Planet

Mercury Venus
Earth
Mars
(c) ESA & MPS for OSIRIS Team MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA, CC BY-SA IGO 3.0
Jupiter Saturn
Uranus Neptune
The eight known planets of the Solar System, according to the IAU definition:
Mercury, Venus, Earth, and Mars
Jupiter and Saturn (gas giants)
Uranus and Neptune (ice giants)

Shown in order from the Sun and in true color. Sizes are not to scale.

A planet is a large astronomical body that is neither a star nor a stellar remnant. There are competing scientific definitions of a 'planet'. In the dynamicist definition adopted by the International Astronomical Union (IAU), a planet is a non-stellar body that is massive enough to be rounded by its own gravity, that directly orbits a star, and that has cleared its orbital zone of competing objects. The IAU has also declared that there are eight planets in the Solar System, independently of the formal definition.[a][1][2] In the geological definition used by most planetologists, a planet is a rounded sub-stellar body, possibly a satellite. In addition to the eight Solar planets accepted by the IAU, these include dwarf planets such as Eris and Pluto and planetary-mass moons.[3] Bodies meeting the geological definition are sometimes called "planetary-mass objects" or "planemos" for short.

The term planet is ancient, with ties to history, astrology, science, mythology and religion. Apart from the Moon, five planets are visible to the naked eye in the night sky. Planets were regarded by many early cultures as emissaries of deities or as divine themselves. As scientific knowledge advanced, human perception of the planets changed, and the invention of the telescope enabled the discovery of additional planetary objects that were diverse in size, shape and orbit. In 2006, the IAU adopted a resolution limiting the number of planets within the Solar System, though they are not followed by all astronomers, especially planetologists. The IAU resolution is controversial because it excludes many geologically active planetary-mass objects due to where or what they orbit.

Ptolemy thought that the planets orbited Earth in deferent and epicycle motions. Although the idea that the planets orbited the Sun had been suggested before, it wasn't until the 17th century that this view was supported by the concrete evidence, in the form of telescopic observations performed by Galileo Galilei. About the same time, by careful analysis of pre-telescopic observational data collected by Tycho Brahe, Johannes Kepler discovered that the planets' orbits were elliptical rather than circular. As observational tools improved, astronomers saw that, like Earth, each of the planets rotated around an axis tilted with respect to its orbital pole, and that some shared such features as ice caps and seasons. Since the dawn of the Space Age, close observations by space probes have found that Earth and other planets share additional characteristics such as volcanism, hurricanes, tectonics and even hydrology.

The eight Solar planets in the IAU definition are divided into two divergent types: large low-density giant planets and small rocky terrestrial planets. In order of increasing distance from the Sun, they are the four terrestrials: Mercury, Venus, Earth, and Mars; and the four giants: Jupiter, Saturn, Uranus, and Neptune. Six are orbited by natural satellites, the two exceptions being the innermost planets Mercury and Venus. Under geophysical definitions, the classification is more complex: the Moon and Jupiter's moons Io and Europa[b] are additional terrestrial planets, but a large number of small icy planets are also added, such as the dwarf planets Ceres and Pluto and the other large giant-planet moons such as Ganymede, Callisto, and Titan.

Beginning at the end of the twentieth century, several thousand planets have been discovered orbiting other stars. These are referred to as "extrasolar planets", or "exoplanets" for short. As of 1 January 2022, 4,905 extrasolar planets have been discovered in 3,629 planetary systems. The count includes 808 multi-planetary systems. Known exoplanets range in size from gas giants about twice as large as Jupiter down to just over the size of the Moon. More than 100 of these planets are approximately the size as Earth, nine of which orbit in the habitable zone of their star.[6][7] In 2011, the Kepler Space Telescope team reported the discovery of the first Earth-sized extrasolar planets orbiting a Sun-like star, Kepler-20e[8] and Kepler-20f.[9][10][11][12] A 2012 study, analyzing gravitational microlensing data, estimates a minimum of 1.6 bound planets on average for every star in the Milky Way.[13] As of 2013, one in five Sun-like[c] stars is thought to have an Earth-sized[d] planet in its habitable[e] zone.[14][15]

History

Printed rendition of a geocentric cosmological model from Cosmographia, Antwerp, 1539

The idea of planets has evolved over its history, from the divine lights of antiquity to the earthly objects of the scientific age. The concept has expanded to include worlds not only in the Solar System, but in hundreds of other extrasolar systems. The ambiguities inherent in defining planets have led to much scientific controversy.

The five classical planets of the Solar System, being visible to the naked eye, have been known since ancient times and have had a significant impact on mythology, religious cosmology, and ancient astronomy. In ancient times, astronomers noted how certain lights moved across the sky, as opposed to the "fixed stars", which maintained a constant relative position in the sky.[16] Ancient Greeks called these lights πλάνητες ἀστέρες (planētes asteres, "wandering stars") or simply πλανῆται (planētai, "wanderers"),[17] from which today's word "planet" was derived.[18][19][20] In ancient Greece, China, Babylon, and indeed all pre-modern civilizations,[21][22] it was almost universally believed that Earth was the center of the Universe and that all the "planets" circled Earth. The reasons for this perception were that stars and planets appeared to revolve around Earth each day[23] and the apparently common-sense perceptions that Earth was solid and stable and that it was not moving but at rest.

Babylon

The first civilization known to have a functional theory of the planets were the Babylonians, who lived in Mesopotamia in the first and second millennia BC. The oldest surviving planetary astronomical text is the Babylonian Venus tablet of Ammisaduqa, a 7th-century BC copy of a list of observations of the motions of the planet Venus, that probably dates as early as the second millennium BC.[24] The MUL.APIN is a pair of cuneiform tablets dating from the 7th century BC that lays out the motions of the Sun, Moon, and planets over the course of the year.[25] The Babylonian astrologers also laid the foundations of what would eventually become Western astrology.[26] The Enuma anu enlil, written during the Neo-Assyrian period in the 7th century BC,[27] comprises a list of omens and their relationships with various celestial phenomena including the motions of the planets.[28][29] Venus, Mercury, and the outer planets Mars, Jupiter, and Saturn were all identified by Babylonian astronomers. These would remain the only known planets until the invention of the telescope in early modern times.[30]

Greco-Roman astronomy

Ptolemy's 7 planetary spheres
1
Moon
☾
2
Mercury
☿
3
Venus
♀
4
Sun
☉
5
Mars
♂
6
Jupiter
♃
7
Saturn
♄

The ancient Greeks initially did not attach as much significance to the planets as the Babylonians. The Pythagoreans, in the 6th and 5th centuries BC appear to have developed their own independent planetary theory, which consisted of the Earth, Sun, Moon, and planets revolving around a "Central Fire" at the center of the Universe. Pythagoras or Parmenides is said to have been the first to identify the evening star (Hesperos) and morning star (Phosphoros) as one and the same (Aphrodite, Greek corresponding to Latin Venus),[31] though this had long been known by the Babylonians. In the 3rd century BC, Aristarchus of Samos proposed a heliocentric system, according to which Earth and the planets revolved around the Sun. The geocentric system remained dominant until the Scientific Revolution.

By the 1st century BC, during the Hellenistic period, the Greeks had begun to develop their own mathematical schemes for predicting the positions of the planets. These schemes, which were based on geometry rather than the arithmetic of the Babylonians, would eventually eclipse the Babylonians' theories in complexity and comprehensiveness, and account for most of the astronomical movements observed from Earth with the naked eye. These theories would reach their fullest expression in the Almagest written by Ptolemy in the 2nd century CE. So complete was the domination of Ptolemy's model that it superseded all previous works on astronomy and remained the definitive astronomical text in the Western world for 13 centuries.[24][32] 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.[20][32][33]

India

In 499 CE, the Indian astronomer Aryabhata propounded a planetary model that explicitly incorporated Earth's rotation about its axis, which he explains as the cause of what appears to be an apparent westward motion of the stars. He also believed that the orbits of planets are elliptical.[34] 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.[35]

In 1500, Nilakantha Somayaji of the Kerala school of astronomy and mathematics, in his Tantrasangraha, revised Aryabhata's model.[36] In his Aryabhatiyabhasya, a commentary on Aryabhata's Aryabhatiya, he developed a planetary model where Mercury, Venus, Mars, Jupiter and Saturn orbit the Sun, which in turn orbits Earth, similar to the Tychonic system later proposed by Tycho Brahe in the late 16th century. Most astronomers of the Kerala school who followed him accepted his planetary model.[36][37]

Medieval Muslim astronomy

In the 11th century, the transit of Venus was observed by Avicenna, who established that Venus was, at least sometimes, below the Sun.[38] In the 12th century, Ibn Bajjah observed "two planets as black spots on the face of the Sun", which was later identified as a transit of Mercury and Venus by the Maragha astronomer Qotb al-Din Shirazi in the 13th century.[39] Ibn Bajjah could not have observed a transit of Venus, because none occurred in his lifetime.[40]

European Renaissance

Renaissance planets,
c. 1543 to 1610
1
Mercury
☿
2
Venus
♀
3
Earth
🜨
4
Moon
☾
5
Mars
♂
6
Jupiter
♃
7
Saturn
♄

With the advent of the Scientific Revolution and the heliocentric model of Copernicus, Galileo and Kepler, use of the term "planet" changed from something that moved around the sky relative to the fixed 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[41] and the Sun was removed. The Copernican count of primary planets stood until 1781, when Uranus was discovered.

