Planet

Artists' impressions

A protoplanetary disk

Asteroids colliding during planet formation

It is not known with certainty how planets are built. The prevailing theory is that they are formed during the collapse of a

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.^[17][18] 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.^[19] 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.^[20][21]

Supernova remnant ejecta producing planet-forming material

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 density, with higher density materials sinking toward the core.^[22] 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.^[23] (Smaller planets will lose any atmosphere they gain through various escape mechanisms.^[24])

With the discovery and observation of

According to the IAU definition, there are eight planets in the Solar System, which are (in increasing distance from the Sun):^[1] Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Jupiter is the largest, at 318 Earth masses, whereas Mercury is the smallest, at 0.055 Earth masses.^[28]

The planets of the Solar System can be divided into categories based on their composition. Terrestrials are similar to Earth, with bodies largely composed of rock and metal: Mercury, Venus, Earth, and Mars. Earth is the largest terrestrial planet.^[29] Giant planets are significantly more massive than the terrestrials: Jupiter, Saturn, Uranus, and Neptune.^[29] They differ from the terrestrial planets in composition. The gas giants, Jupiter and Saturn, are primarily composed of hydrogen and helium and are the most massive planets in the Solar System. Saturn is one third as massive as Jupiter, at 95 Earth masses.^[30] The 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).^[30]

Dwarf planets are gravitationally rounded, but have not cleared their orbits of other bodies. In increasing order of average distance from the Sun, the ones generally agreed among astronomers are Ceres, Pluto, Haumea, Quaoar, Makemake, Gonggong, Eris, and Sedna.^[31][32] Ceres is the largest object in the asteroid belt, located between the orbits of Mars and Jupiter. The other seven all orbit beyond Neptune. Pluto, Haumea, Quaoar, and Makemake orbit in the Kuiper belt, which is a second belt of small Solar System bodies beyond the orbit of Neptune. Gonggong and Eris orbit in the scattered disc, which is somewhat further out and, unlike the Kuiper belt, is unstable towards interactions with Neptune. Sedna is the largest known detached 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 eight 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 known.^[33][34]

There are at least nineteen planetary-mass moons or satellite planets—moons large enough to take on ellipsoidal shapes:^[3]

• One satellite of Earth: the Moon
• Four satellites of Jupiter: Io, Europa, Ganymede, and Callisto
• Seven satellites of Saturn:
  Mimas, Enceladus, Tethys, Dione, Rhea, Titan, and Iapetus
• Five satellites of Uranus: Miranda, Ariel, Umbriel, Titania, and Oberon
• One satellite of Neptune: Triton
• One satellite of Pluto: Charon

The Moon, Io, and Europa have compositions similar to the terrestrial planets; the others are made of ice and rock like the dwarf planets, with Tethys 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.^[3][35]) 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 a geophysical planet, at about six millionths of Earth's mass, though there are many larger bodies that may not be geophysical planets (e.g. Orcus and Salacia).^[31]

Exoplanets

An exoplanet (exoplanet) is a planet outside the Solar System. As of 1 April 2024, there are 5,653 confirmed exoplanets in 4,161 planetary systems, with 896 systems having more than one planet.^[37] Known exoplanets range in size from gas giants about twice as large as Jupiter down to just over the size of the Moon. Analysis of gravitational microlensing data suggests a minimum average of 1.6 bound planets for every star in the Milky Way.^[38]

In early 1992, radio astronomers

There are types of planets that do not exist in the Solar System: super-Earths and mini-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.^[53] The planet Gliese 581c, with mass 5.5–10.4 times the mass of Earth,^[54] attracted attention upon its discovery for potentially being in the habitable zone,^[55] though later studies concluded that it is actually too close to its star to be habitable.^[56] Planets more massive than Jupiter are also known, extending seamlessly into the realm of brown dwarfs.^[57]

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, but ultra-short period planets can orbit in less than a day. The Kepler-11 system has five of its planets in shorter orbits than Mercury's, all of them much more massive than Mercury. There are hot Jupiters, such as 51 Pegasi b,^[41] that orbit very close to their star and may evaporate to become chthonian 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.^[58]

Attributes

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 commonly observed in exoplanets.^[59]

Dynamic characteristics

Orbit

In the Solar System, all the planets orbit the Sun in the same direction as the Sun rotates:

