy of Sciences and Humanities.Archived from the original on 6 June 2021. Retrieved12 July 2022.
^"earth".Oxford English Dictionary.Archived from the original on 10 May 2021. Retrieved7 May 2021.
^Harper, Douglas (September 2001)."Etymology of "terrain"".Online Etymology Dictionary.Archived from the original on 23 August 2012. Retrieved30 January 2008.
^Kambas, Michael (2004).Greek-English, English-Greek Dictionary. Hippocrene Books. p. 259.ISBN978-0781810029.
^Ragep, F.J.; Hartner, W. (24 April 2012)."Zuhara".Encyclopaedia of Islam (2nd ed.).Archived from the original on 9 July 2021. Retrieved16 January 2019 – via referenceworks.brillonline.com.
^Galter, Hannes D. (23–27 September 1991)."Die Rolle der Astronomie in den Kulturen Mesopotamiens" [The role of astronomy in the cultures of the Mesopotamians].Beiträge Zum 3. Grazer Morgenländischen Symposion ( 23–27 September 1991). 3. Grazer Morgenländischen Symposion [Third Graz Oriental Symposium]. Graz, Austria: GrazKult (published 31 July 1993).ISBN978-3853750094 – via Google Books.
^abJones, Alexander (1999).Astronomical Papyri from Oxyrhynchus. American Philosophical Society. pp. 62–63.ISBN978-0-87169-233-7.
^"Bianchini's planisphere". Florence, Italy: Istituto e Museo di Storia della Scienza [Institute and Museum of the History of Science].Archived from the original on 27 February 2018. Retrieved20 August 2018.
^abMaunder, A.S.D. (1934). "The origin of the symbols of the planets".The Observatory. Vol. 57. pp. 238–247.Bibcode:1934Obs....57..238M.
.jpg)
.svg)
.svg)
.svg)
.svg)
.svg)
.svg)
.svg)
.svg)
.svg)
.svg)
.svg)
.svg)
.svg)
.svg)
.svg)
.svg)
.svg)
.svg)
.svg)
.svg)
.svg)
.svg)
.svg)
.svg)
.svg)
.svg)