Satellite planets, 1787
TerrestrialJovianSaturnianUranian
Moon
☾
Jupiter I (Io)
Jupiter II (Europa)
Jupiter III (Ganymede)
Jupiter IV (Callisto)
Saturn I (now III Tethys)
Saturn II (now IV Dione)
Saturn III (now V Rhea)
Saturn IV (now VI Titan)
Saturn V (now VIII Iapetus)
Uranus I (now III Titania)
Uranus II (now IV Oberon)

When four satellites of Jupiter 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, and it's not always clear whether they were still considered to be planets. The last satellites to be explicitly called "planets" in their discovery reports were Uranus' Titania and Oberon in 1787,[42] though references to "secondary planets" can be found even after that.[43]

19th century

Primary planets, 1807–1845
1
Mercury
☿
2
Venus
♀
3
Earth
🜨
4
Mars
♂
5
Vesta
⚶
6
Juno
⚵
7
Ceres
⚳
8
Pallas
⚴
9
Jupiter
♃
10
Saturn
♄
11
Uranus
⛢

In the first decade of the 19th century, four new planets were discovered: Ceres, Pallas, Juno, and Vesta. However, 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, where only one planet had been expected, and they were much much smaller; indeed, it was suspected that they might be shards of a larger planet that had broken up. They were called "asteroid" because even in the largest telescopes they resembled stars, without a resolvable disk.

Major planets, 1854–1930
1
Mercury
☿
2
Venus
♀
3
Earth
🜨
4
Mars
♂
5
Jupiter
♃
6
Saturn
♄
7
Uranus
⛢
8
Neptune
♆

The situation was stable for four decades, but in the mid-1840s several additional asteroids were discovered, and soon new "planets" were discovered every year. As a result, and although they would continue to be called "planets" into the 21st century, astronomers began tabulating the asteroids (minor planets) separately from the major planets, and assigning them numbers instead of abstract planetary symbols.[44]

20th century

Major Solar planets, 1930–2006
1
Mercury
☿
2
Venus
♀
3
Earth
🜨
4
Mars
♂
5
Jupiter
♃
6
Saturn
♄
7
Uranus
⛢
8
Neptune
♆
9
Pluto
♇

Pluto was discovered in 1930. After initial observations led to the belief that it was larger than Earth,[45] 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 of Neptune,[46] and Fred Whipple suggested in 1964 that Pluto may be a comet.[47] As it was still larger than all known asteroids and the population of dwarf planets and other trans-Neptunian objects was not well observed,[48] it kept its status until 2006.

In 1992, astronomers Aleksander Wolszczan and Dale Frail announced the discovery of planets around a pulsar, PSR B1257+12.[49] This discovery is generally considered to be the first definitive detection of a planetary system around another star. Then, on October 6, 1995, Michel Mayor and Didier Queloz of the Geneva Observatory announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[50]

The discovery of extrasolar planets led to another ambiguity in defining a planet: the point at which a planet becomes a star. Many known extrasolar planets are many times the mass of Jupiter, approaching that of stellar objects known as brown dwarfs. Brown dwarfs are generally considered stars due to their 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.[51]

21st century

Solar planets 2006–present (dynamical definition)
1
Mercury
☿
2
Venus
♀
3
Earth
🜨
4
Mars
♂
5
Jupiter
♃
6
Saturn
♄
7
Uranus
⛢
8
Neptune
♆
Consensus dwarf planets 2007–present
Ceres
⚳
Orcus
Orcus symbol (Moskowitz, fixed width).svg
Pluto
⯓
Haumea
Haumea symbol (Moskowitz, fixed width).svg
Quaoar
Quaoar symbol (Moskowitz, fixed width).svg
Makemake
Makemake symbol (Moskowitz, fixed width).svg
Gonggong
Gonggong symbol (Moskowitz, fixed width).svg
Eris
⯰
Sedna
⯲
Satellite planets 1978–present
EarthJupiterSaturnUranusNeptunePluto
Moon
☾
Io
Europa
Ganymede
Callisto
Mimas
Enceladus
Tethys
Dione
Rhea
Titan
Iapetus
Miranda
Ariel
Umbriel
Titania
Oberon
TritonCharon

With the discovery during the latter half of the 20th century of more objects within the Solar System and large objects around other stars, disputes arose over what should constitute a planet. There were particular disagreements over whether an object should be considered a planet if it was part of a distinct population such as a belt, or if it was large enough to generate energy by the thermonuclear fusion of deuterium.

A growing number of astronomers argued for Pluto to be declassified as a planet, because many similar objects approaching its size had been found in the same region of the Solar System (the Kuiper belt) during the 1990s and early 2000s. Pluto was found to be just one small body in a population of thousands.

Some of them, such as Quaoar, Sedna, and Eris, were heralded in the popular press as the tenth planet. The announcement of Eris in 2005, an object 27% more massive than Pluto, created the impetus for an official definition of a planet.

Acknowledging the problem, the IAU set about creating the definition of planet, and produced one in August 2006. Their definition dropped to the eight significantly larger bodies that had cleared their orbit (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), and a new class of dwarf planets was created, initially containing three objects (Ceres, Pluto and Eris).[52]

This definition has not been universally accepted. 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.[53] The number of dwarf planets even among known objects is not certain, but there is general consensus on Ceres in the asteroid belt and on at least eight trans-Neptunians: Quaoar, Sedna, Orcus, Pluto, Haumea, Eris, Makemake, and Gonggong.[54] Planetary geologists also typically include the nineteen known planetary-mass moons as "satellite planets", including Earth's Moon, like the early modern astronomers.[55] Some go even further and include relatively large, geologically evolved bodies that are nonetheless not very round today, such as Pallas and Vesta, though not all planetary geologists do so.[4]

Extrasolar planets

There is no official definition of extrasolar planets. In 2003, the International Astronomical Union (IAU) Working Group on Extrasolar Planets issued a position statement, but this position statement was never proposed as an official IAU resolution and was never voted on by IAU members. The positions statement incorporates the following guidelines, mostly focused upon the boundary between planets and brown dwarfs:[2]

  1. Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 times the mass of Jupiter for objects with the same isotopic abundance as the Sun[56]) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass and size required for an extrasolar object to be considered a planet should be the same as that used in the Solar System.[2]
  2. Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed or where they are located.[2]
  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).[2]

This working definition was amended by the IAU's Commission F2: Exoplanets and the Solar System in August 2018.[57] The official working definition of an exoplanet is now as follows:

  • Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars, brown dwarfs or stellar remnants and that have a mass ratio with the central object below the L4/L5 instability (M/Mcentral < 2/(25+621) are "planets" (no matter how they formed).
  • The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System.[57]

The IAU noted that this definition could be expected to evolve as knowledge improves.[57]

One definition of a sub-brown dwarf is a planet-mass object that formed through cloud collapse rather than accretion. This formation distinction between a sub-brown dwarf and a planet is not universally agreed upon; astronomers are divided into two camps as whether to consider the formation process of a planet as part of its division in classification.[58] One reason for the dissent is that often it may not be possible to determine the formation process. For example, a planet formed by accretion around a star may get ejected from the system to become free-floating, and likewise a sub-brown dwarf that formed on its own in a star cluster through cloud collapse may get captured into orbit around a star.

One study suggests that objects above 10 MJup formed through gravitational instability and should not be thought of as planets.[59]

The 13 Jupiter-mass cutoff represents an average mass rather than a precise threshold value. Large objects will fuse most of their deuterium and smaller ones will fuse only a little, and the 13 MJ value is somewhere in between. In fact, calculations show that an object fuses 50% of its initial deuterium content when the total mass ranges between 12 and 14 MJ.[60] The amount of deuterium fused depends not only on mass but also on the composition of the object, on the amount of helium and deuterium present.[61] As of 2011 the Extrasolar Planets Encyclopaedia included objects up to 25 Jupiter masses, saying, "The fact that there is no special feature around 13 MJup in the observed mass spectrum reinforces the choice to forget this mass limit".[62] As of 2016 this limit was increased to 60 Jupiter masses[63] based on a study of mass–density relationships.[64] The Exoplanet Data Explorer includes objects up to 24 Jupiter masses with the advisory: "The 13 Jupiter-mass distinction by the IAU Working Group is physically unmotivated for planets with rocky cores, and observationally problematic due to the sin i ambiguity."[65] The NASA Exoplanet Archive includes objects with a mass (or minimum mass) equal to or less than 30 Jupiter masses.[66]

Another criterion for separating planets and brown dwarfs, rather than deuterium fusion, formation process or location, is whether the core pressure is dominated by coulomb pressure or electron degeneracy pressure.[67][68]

In close binary star systems one of the stars can lose mass to a heavier companion. Accretion-powered pulsars may drive mass loss. The shrinking star can then become a planetary-mass object. An example is a Jupiter-mass object orbiting the pulsar PSR J1719-1438.[69] These shrunken white dwarfs may become a helium planet or carbon planet.