• The eccentricity 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.^[61] The comets and many Kuiper belt objects, as well as several exoplanets, have very high eccentricities, and thus exceedingly elliptical orbits.^[63][64]
• The
  semi-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.^[61]
• The
  inclination 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 the ecliptic. For exoplanets, the plane, known as the sky plane or plane of the sky, is the plane perpendicular to the observer's line of sight from Earth.^[65] 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.^[66] 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 orbits retrograde, i.e. in the direction opposite to its parent planet's rotation.^[67]
• The points at which a planet crosses above and below its reference plane are called its
  descending nodes.^[61] The longitude of the ascending node is the angle between the reference plane's 0 longitude and the planet's ascending node. The argument of periapsis (or perihelion in the Solar System) is the angle between a planet's ascending node and its closest approach to its star.^[61]

Planets have varying degrees of axial tilt; they spin at an angle to the

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 contributes 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.^[80] 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.^[81] The rotational periods of exoplanets are not known, but for hot Jupiters, their proximity to their stars means that they are tidally 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.^[82] 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),^[83] and Venus' rotation may be in equilibrium between tidal forces slowing it down and atmospheric tides created by solar heating speeding it up.^[84][85]

All the large moons are tidally locked to their parent planets;

The defining dynamic characteristic of a planet, according to the IAU definition, 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

A planet's defining physical characteristic is that it is massive enough for the force of its own gravity to dominate over the

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

All of the Solar System planets except Mercury^[116] have substantial atmospheres because their gravity is strong enough to keep gases close to the surface. Saturn's largest moon Titan also has a substantial atmosphere thicker than that of Earth;^[117] Neptune's largest moon Triton^[118] and the dwarf planet Pluto have more tenuous atmospheres.^[119] 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.^[120] Analysis of exoplanets suggests that the threshold for being able to hold on to these light gases occurs at about 2.0+0.7
−0.6 M_E, so that Earth and Venus are near the maximum size for rocky planets.^[53]

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.^[121] The atmospheres of Mars and Venus are both dominated by carbon dioxide, but differ drastically in density: the average surface pressure of Mars' atmosphere is less than 1% that of Earth's (too low to allow liquid water to exist),^[122] while the average surface pressure of Venus' atmosphere is about 92 times that of Earth's.^[123] It is likely that Venus' atmosphere was the result of a runaway greenhouse effect in its history, which today makes it the hottest planet by surface temperature, hotter even than Mercury.^[124] Despite hostile surface conditions, temperature, and pressure at about 50–55 km altitude in Venus' 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 future human exploration.^[125] Titan has the only nitrogen-rich planetary atmosphere in the Solar System other than Earth's. Just as Earth's conditions are close to the triple point of water, allowing it to exist in all three states on the planet's surface, so Titan's are to the triple point of methane.^[126]

Planetary atmospheres are affected by the varying

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.[131]^[132] These planets may have vast differences in temperature between their day and night sides that produce supersonic winds,^[133] although multiple factors are involved and the details of the atmospheric dynamics that affect the day-night temperature difference are complex.^[134][135]

Magnetosphere

One important characteristic of the planets is their intrinsic

In 2003, a team of astronomers in Hawaii observing the star HD 179949 detected a bright spot on its surface, apparently created by the magnetosphere of an orbiting hot Jupiter.^[138][139]

Secondary characteristics

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 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 have natural 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).^{[140][141][142][143][144]}

The four giant planets are orbited by

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 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 encompassed asteroids, moons, and dwarf planets like Pluto,^{[151][152][153]} and there continues to be some disagreement today.^[153]

Ancient civilizations and classical planets

The five

The first civilization known to have a functional theory of the planets were the

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.^[163][173] 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.^{[158][173][174]}

Medieval astronomy

After the fall of the Western Roman Empire, astronomy developed further in India and the medieval Islamic world. 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 theorised that the orbits of planets were elliptical.^[175] 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.^[176]

The astronomy of the

With the advent of the

When four satellites of Jupiter (the Galilean moons) and five of Saturn were discovered in the 17th century, they were thought of as "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.^[153]