A 2016 study suggests that when classified by mass and radius, brown dwarfs are indistinguishable from high-mass planets, and that a change happens only with the onset of hydrogen burning at about 0.080 ± 0.008 M.[70]

Several computer simulations of stellar and planetary system formation have suggested that some objects of planetary mass would be ejected into interstellar space.[71] Such objects are typically called rogue planets or free-floating planets.[72] Sub-brown dwarfs might be considered as rogue planets, or they might be considered as planetary-mass brown dwarfs.[73] Rogue planets in stellar clusters have similar velocities to the stars and so can be recaptured. They are typically captured into wide orbits between 100 and 105 AU. The capture efficiency decreases with increasing cluster volume, and for a given cluster size it increases with the host/primary mass. It is almost independent of the planetary mass. Single and multiple planets could be captured into arbitrary unaligned orbits, non-coplanar with each other or with the stellar host spin, or pre-existing planetary system.[74]

2006 IAU definition of planet

Euler diagram showing the types of bodies in the Solar System.

The matter of the lower limit was addressed during the 2006 meeting of the IAU's General Assembly. After much debate and one failed proposal, a large majority of those remaining at the meeting voted to pass a resolution. The 2006 resolution defines planets within the Solar System as follows:[1]

A "planet" [1] is a celestial body inside the Solar System that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.


[1] The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

Under this definition, the Solar System is considered to have eight planets. Bodies that fulfill the first two conditions but not the third (such as Ceres, Pluto, and Eris) are classified as dwarf planets, provided they are not also natural satellites of other planets. Originally an IAU committee had proposed a definition that would have included a much larger number of planets as it did not include (c) as a criterion.[75] After much discussion, it was decided via a vote that those bodies should instead be classified as dwarf planets.[76]

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

The end product of secondary disk accretion is a small number of relatively large bodies (planets) in either non-intersecting or resonant orbits, which prevent collisions between them. Minor planets and comets, including KBOs [Kuiper belt objects], differ from planets in that they can collide with each other and with planets.[77]

The 2006 IAU definition presents some challenges for exoplanets because the language is specific to the Solar System and because the criteria of roundness and orbital zone clearance are not presently observable.

Margot's criterion

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.[78][79] The formula produces a value[f] called π that is greater than 1 for planets. The eight known planets and all known exoplanets have π values above 100, while Ceres, Pluto, and Eris have π values of 0.1, or less. Objects with π values of 1 or more are also expected to be approximately spherical, so that objects that fulfill the orbital zone clearance requirement automatically fulfill the roundness requirement.[80]

Geophysical definitions

The planetary-mass moons compared in size with Mercury, Venus, Earth, Mars, and Pluto. Also included are Neptune's moons Proteus and Nereid, as they are similar in size to Saturn's smallest round moon Mimas, although Proteus is known not to be round and smaller Nereid is not expected to be round either.

The IAU definition is not fully accepted by all astronomers and planetary scientists. Planetary scientists are often interested in planetary geology rather than dynamics: a celestial body may have a dynamic (planetary) geology at approximately the mass required for its mantle to become plastic under its own weight (hydrostatic equilibrium), which results in the body acquiring a round shape. This is adopted as the hallmark of planethood by geophysical definitions, for example:[81]

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.[82]

In the Solar System, this mass is generally less than the mass required for a body to clear its orbit, and 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] Proponents of such definitions often argue that location should not matter and that planethood should be defined by the intrinsic properties of an object.

Geophysical definitions also often do not require planets to orbit stars, so that round satellites such as our moon or Jupiter's Galilean moons are also considered planets.[4] They are then sometimes called "satellite planets".

Some other words have been used for the bodies meeting geophysical definitions of "planet", such as "planetary-mass object", "planemo",[83] "world",[84] or "planetary body".

Mythology and naming

The Greek gods of Olympus, after whom the Solar System's Roman names of the planets are derived

The names for the planets in the Western world are derived from the naming practices of the Romans, which ultimately derive from those of the Greeks and the Babylonians. In ancient Greece, the two great luminaries the Sun and the Moon were called Helios and Selene, two ancient Titanic deities; the slowest planet (Saturn) was called Phainon, the shiner; followed by Phaethon (Jupiter), "bright"; the red planet (Mars) was known as Pyroeis, the "fiery"; the brightest (Venus) was known as Phosphoros, the light bringer; and the fleeting final planet (Mercury) was called Stilbon, the gleamer. The Greeks also assigned each planet to one among their pantheon of gods, the Olympians and the earlier Titans:

  • 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.[24]

The Greek practice of grafting their gods' names onto the planets was almost certainly borrowed from the Babylonians. The Babylonians named Phosphoros [Venus] after their goddess of love, Ishtar; Pyroeis [Mars] after their god of war, Nergal, Stilbon [Saturn] after their god of wisdom Nabu, and Phaethon [Jupiter] after their chief god, Marduk.[85] There are too many concordances between Greek and Babylonian naming conventions for them to have arisen separately.[24] The translation was not perfect. For instance, the Babylonian Nergal was a god of war, and thus the Greeks identified him with Ares. Unlike Ares, Nergal was also god of pestilence and the underworld.[86]

Today, most people in the western world know the planets by names derived from the Olympian pantheon of gods. Although modern Greeks still use their ancient names for the planets, other European languages, because of the influence of the Roman Empire and, later, the Catholic Church, use the Roman (Latin) names rather than the Greek ones. The Romans, who, like the Greeks, were Indo-Europeans, shared with them a common pantheon under different names but lacked the rich narrative traditions that Greek poetic culture had given their gods. During the later period of the Roman Republic, Roman writers borrowed much of the Greek narratives and applied them to their own pantheon, to the point where they became virtually indistinguishable.[87] When the Romans studied Greek astronomy, they gave the planets their own gods' names: Mercurius (for Hermes), Venus (Aphrodite), Mars (Ares), Iuppiter (Zeus) and Saturnus (Cronus). When subsequent planets were discovered in the 18th and 19th centuries, the naming practice was retained with Neptūnus (Poseidon). Uranus is unique in that it is named for a Greek deity rather than his Roman counterpart.

Ceres, Orcus, Pluto, and Eris continued the Roman and Greek scheme; however, the other consensus dwarf planets are named after gods and goddesses from other cultures (e.g. Quaoar is named after a Tongva god). Objects beyond Neptune follow various naming conventions depending on their orbits: those in the 2:3 resonance with Neptune (the plutinos) are given names from underworld myths, while others are given names from creation myths.

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 from Shakespeare and Pope (originally specifically from fairy mythology, befitting Uranus as god of the sky and air, but that ended with the naming of Miranda). Neptune's planetary-mass moon Triton is named after the god's son; Pluto's planetary-mass moon Charon is named after the ferryman of the dead, who carries the souls of the newly deceased to the underworld (Pluto's domain).

Some Romans, following a belief possibly originating in Mesopotamia but developed in Hellenistic Egypt, believed that the seven gods after whom the planets were named took hourly shifts in looking after affairs on Earth. The order of shifts went Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon (from the farthest to the closest planet).[88] Therefore, the first day was started by Saturn (1st hour), second day by Sun (25th hour), followed by Moon (49th hour), Mars, Mercury, Jupiter and Venus. Because each day was named by the god that started it, this is also the order of the days of the week in the Roman calendar after the Nundinal cycle was rejected – and still preserved in many modern languages.[89] In English, Saturday, Sunday, and Monday are straightforward translations of these Roman names. The other days were renamed after Tīw (Tuesday), Wōden (Wednesday), Þunor (Thursday), and Frīġ (Friday), the Anglo-Saxon gods considered similar or equivalent to Mars, Mercury, Jupiter, and Venus, respectively.

Earth is the only planet whose name in English is not derived from Greco-Roman mythology. Because it was only generally accepted as a planet in the 17th century,[41] there is no tradition of naming it after a god. (The same is true, in English at least, of the Sun and the Moon, though they are no longer generally considered planets.) The name originates from the Old English word eorþe, which was the word for "ground" and "dirt" as well as the Earth itself.[90] As with its equivalents in the other Germanic languages, it derives ultimately from the Proto-Germanic word erþō, as can be seen in the English earth, the German Erde, the Dutch aarde, and the Scandinavian jord. Many of the Romance languages retain the old Roman word terra (or some variation of it) that was used with the meaning of "dry land" as opposed to "sea".[91] The non-Romance languages use their own native words. The Greeks retain their original name, Γή (Ge).

Non-European cultures use other planetary-naming systems. India uses a system based on the Navagraha, which incorporates the seven traditional planets (Surya for the Sun, Chandra for the Moon, Budha for Mercury, Shukra for Venus, Mangala for Mars, Bṛhaspati for Jupiter, and Shani for Saturn) and the ascending and descending lunar nodes Rahu and Ketu.

China and the countries of eastern Asia historically subject to Chinese cultural influence (such as Japan, Korea and Vietnam) use a naming system based on the five Chinese elements: water (Mercury), metal (Venus), fire (Mars), wood (Jupiter) and earth (Saturn).[89]

In traditional Hebrew astronomy, the seven traditional planets have (for the most part) descriptive names – the Sun is חמה Ḥammah or "the hot one," the Moon is לבנה Levanah or "the white one," Venus is כוכב נוגה Kokhav Nogah or "the bright planet," Mercury is כוכב Kokhav or "the planet" (given its lack of distinguishing features), Mars is מאדים Ma'adim or "the red one," and Saturn is שבתאי Shabbatai or "the resting one" (in reference to its slow movement compared to the other visible planets).[92] The odd one out is Jupiter, called צדק Tzedeq or "justice". Steiglitz suggests that this may be a euphemism for the original name of כוכב בעל Kokhav Ba'al or "Baal's planet", seen as idolatrous and euphemized in a similar manner to Ishbosheth from II Samuel.[92]

In Arabic, Mercury is عُطَارِد (ʿUṭārid, cognate with Ishtar / Astarte), Venus is الزهرة (az-Zuhara, "the bright one",[93] an epithet of the goddess Al-'Uzzá[94]), Earth is الأرض (al-ʾArḍ, from the same root as eretz), Mars is اَلْمِرِّيخ (al-Mirrīkh, meaning "featherless arrow" due to its retrograde motion[95]), Jupiter is المشتري (al-Muštarī, "the reliable one", from Akkadian[96]) and Saturn is زُحَل (Zuḥal, "withdrawer"[97]).[98][99]

Formation

An artist's impression of protoplanetary disk

It is not known with certainty how planets are formed. The prevailing theory is that they are formed during the collapse of a nebula into a thin disk of gas and dust. A protostar forms at the core, surrounded by a rotating protoplanetary disk. Through accretion (a process of sticky collision) dust particles in the disk steadily accumulate mass to form ever-larger bodies. Local concentrations of mass known as planetesimals form, and these accelerate the accretion process by drawing in additional material by their gravitational attraction. These concentrations become ever denser until they collapse inward under gravity to form protoplanets.[100] After a planet reaches a mass somewhat larger than Mars' mass, it begins to accumulate an extended atmosphere,[101] greatly increasing the capture rate of the planetesimals by means of atmospheric drag.[102][103] Depending on the accretion history of solids and gas, a giant planet, an ice giant, or a terrestrial planet may result.[104][105][106] It is thought that the regular satellites of Jupiter, Saturn, and Uranus formed in a similar way; however, Triton was likely captured by Neptune, and Earth's Moon and Pluto's Charon might have formed in collisions.

Asteroid collision - building planets (artist concept).

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.[107][108] Thereafter there still may be many protoplanets orbiting the star or each other, but over time many will collide, either to form a single larger planet or release material for other larger protoplanets or planets to absorb.[109] Those objects that have become massive enough will capture most matter in their orbital neighbourhoods to become planets. Protoplanets that have avoided collisions may become natural satellites of planets through a process of gravitational capture, or remain in belts of other objects to become either dwarf planets or small bodies.

The energetic impacts of the smaller planetesimals (as well as radioactive decay) will heat up the growing planet, causing it to at least partially melt. The interior of the planet begins to differentiate by mass, developing a denser core.[110] 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.[111] (Smaller planets will lose any atmosphere they gain through various escape mechanisms.)

With the discovery and observation of planetary systems around stars other than the Sun, it is becoming possible to elaborate, revise or even replace this account. The level of metallicity—an astronomical term describing the abundance of chemical elements with an atomic number greater than 2 (helium)—is now thought to determine the likelihood that a star will have planets.[112] Hence, it is thought that a metal-rich population I star will likely have a more substantial planetary system than a metal-poor, population II star.

Supernova remnant ejecta producing planet-forming material.

Solar System

Solar System – sizes but not distances are to scale
The Sun and the eight planets of the Solar System
The inner planets, Mercury, Venus, Earth, and Mars
The four giant planets Jupiter, Saturn, Uranus, and Neptune against the Sun and some sunspots

According to the IAU definition, there are eight planets in the Solar System, which are in increasing distance from the Sun:[1]

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

Jupiter is the largest, at 318 Earth masses, whereas Mercury is the smallest, at 0.055 Earth masses.

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

  • Terrestrials: Planets that are similar to Earth, with bodies largely composed of rock and metal: Mercury, Venus, Earth, and Mars. At 0.055 Earth masses, Mercury is the smallest terrestrial planet (and smallest planet) in the Solar System. Earth is the largest terrestrial planet.
  • Giant planets (Jovians): Massive planets significantly more massive than the terrestrials: Jupiter, Saturn, Uranus, and Neptune.
    • Gas giants: Jupiter and Saturn, are giant planets primarily composed of hydrogen and helium and are the most massive planets in the Solar System. Jupiter, at 318 Earth masses, is the largest planet in the Solar System. Saturn is one third as massive, at 95 Earth masses.
    • Ice giants: Uranus and Neptune, are primarily composed of low-boiling-point materials such as water, methane, and ammonia, with thick atmospheres of hydrogen and helium. They have a significantly lower mass than the gas giants (only 14 and 17 Earth masses).

The number of geophysical planets in the Solar System is unknown - previously considered to be potentially in the hundreds, but now only estimated at only the low double digits.[113] These include the eight classical planets, as well as two more populations. Nine objects are generally agreed to be dwarf planets, with some others being disputed candidates. Dwarf planets are gravitationally rounded, but do not clear their orbits. In increasing order of average distance from the Sun, they are:

  1. Ceres symbol (fixed width).svg Ceres
  2. Orcus symbol (Moskowitz, fixed width).svg Orcus
  3. Pluto monogram.svg Pluto symbol (fixed width).svg Pluto
  4. Haumea symbol (Moskowitz, fixed width).svg Haumea
  5. Quaoar symbol (Moskowitz, fixed width).svg Quaoar
  6. Makemake symbol (Moskowitz, fixed width).svg Makemake
  7. Gonggong symbol (Moskowitz, fixed width).svg Gonggong
  8. Eris symbol (Moskowitz, fixed width).svg Eris
  9. Sedna symbol (Moskowitz, fixed width).svg Sedna

Ceres is the largest object in the asteroid belt, between the orbits of Mars and Jupiter. The other eight all orbit beyond Neptune. Orcus, Pluto, Haumea, Quaoar, and Makemake orbit in the Kuiper belt, which is a second asteroid belt 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 nine of these would be considered icy planets: they are similar to terrestrial planets in having a solid surface, but they are made of ice and rock, rather than rock and metal. All of them are smaller than Mercury, with Pluto being the largest known dwarf planet, and Eris being the most massive known.

There are also at least nineteen planetary-mass moons or satellite planets, i.e. moons large enough to take on ellipsoidal shapes. The nineteen generally agreed are:

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.) 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.

Planetary attributes

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)
NameEquatorial
diameter[g]
Mass[g]Semi-major axis (AU)Orbital period
(years)
Inclination
to Sun's equator
(°)
Orbital
eccentricity
Rotation period
(days)
Confirmed
moons
Axial tilt (°)RingsAtmosphere
1.Mercury0.3830.060.390.243.380.20658.6500.10nominimal
2.Venus0.9490.810.720.623.860.007−243.020177.30noCO2, N2
3.Earth(a)1.0001.001.001.007.250.0171.00123.44noN2, O2, Ar
4.Mars0.5320.111.521.885.650.0931.03225.19noCO2, N2, Ar
5.Jupiter11.209317.835.2011.866.090.0480.41803.12yesH2, He
6.Saturn9.44995.169.5429.455.510.0540.448326.73yesH2, He
7.Uranus4.00714.5419.1984.026.480.047−0.722797.86yesH2, He, CH4
8.Neptune3.88317.1530.07164.796.430.0090.671429.60yesH2, He, CH4
Color legend:  terrestrial planets   gas giants   ice giants (both are giant planets).

(a) Find absolute values in article Earth

Exoplanets

Exoplanets, by year of discovery, through September 2014.

An exoplanet (extrasolar planet) is a planet outside the Solar System. As of 1 January 2022, there are 4,905 confirmed exoplanets in 3,629 planetary systems, with 808 systems having more than one planet.[114][115][116][117]

In early 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12.[49] This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of giant planets that survived the supernova and then decayed into their current orbits.

Sizes of Kepler Planet Candidates – based on 2,740 candidates orbiting 2,036 stars as of 4 November 2013 (NASA).

The first confirmed discovery of an extrasolar planet orbiting an ordinary main-sequence star occurred on 6 October 1995, when Michel Mayor and Didier Queloz of the University of Geneva announced the detection of an exoplanet around 51 Pegasi. From then until the Kepler mission most known extrasolar planets were gas giants comparable in mass to Jupiter or larger as they were more easily detected. The catalog of Kepler candidate planets consists mostly of planets the size of Neptune and smaller, down to smaller than Mercury.

There are types of planets that do not exist in the Solar System: super-Earths and mini-Neptunes, which could be rocky like Earth or a mixture of volatiles and gas like Neptune—the dividing line between the two is currently thought to occur at about twice the mass of Earth.[70] There are hot Jupiters that orbit very close to their star and may evaporate to become chthonian planets, which are the leftover cores. Another possible type of planet is carbon planets, which form in systems with a higher proportion of carbon than in the Solar System.

A 2012 study, analyzing gravitational microlensing data, estimates an average of at least 1.6 bound planets for every star in the Milky Way.[13]

On 20 December 2011, the Kepler Space Telescope team reported the discovery of the first Earth-size exoplanets, Kepler-20e[8] and Kepler-20f,[9] orbiting a Sun-like star, Kepler-20.[10][11][12]

Around 1 in 5 Sun-like stars have an "Earth-sized"[d] planet in the habitable[e] zone, so the nearest would be expected to be within 12 light-years distance from Earth.[14][118] The frequency of occurrence of such terrestrial planets is one of the variables in the Drake equation, which estimates the number of intelligent, communicating civilizations that exist in the Milky Way.[119]

There are exoplanets that are much closer to their parent star than any planet in the Solar System is to the Sun, and there are also exoplanets that are much farther from their star. Mercury, the closest planet to the Sun at 0.4 AU, takes 88 days for an orbit, but the shortest known orbits for exoplanets take only a few hours, see Ultra-short period planet. The Kepler-11 system has five of its planets in shorter orbits than Mercury's, all of them much more massive than Mercury. Neptune is 30 AU from the Sun and takes 165 years to orbit, but there are exoplanets that are hundreds of AU from their star and take more than a thousand years to orbit, e.g. 1RXS1609 b.