In the first decade of the 19th century, four new 'planets' were discovered: Ceres (in 1801), Pallas (in 1802), Juno (in 1804), and Vesta (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 (the asteroid belt), with sometimes overlapping orbits. This was an area where only one planet had been expected, and they were much 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 them asteroids (from the Greek for "starlike") because even in the largest telescopes they resembled stars, without a resolvable disk.^[152][182]

The situation was stable for four decades, but in the 1840s several additional asteroids were discovered (

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 be another 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's general relativity.^[184][185]

In the 1950s, Gerard Kuiper published papers on the origin of the asteroids. He recognised that asteroids were typically not spherical, as had previously been thought, and that the asteroid 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.^[183]

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.^[153] (This is not exactly the same as the definition used in the previous century, which classed all satellites as secondary planets, even non-round ones like Saturn's Hyperion or Mars' Phobos and Deimos.)^[192][193]

Redefining the term planet

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

The announcement of Eris in 2005, an object 27% more massive than Pluto, created the impetus for an official definition of a planet,^[194] 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.^[153] 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.^[196][197] Many astronomers argued that the public would not accept a definition creating a large number of planets.^[153]

To acknowledge the problem, the International Astronomical Union (IAU) set about creating the definition 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 as dwarf planets, provided they are not natural 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.^[198] After much discussion, it was decided via a vote that those bodies should instead be classified as dwarf planets.^[191][199]

Criticisms and alternatives to IAU definition

The IAU definition has not been universally used or accepted. In planetary geology, celestial objects have been assessed and defined as planets by geophysical characteristics. Planetary scientists are more interested in planetary geology than dynamics, so they classify planets based on their geological properties. 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 of hydrostatic equilibrium where the body acquires a stable, round shape, which is adopted as the hallmark of planethood by geophysical definitions. For example:^[200]

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.^[201]

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.^[3] (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,^[202] but is universally included as a planet regardless.)^[203] Proponents of such definitions often argue that location should not matter and that planethood should be defined by the intrinsic properties of an object.^[3] Dwarf planets had been proposed as a category of small planet (as opposed to planetoids as sub-planetary objects) and planetary geologists continue to treat them as planets despite the IAU definition.^[31]

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.^[31] There is general consensus on Ceres in the asteroid belt^[204] and on the seven trans-Neptunians that probably cross this threshold – Quaoar, Sedna, Pluto, Haumea, Eris, Makemake, and Gonggong.^[205][32]

Planetary geologists may include the nineteen known

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.^[209] The formula produces a value called π that is greater than 1 for planets.^[b] 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 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.^[210]

Exoplanets

Even before the discovery of exoplanets, there were particular disagreements over whether an object should be considered a planet if it was part of a distinct population such as a

In 1992, astronomers Aleksander Wolszczan and Dale Frail announced the discovery of planets around a pulsar, PSR B1257+12.^[39] This discovery is generally considered to be the first definitive detection of a planetary system around another star. Then, on 6 October 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).^[212]

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 as brown dwarfs. Brown dwarfs are generally considered stars due to their theoretical ability to fuse deuterium, a heavier isotope of hydrogen. 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.^[213]

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.^[214] In 2018, this definition was reassessed and updated as knowledge of exoplanets increased.^[215] The current official working definition of an exoplanet is as follows:^[98]

1. 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/M_central < 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.
2. Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed nor where they are located.
3. Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).^[98]

The IAU noted that this definition could be expected to evolve as knowledge improves.^[98] 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.^[215]

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.^[216][217] 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.^[57] 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.^[57][53]

Thus, many catalogues of exoplanets include objects heavier than 13 Jupiter masses, sometimes going up to 60 Jupiter masses.^{[218][99][100][219]} (The limit for hydrogen burning and becoming a red dwarf star is about 80 Jupiter masses.)^[57] 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.^[57]

Mythology and naming

The naming of planets differs between planets of the Solar System and exoplanets (planets of other planetary systems). exoplanets are commonly named after their parent star and their order of discovery within its planetary system, such as Proxima Centauri b.

The names for the planets of the

• Helios and Selene were the names of both planets and gods, both of them Titans (later supplanted by Olympians Apollo 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.^[163]

Although modern Greeks still use their ancient names for the planets, other European languages, because of the influence of the

Anglo-Saxon gods considered similar or equivalent to Mars, Mercury, Jupiter, and Venus, respectively.^[227]

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,