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 also commonly observed in extrasolar planets.

Dynamic characteristics

Orbit

The orbit of the planet Neptune compared to that of Pluto. 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 the Sun rotates (counter-clockwise as seen from above the Sun's north pole). At least one extrasolar planet, WASP-17b, has been found to orbit in the opposite direction to its star's rotation.[120] The period of one revolution of a planet's orbit is known as its sidereal period or year.[121] A planet's year depends on its distance from its star; the farther a planet is from its star, not only the longer the distance it must travel, but also the slower its speed, because it is less affected by its star's gravity. No planet's orbit is perfectly circular, and hence the distance of each varies over the course of its year. The closest approach to its star is called its periastron (perihelion in the Solar System), whereas its farthest separation from the star is called its apastron (aphelion). As a planet approaches periastron, its speed increases as it trades gravitational potential energy for kinetic energy, just as a falling object on Earth accelerates as it falls; as the planet reaches apastron, its speed decreases, just as an object thrown upwards on Earth slows down as it reaches the apex of its trajectory.[122]

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

  • The eccentricity of an orbit describes how elongated a planet's orbit is. Planets with low eccentricities have more circular orbits, whereas planets with high eccentricities have more elliptical orbits. The planets in the Solar System have very low eccentricities, and thus nearly circular orbits.[121] Comets and Kuiper belt objects (as well as several extrasolar planets) have very high eccentricities, and thus exceedingly elliptical orbits.[123][124]
  • Illustration of the semi-major axis
    The semi-major axis is the distance from a planet to the half-way point along the longest diameter of its elliptical orbit (see image). This distance is not the same as its apastron, because no planet's orbit has its star at its exact centre.[121]
  • The inclination of a planet tells how far above or below an established reference plane its orbit lies. In the Solar System, the reference plane is the plane of Earth's orbit, called the ecliptic. For extrasolar planets, 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.[125] The eight planets of the Solar System all lie very close to the ecliptic; comets and Kuiper belt objects like Pluto are at far more extreme angles to it.[126] The points at which a planet crosses above and below its reference plane are called its ascending and descending nodes.[121] 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.[121]

Axial tilt

Earth's axial tilt is about 23.4°. It oscillates between 22.1° and 24.5° on a 41,000-year cycle and is currently decreasing.

Planets also have varying degrees of axial tilt; they lie at an angle to the plane of their stars' equators. This causes the amount of light received by each hemisphere to vary over the course of its year; when the northern hemisphere points away from its star, the southern hemisphere points towards it, and vice versa. Each planet therefore has seasons, changes to the climate over the course of its year. The time at which each hemisphere points farthest or nearest from its star is known as its solstice. Each planet has two in the course of its orbit; when one hemisphere has its summer solstice, when its day is longest, the other has its winter solstice, when its day is shortest. The varying amount of light and heat received by each hemisphere creates annual changes in weather patterns for each half of the planet. Jupiter's axial tilt is very small, so its seasonal variation is minimal; Uranus, on the other hand, has an axial tilt so extreme it is virtually on its side, which means that its hemispheres are either perpetually in sunlight or perpetually in darkness around the time of its solstices.[127] Among extrasolar planets, axial tilts are not known for certain, though most hot Jupiters are believed to have negligible to no axial tilt as a result of their proximity to their stars.[128]

Rotation

The planets rotate around invisible axes through their centres. A planet's rotation period is known as a stellar day. Most of the planets in the Solar System rotate in the same direction as they orbit the Sun, which is counter-clockwise as seen from above the Sun's north pole, the exceptions being Venus[129] and Uranus,[130] 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.[131] Regardless of which convention is used, Uranus has a retrograde rotation relative to its orbit.

The rotation of a planet can be induced by several factors during formation. A net angular momentum can be induced by the individual angular momentum contributions of accreted objects. The accretion of gas by the giant planets can also contribute to the angular momentum. Finally, during the last stages of planet building, a stochastic process of protoplanetary accretion can randomly alter the spin axis of the planet.[132] 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.[133] The rotational periods of extrasolar planets are not known. However, for "hot" Jupiters, their proximity to their stars means that they are tidally locked (i.e., their orbits are in sync with their rotations). This means, they always show one face to their stars, with one side in perpetual day, the other in perpetual night.[134]

Orbital clearing

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 planetesimals in its orbit. In effect, it orbits its star in isolation, as opposed to sharing its orbit with a multitude of similar-sized objects. This characteristic was mandated as part of the IAU's official definition of a planet in August 2006. This criterion excludes such planetary bodies as Pluto, Eris and Ceres from full-fledged planethood, making them instead dwarf planets.[1] Although to date this criterion only applies to the Solar System, a number of young extrasolar systems have been found in which evidence suggests orbital clearing is taking place within their circumstellar discs.[135]

Physical characteristics

Size and shape

A planet's size is defined at least by an average radius (e.g., Earth radius, Jupiter radius, etc.); polar and equatorial radii of a spheroid or more general triaxial ellipsoidal shapes are often estimated (e.g., reference ellipsoid). Derived quantities include the flattening, surface area, and volume. Knowing further the rotation rate and mass, allows the calculation of normal gravity.

Mass

A planet's defining physical characteristic is that it is massive enough for the force of its own gravity to dominate over the electromagnetic forces binding its physical structure, leading to a state of hydrostatic equilibrium. 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.[136]

Mass is also the prime attribute by which planets are distinguished from stars. While the lower stellar mass limit is estimated to be around 75 times that of Jupiter (MJ), the upper planetary mass limit for planethood is only roughly 13 MJ for objects with solar-type isotopic abundance, beyond which it achieves conditions suitable for nuclear fusion. Other than the Sun, no objects of such mass exist in the Solar System; but there are exoplanets of this size. The 13 MJ limit is not universally agreed upon and the Extrasolar Planets Encyclopaedia includes objects up to 60 MJ,[63] and the Exoplanet Data Explorer up to 24 MJ.[137]

The smallest known exoplanet with an accurately known mass is PSR B1257+12A, one of the first extrasolar planets discovered, which was found in 1992 in orbit around a pulsar. Its mass is roughly half that of the planet Mercury.[7] Even smaller is WD 1145+017 b, orbiting a white dwarf; its mass is roughly that of the dwarf planet Haumea. That said, this object might not qualify as a planet under all definitions. The smallest known planet orbiting a main-sequence star other than the Sun is Kepler-37b, with a mass (and radius) that is probably slightly higher than that of the Moon.

Internal differentiation

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 a differentiated interior consisting of a dense planetary core surrounded by a mantle that either is or was a fluid. The terrestrial planets are sealed within hard crusts,[138] but in the giant planets the mantle simply blends into the upper cloud layers. The terrestrial planets have cores of elements such as iron and nickel, and mantles of silicates. Jupiter and Saturn are believed to have cores of rock and metal surrounded by mantles of metallic hydrogen.[139] Uranus and Neptune, which are smaller, have rocky cores surrounded by mantles of water, ammonia, methane and other ices.[140] The fluid action within these planets' cores creates a geodynamo that generates a magnetic field.[138]

Atmosphere

Earth's atmosphere

All of the Solar System planets except Mercury[141] have substantial atmospheres because their gravity is strong enough to keep gases close to the surface. The larger giant planets are massive enough to keep large amounts of the light gases hydrogen and helium, whereas the smaller planets lose these gases into space.[142] 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.[143]

Planetary atmospheres are affected by the varying insolation or internal energy, leading to the formation of dynamic weather systems such as hurricanes (on Earth), planet-wide dust storms (on Mars), a greater-than-Earth-sized anticyclone on Jupiter (called the Great Red Spot), and holes in the atmosphere (on Neptune).[127] At least one extrasolar planet, HD 189733 b, has been claimed to have such a weather system, similar to the Great Red Spot but twice as large.[144]

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.[145][146] These planets may have vast differences in temperature between their day and night sides that produce supersonic winds,[147] although the day and night sides of HD 189733 b appear to have very similar temperatures, indicating that that planet's atmosphere effectively redistributes the star's energy around the planet.[144]

Magnetosphere

One important characteristic of the planets is their intrinsic magnetic moments, which in turn give rise to magnetospheres. The presence of a magnetic field indicates that the planet is still geologically alive. In other words, magnetized planets have flows of electrically conducting material in their interiors, which generate their magnetic fields. These fields significantly change the interaction of the planet and solar wind. A magnetized planet creates a cavity in the solar wind around itself called the magnetosphere, which the wind cannot penetrate. The magnetosphere can be much larger than the planet itself. In contrast, non-magnetized planets have only small magnetospheres induced by interaction of the ionosphere with the solar wind, which cannot effectively protect the planet.[148]

Of the eight planets in the Solar System, only Venus and Mars lack such a magnetic field.[148] In addition, the moon of Jupiter Ganymede also has one. Of the magnetized planets the magnetic field of Mercury is the weakest, and is barely able to deflect the solar wind. Ganymede's magnetic field is several times larger, and Jupiter's is the strongest in the Solar System (so strong in fact that it poses a serious health risk to future crewed missions to all its moons but Callisto). The magnetic fields of the other giant planets are roughly similar in strength to that of Earth, but their magnetic moments are significantly larger. The magnetic fields of Uranus and Neptune are strongly tilted relative the rotational axis and displaced from the centre of the planet.[148]

In 2004, a team of astronomers in Hawaii observed an extrasolar planet around the star HD 179949, which appeared to be creating a sunspot on the surface of its parent star. The team hypothesized that the planet's magnetosphere was transferring energy onto the star's surface, increasing its already high 7,760 °C temperature by an additional 400 °C.[149]

Several planets or dwarf planets in the Solar System (such as Neptune and Pluto) have orbital periods that are in resonance with each other or with smaller bodies. This is also common in satellite systems (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. 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).[150][151][152]

The four giant planets are also orbited by planetary rings of varying size and complexity. The rings are composed primarily of dust or particulate matter, but can host tiny 'moonlets' whose gravity shapes and maintains their structure. Although the origins of planetary rings is not precisely known, they are believed to be the result of natural satellites that fell below their parent planet's Roche limit and were torn apart by tidal forces.[153][154]

No secondary characteristics have been observed around extrasolar planets. The sub-brown dwarf Cha 110913-773444, which has been described as a rogue planet, is believed to be orbited by a tiny protoplanetary disc[155] and the sub-brown dwarf OTS 44 was shown to be surrounded by a substantial protoplanetary disk of at least 10 Earth masses.[156]

See also

  • Double planet – A binary system where two planetary-mass objects share an orbital axis external to both – Two planetary mass objects orbiting each other
  • List of exoplanets
  • List of hypothetical Solar System objects
  • List of landings on extraterrestrial bodies
  • Lists of planets – A list of lists of planets sorted by diverse attributes
  • Mesoplanet – Planetary objects that have a mass smaller than Mercury but larger than Ceres
  • Minor planet – Astronomical object in direct orbit around the Sun that is neither a planet or a comet
  • Planetary habitability – Extent to which a planet is suitable for life as we know it
  • Planetary mnemonic – Phrase used to remember the names of planets
  • Planetary science – Science of planets and planetary systems
  • Planets in astrology – Role and significance of celestial objects in the field of astrology
  • Planets in science fiction – Planet that only appears in works of fiction
  • Theoretical planetology

Notes

  1. ^ This definition is drawn from two separate IAU declarations; a formal definition agreed by the IAU in 2006 (IAU Resolution 5A), and an informal working definition proposed in a position statement by an IAU Working Group in 2001/2003 for objects outside of the Solar System (no corresponding IAU resolution). The official 2006 definition applies only to the Solar System, whereas the 2003 working definition applies to planets of other stars. The extrasolar planet issue was deemed too complex to resolve at the 2006 IAU conference. Rogue planets are not addressed.
  2. ^ Some authors consider Europa icy because of its surface ice layer,[4] but its high density indicates that it is mostly rocky.[5]
  3. ^ Data for G-type stars like the Sun is not available. This statistic is an extrapolation from data on K-type stars.
  4. ^ a b For the purpose of this 1 in 5 statistic, Earth-sized means 1–2 Earth radii
  5. ^ a b For the purpose of this 1 in 5 statistic, "habitable zone" means the region with 0.25 to 4 times Earth's stellar flux (corresponding to 0.5–2 AU for the Sun).
  6. ^ Margot's parameter[80] is not to be confused with the famous mathematical constant π≈3.14159265 ... .
  7. ^ a b Measured relative to Earth.

References

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External links

Media files used on this page

Earth symbol (fixed width).svg
Author/Creator: Denis Moskowitz, Licence: CC BY-SA 4.0
U+1F728 🜨: Planetary symbol for the Earth; Daltonian symbol for sulphur when red
Wiktionary-logo-en-v2.svg
Author/Creator: Dan Polansky based on work currently attributed to Wikimedia Foundation but originally created by Smurrayinchester, Licence: CC BY-SA 4.0
A logo derived from File:WiktionaryEn.svg, a logo showing a 3 x 3 matrix of variously rotated tiles with a letter or character on each tile. The derivation consisted in removing the tiles that form the background of each of the shown characters. File:WiktionaryEn.svg is under Creative Commons Attribution-Share Alike, created by Smurrayinchester, and attributed to Wikimedia Foundation. This is the version without the wordmark.
Solar system.jpg
This is a montage of planetary images taken by spacecraft managed by the Jet Propulsion Laboratory in Pasadena, CA. Included are (from top to bottom) images of Mercury, Venus, Earth (and Moon), Mars, Jupiter, Saturn, Uranus and Neptune. The spacecraft responsible for these images are as follows:
  • the Mercury image was taken by Mariner 10,
  • the Venus image by Magellan,
  • the Earth and Moon images by Galileo,
  • the Mars image by Mars Global Surveyor,
  • the Jupiter image by Cassini, and
  • the Saturn, Uranus and Neptune images by Voyager.
  • Pluto is not shown as it is no longer a planet. The inner planets (Mercury, Venus, Earth, Moon, and Mars) are roughly to scale to each other; the outer planets (Jupiter, Saturn, Uranus, and Neptune) are roughly to scale to each other. PIA 00545 is the same montage with Neptune shown larger in the foreground. Actual diameters are given below:
  • Sun (to photosphere) 1,392,684 km
  • Mercury 4,879.4 km
  • Venus 12,103.7 km
  • Earth 12,756.28 km
  • Moon 3,476.2 km
  • Mars 6,804.9 km
  • Jupiter 142,984 km
  • Saturn 120,536 km
  • Uranus 51,118 km
  • Neptune 49,528 km
Crab Nebula.jpg
This is a mosaic image, one of the largest ever taken by NASA's Hubble Space Telescope, of the Crab Nebula, a six-light-year-wide expanding remnant of a star's supernova explosion. Japanese and Chinese astronomers recorded this violent event in 1054 CE, as did, almost certainly, Native Americans.

The orange filaments are the tattered remains of the star and consist mostly of hydrogen. The rapidly spinning neutron star embedded in the center of the nebula is the dynamo powering the nebula's eerie interior bluish glow. The blue light comes from electrons whirling at nearly the speed of light around magnetic field lines from the neutron star. The neutron star, like a lighthouse, ejects twin beams of radiation that appear to pulse 30 times a second due to the neutron star's rotation. A neutron star is the crushed ultra-dense core of the exploded star.

The Crab Nebula derived its name from its appearance in a drawing made by Irish astronomer Lord Rosse in 1844, using a 36-inch telescope. When viewed by Hubble, as well as by large ground-based telescopes such as the European Southern Observatory's Very Large Telescope, the Crab Nebula takes on a more detailed appearance that yields clues into the spectacular demise of a star, 6,500 light-years away.

The newly composed image was assembled from 24 individual Wide Field and Planetary Camera 2 exposures taken in October 1999, January 2000, and December 2000. The colors in the image indicate the different elements that were expelled during the explosion. Blue in the filaments in the outer part of the nebula represents neutral oxygen, green is singly-ionized sulfur, and red indicates doubly-ionized oxygen.
The Earth seen from Apollo 17 with transparent background.png
"The Blue Marble" is a famous photograph of the Earth taken on December 7, 1972 by the crew of the Apollo 17 spacecraft en route to the Moon at a distance of about 29,000 kilometers (18,000 statute miles). It shows Africa, Antarctica, and the Arabian Peninsula.
PDS 70.jpg
(c) ESO/A. Müller et al., CC BY 4.0
This spectacular image from the SPHERE instrument on ESO's Very Large Telescope is the first clear image of a planet caught in the very act of formation around the dwarf star PDS 70. The planet stands clearly out, visible as a bright point to the right of the centre of the image, which is blacked out by the coronagraph mask used to block the blinding light of the central star.
He1523a.jpg
Author/Creator: ESO, European Southern Observatory, Licence: CC BY 4.0
Artist's impression of "the oldest star of our Galaxy": HE 1523-0901
  • About 13.2 billion years old
  • Approximately 7500 light years far from Earth
  • Published as part of Hamburg/ESO Survey in the May 10 2007 issue of The Astrophysical Journal
RocketSunIcon.svg
Author/Creator: Me, Licence: Copyrighted free use
SVG replacement for File:Spaceship and the Sun.jpg. A stylized illustration of a spaceship and the sun, based on the description of the emblem of the fictional Galactic Empire in Isaac Asimov's Foundation series ("The golden globe with its conventionalized rays, and the oblique cigar shape that was a space vessel"). This image could be used as a icon for science-fiction related articles.
Earth-moon.jpg
This view of the rising Earth greeted the Apollo 8 astronauts as they came from behind the Moon after the fourth nearside orbit. Earth is about five degrees above the horizon in the photo. The unnamed surface features in the foreground are near the eastern limb of the Moon as viewed from Earth. The lunar horizon is approximately 780 kilometers from the spacecraft. Width of the photographed area at the horizon is about 175 kilometers. On the Earth 240,000 miles away, the sunset terminator bisects Africa.
Gas Giants & The Sun in 1,000 km.jpg
The gas giants against the Sun's limb, at 1 px = 1 Mm The diameters are to scale. The limb of the Sun is in the background. From left to right, Jupiter, Saturn, Uranus and Neptune.
Top of Atmosphere.jpg
View of the crescent moon through the top of the Earth's atmosphere. Photographed above 21.5°N, 113.3°E by International Space Station crew Expedition 13 over the South China Sea, just south of Macau (NASA image ID: ISS013-E-54329).
Saturn symbol (fixed width).svg
Author/Creator: Denis Moskowitz, Licence: CC BY-SA 4.0
Planetary symbol for Saturn, ♄ U+2644
Sedna symbol (Moskowitz, fixed width).svg
Author/Creator: Denis Moskowitz, Licence: CC BY-SA 4.0
Planetary symbol for Sedna. It's common in astrology (e.g. in the default and free Astronomicon fonts used by Astrolog, the oldest and most popular shareware (free) astrology program) and is supported by Unicode at U+2BF2 ⯲
Sun symbol (fixed width).svg
Author/Creator: Denis Moskowitz, Licence: CC BY-SA 4.0
Astronomical/planetary symbol for the Sun and botanical symbol for annual plants; Daltonian symbol for hydrogen when red. Philatelic symbol for a used stamp.
Makemake symbol (Moskowitz, fixed width).svg
Author/Creator: Denis Moskowitz, Licence: CC BY-SA 4.0
Planetary symbol for Haumea. It's common in astrology and has been used by NASA.[1]
Eris symbol (Moskowitz, fixed width).svg
Author/Creator: Denis Moskowitz, Licence: CC BY-SA 4.0
U+2BF0 ⯰: Planetary symbol for Eris, 0.8px lines. It's common in astrology and has been used by NASA.[1]
Mercury symbol (fixed width).svg
Author/Creator: Denis Moskowitz, Licence: CC BY-SA 4.0
Planetary symbol for Mercury
Quaoar symbol (Moskowitz, fixed width).svg
Author/Creator: Denis Moskowitz, Licence: CC BY-SA 4.0
Quaoar's planetary symbol, which is common in astrology (e.g. in the default and free Astronomicon fonts used by Astrolog, the oldest and most popular shareware (free) astrology program)
Mars symbol (fixed width).svg
Author/Creator: Denis Moskowitz, Licence: CC BY-SA 4.0
Planetary symbol for Mars
Planets2013.svg
Author/Creator: WP, Licence: CC BY-SA 3.0
Positions and names of planets in the Solar System.
Ceres symbol (fixed width).svg
Author/Creator: Denis Moskowitz, Licence: CC BY-SA 4.0
Planetary symbol for 1 Ceres (U+26B3 ⚳). 0.8px lines, capped.
Olympians.jpg
The Olympian gods. Depicted clockwise from top center are: Zeus, Hephaestus, Athena, Apollo, Hermes, Artemis, Poseidon, Eros, Aphrodite, Ares, Dionysus, Hades, Hestia, Demeter, Hera.
The Blue Marble (remastered).jpg
Full disk view of the Earth taken on December 7, 1972, by the crew of the Apollo 17 spacecraft en route to the Moon at a distance of about 29,000 kilometres (18,000 mi). It shows Africa, Antarctica, and the Arabian Peninsula.
Mercury in color - Prockter07-edit1.jpg
Enhanced-color image of Mercury from first MESSENGER flyby.
Orcus symbol (Moskowitz, fixed width).svg
Author/Creator: Denis Moskowitz, Licence: CC BY-SA 4.0
Planetary symbol for Orcus. It's common in astrology (e.g. in the default and free Astronomicon fonts used by Astrolog, the oldest and most popular shareware (free) astrology program)
Jupiter and its shrunken Great Red Spot.jpg
This full-disc image of Jupiter was taken on 21 April 2014 with Hubble's Wide Field Camera 3 (WFC3).
15-044b-SuperNovaRemnant-PlanetFormation-SOFIA-20150319.jpg
March 19, 2015

RELEASE 15-044 NASA’s SOFIA Finds Missing Link Between Supernovae and Planet Formation

http://www.nasa.gov/press/2015/march/nasa-s-sofia-finds-missing-link-between-supernovae-and-planet-formation/

SOFIA data on a supernova http://www.nasa.gov/sites/default/files/thumbnails/image/15-044a.jpg SOFIA data reveal warm dust (white) surviving inside a supernova remnant. The SNR Sgr A East cloud is traced in X-rays (blue). Radio emission (red) shows expanding shock waves colliding with surrounding interstellar clouds (green).

Using NASA's Stratospheric Observatory for Infrared Astronomy (SOFIA), an international scientific team discovered that supernovae are capable of producing a substantial amount of the material from which planets like Earth can form.

These findings are published in the March 19 online issue of Science magazine.

"Our observations reveal a particular cloud produced by a supernova explosion 10,000 years ago contains enough dust to make 7,000 Earths," said Ryan Lau of Cornell University in Ithaca, New York.

The research team, headed by Lau, used SOFIA's airborne telescope and the Faint Object InfraRed Camera for the SOFIA Telescope, FORCAST, to take detailed infrared images of an interstellar dust cloud known as Supernova Remnant Sagittarius A East, or SNR Sgr A East.

Supernova remnant dust as seen by SOFIA http://www.nasa.gov/sites/default/files/thumbnails/image/15-044b.jpg Supernova remnant dust detected by SOFIA (yellow) survives away from the hottest X-ray gas (purple). The red ellipse outlines the supernova shock wave. The inset shows a magnified image of the dust (orange) and gas emission (cyan).

The team used SOFIA data to estimate the total mass of dust in the cloud from the intensity of its emission. The investigation required measurements at long infrared wavelengths in order to peer through intervening interstellar clouds and detect the radiation emitted by the supernova dust.

Astronomers already had evidence that a supernova’s outward-moving shock wave can produce significant amounts of dust. Until now, a key question was whether the new soot- and sand-like dust particles would survive the subsequent inward “rebound” shock wave generated when the first, outward-moving shock wave collides with surrounding interstellar gas and dust.

"The dust survived the later onslaught of shock waves from the supernova explosion, and is now flowing into the interstellar medium where it can become part of the 'seed material' for new stars and planets," Lau explained.

These results also reveal the possibility that the vast amount of dust observed in distant young galaxies may have been made by supernova explosions of early massive stars, as no other known mechanism could have produced nearly as much dust.

"This discovery is a special feather in the cap for SOFIA, demonstrating how observations made within our own Milky Way galaxy can bear directly on our understanding of the evolution of galaxies billions of light years away," said Pamela Marcum, a SOFIA project scientist at Ames Research Center in Moffett Field, California.

SOFIA is a heavily modified Boeing 747 Special Performance jetliner that carries a telescope with an effective diameter of 100 inches (2.5 meters) at altitudes of 39,000 to 45,000 feet (12 to 14 km). SOFIA is a joint project of NASA and the German Aerospace Center. The aircraft observatory is based at NASA's Armstrong Flight Research Center facility in Palmdale, California. The agency’s Ames Research Center in Moffett Field, California, is home to the SOFIA Science Center, which is managed by NASA in cooperation with the Universities Space Research Association in Columbia, Maryland, and the German SOFIA Institute at the University of Stuttgart.

For more information about SOFIA, visit: http://www.nasa.gov/sofia or http://www.dlr.de/en/sofia For information about SOFIA's science mission and scientific instruments, visit: http://www.sofia.usra.edu or

http://www.dsi.uni-stuttgart.de/index.en.html
Vesta symbol (original, fixed width).svg
Author/Creator: Kwamikagami, Licence: CC BY-SA 4.0
The earliest symbol for asteroid (4) Vesta, dating from at least 1807. Stylized to match other fixed-width astronomical symbols.
Haumea symbol (Moskowitz, fixed width).svg
Author/Creator: Denis Moskowitz, Licence: CC BY-SA 4.0
Planetary symbol for Haumea. It's common in astrology and has been used by NASA.[1]
OSIRIS Mars true color.jpg
(c) ESA & MPS for OSIRIS Team MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA, CC BY-SA IGO 3.0
True color image of Mars taken by the OSIRIS instrument on the ESA Rosetta spacecraft during its February 2007 flyby of the planet. The image was generated using the OSIRIS orange (red), green, and blue filters.
Alternative description: The first true-colour image generated using the OSIRIS orange (red), green and blue colour filters. The image was acquired on 24 February 2007 at 19:28 CET from a distance of about 240 000 km; image resolution is about 5 km/pixel.
Pluto symbol (fixed width).svg
Author/Creator: Denis Moskowitz, Licence: CC BY-SA 4.0
Planetary symbol for Pluto, U+2BD3 ⯓. It's common in astrology and has been used by NASA.[1]
15-044a-SuperNovaRemnant-PlanetFormation-SOFIA-20150319.jpg
March 19, 2015

RELEASE 15-044 NASA’s SOFIA Finds Missing Link Between Supernovae and Planet Formation

http://www.nasa.gov/press/2015/march/nasa-s-sofia-finds-missing-link-between-supernovae-and-planet-formation/

SOFIA data on a supernova http://www.nasa.gov/sites/default/files/thumbnails/image/15-044a.jpg SOFIA data reveal warm dust (white) surviving inside a supernova remnant. The SNR Sgr A East cloud is traced in X-rays (blue). Radio emission (red) shows expanding shock waves colliding with surrounding interstellar clouds (green).

Using NASA's Stratospheric Observatory for Infrared Astronomy (SOFIA), an international scientific team discovered that supernovae are capable of producing a substantial amount of the material from which planets like Earth can form.

These findings are published in the March 19 online issue of Science magazine.

"Our observations reveal a particular cloud produced by a supernova explosion 10,000 years ago contains enough dust to make 7,000 Earths," said Ryan Lau of Cornell University in Ithaca, New York.

The research team, headed by Lau, used SOFIA's airborne telescope and the Faint Object InfraRed Camera for the SOFIA Telescope, FORCAST, to take detailed infrared images of an interstellar dust cloud known as Supernova Remnant Sagittarius A East, or SNR Sgr A East.

Supernova remnant dust as seen by SOFIA http://www.nasa.gov/sites/default/files/thumbnails/image/15-044b.jpg Supernova remnant dust detected by SOFIA (yellow) survives away from the hottest X-ray gas (purple). The red ellipse outlines the supernova shock wave. The inset shows a magnified image of the dust (orange) and gas emission (cyan).

The team used SOFIA data to estimate the total mass of dust in the cloud from the intensity of its emission. The investigation required measurements at long infrared wavelengths in order to peer through intervening interstellar clouds and detect the radiation emitted by the supernova dust.

Astronomers already had evidence that a supernova’s outward-moving shock wave can produce significant amounts of dust. Until now, a key question was whether the new soot- and sand-like dust particles would survive the subsequent inward “rebound” shock wave generated when the first, outward-moving shock wave collides with surrounding interstellar gas and dust.

"The dust survived the later onslaught of shock waves from the supernova explosion, and is now flowing into the interstellar medium where it can become part of the 'seed material' for new stars and planets," Lau explained.

These results also reveal the possibility that the vast amount of dust observed in distant young galaxies may have been made by supernova explosions of early massive stars, as no other known mechanism could have produced nearly as much dust.

"This discovery is a special feather in the cap for SOFIA, demonstrating how observations made within our own Milky Way galaxy can bear directly on our understanding of the evolution of galaxies billions of light years away," said Pamela Marcum, a SOFIA project scientist at Ames Research Center in Moffett Field, California.

SOFIA is a heavily modified Boeing 747 Special Performance jetliner that carries a telescope with an effective diameter of 100 inches (2.5 meters) at altitudes of 39,000 to 45,000 feet (12 to 14 km). SOFIA is a joint project of NASA and the German Aerospace Center. The aircraft observatory is based at NASA's Armstrong Flight Research Center facility in Palmdale, California. The agency’s Ames Research Center in Moffett Field, California, is home to the SOFIA Science Center, which is managed by NASA in cooperation with the Universities Space Research Association in Columbia, Maryland, and the German SOFIA Institute at the University of Stuttgart.

For more information about SOFIA, visit: http://www.nasa.gov/sofia or http://www.dlr.de/en/sofia For information about SOFIA's science mission and scientific instruments, visit: http://www.sofia.usra.edu or

http://www.dsi.uni-stuttgart.de/index.en.html
Solar system bodies rotation animation.svg
Author/Creator: cmglee, NASA and Solar System Scope, Licence: CC BY-SA 4.0
CSS3 SVG animation comparing the rotation period (sped up 10 000 times, negative values denoting retrograde), flattening and axial tilt of some bodies in the solar system by CMG Lee, with data from http://nssdc.gsfc.nasa.gov/planetary/factsheet and http://en.wikipedia.org/wiki/Axial_tilt#Solar_System_bodies and bitmaps from NASA and Solar System Scope.
Ptolemaicsystem-small.png
The scheme of the aforementioned division of spheres. · The empyrean (fiery) heaven, dwelling of God and of all the selected · 10 Tenth heaven, first cause · 9 Ninth heaven, crystalline · 8 Eighth heaven of the firmament · 7 Heaven of Saturn · 6 Jupiter · 5 Mars · 4 Sun · 3 Venus · 2 Mercury · 1 Moon
Solar System Template Final.png
Major Solar System objects. Sizes of planets and Sun are roughly to scale, but distances are not. This is not a diagram of all known moons – small gas giants' moons and Pluto's S/2011 P 1 moon are not shown.
Jupiter symbol (fixed width).svg
Author/Creator: Denis Moskowitz, Licence: CC BY-SA 4.0
Planetary symbol for Jupiter, ♃ U+2643
PIA23791-Venus-RealAndEnhancedContrastViews-20200608 (cropped2).jpg
PIA23791: Venus from Mariner 10

https://photojournal.jpl.nasa.gov/catalog/PIA23791


Click here for the combined view for PIA23791

Click on an individual image below for the larger versions:

Click here for Figure A/OLD for PIA23791

Click here for Figure B/NEW for PIA23791


As it sped away from Venus, NASA's Mariner 10 spacecraft captured this seemingly peaceful view of a planet the size of Earth, wrapped in a dense, global cloud layer. But, contrary to its serene appearance, the clouded globe of Venus is a world of intense heat, crushing atmospheric pressure and clouds of corrosive acid.

This newly processed image revisits the original data with modern image processing software. A contrast-enhanced version of this view, also provided here, makes features in the planet's thick cloud cover visible in greater detail.

The clouds seen here are located about 40 miles (60 kilometers) above the planet's surface, at altitudes where Earth-like atmospheric pressures and temperatures exist. They are comprised of sulfuric acid particles, as opposed to water droplets or ice crystals, as on Earth. These cloud particles are mostly white in appearance; however, patches of red-tinted clouds also can be seen. This is due to the presence of a mysterious material that absorbs light at blue and ultraviolet wavelengths. Many chemicals have been suggested for this mystery component, from sulfur compounds to even biological materials, but a consensus has yet to be reached among researchers.

The clouds of Venus whip around the planet at nearly over 200 miles per hour (100 meters per second), circling the globe in about four and a half days. That these hurricane-force winds cover nearly the entire planet is another unexplained mystery, especially given that the solid planet itself rotates at a very slow 4 mph (less than 2 meters per second) — much slower than Earth's rotation rate of about 1,000 mph (450 meters per second).

The winds and clouds also blow to the west, not to the east as on the Earth. This is because the planet itself rotates to the west, backward compared to Earth and most of the other planets. As the clouds travel westward, they also typically progress toward the poles; this can be seen in the Mariner 10 view as a curved spiral pattern at mid latitudes. Near the equator, instead of long streaks, areas of more clumpy, discrete clouds can be seen, indicating enhanced upwelling and cloud formation in the equatorial region, spurred on by the enhanced power of sunlight there.

This view is a false color composite created by combining images taken using orange and ultraviolet spectral filters on the spacecraft's imaging camera. These were used for the red and blue channels of the color image, respectively, with the green channel synthesized by combining the other two images.

Flying past Venus en route to the first-ever flyby of Mercury, Mariner 10 became the first spacecraft to use a gravity assist to change its flight path in order to reach another planet. The images used to create this view were acquired by Mariner 10 on Feb. 7 and 8, 1974, a couple of days after the spacecraft's closest approach to Venus on Feb. 5.

Despite their many differences, comparisons between Earth and Venus are valuable for helping to understand their distinct climate histories. Nearly 50 years after this view was obtained, many fundamental questions about Venus remain unanswered. Did Venus have oceans long ago? How has its atmosphere evolved over time, and when did its runaway greenhouse effect begin? How does Venus lose its heat? How volcanically and tectonically active has Venus been over the last billion years?

This image was processed from archived Mariner 10 data by JPL engineer Kevin M. Gill.

The Mariner 10 mission was managed by NASA's Jet Propulsion Laboratory.
Pallas symbol (fixed width).svg
Author/Creator: Kwamikagami, Licence: CC BY-SA 4.0
U+26B4 ⚴: symbol for asteroid (2) Pallas. 0.8px line weight
Euler diagram of solar system bodies.svg
Author/Creator: SounderBruce (translated version), Ariel Provost (French version), Tahc (original version), Licence: CC BY-SA 3.0
An Euler diagram showing the relationship between objects in the Solar System.
The central star has been excluded. Also excluded are artificial satellites.

Cosmic dust found within the Solar System falls under two categories depicted here: Small Solar System bodies and natural satellites. If the dust is orbiting the Sun, as with zodiacal dust, then it fits with small solar system bodies, which have no lower limit defined. And if the dust is orbiting a body other than the Sun, as with the countless particles of Saturn's rings, then these fit with natural satellites, which also have no lower limit defined. This diagram could be further refined to show how moonlets and dust fall within the category of natural satellites, with a separate set of cosmic dust orbiting the Sun falling within the category of small Solar System bodies.
W3C grn.svg The SVG code is valid.
Semimajoraxis.svg
Author/Creator: , Licence: CC BY-SA 4.0
Image shows the semi-major axis of an ellipse.
Pluto monogram.svg
Traditinoal astronomical planetary symbol for Pluto; see File:Pluto astrological - dwarf planet symbol.svg for Pluto's astrological and dwarf-planetary symbol. In Unicode this is reserved at U+2647 which renders as ♇ .
Terrestrial planet sizes2.jpg
This diagram shows the approximate relative sizes of the terrestrial planets, from left to right: Mercury, Venus, Earth and Mars. Distances are not to scale. A terrestrial planet is a planet that is primarily composed of silicate rocks. The term is derived from the Latin word for Earth, "Terra", so an alternate definition would be that these are planets which are, in some notable fashion, "Earth-like". Terrestrial planets are substantially different from gas giants, which might not have solid surfaces and are composed mostly of some combination of hydrogen, helium, and water existing in various physical states. Terrestrial planets all have roughly the same structure: a central metallic core, mostly iron, with a surrounding silicate mantle. Terrestrial planets have canyons, craters, mountains, volcanoes and secondary atmospheres.
Uranus2.jpg
This is an image of the planet Uranus taken by the spacecraft Voyager 2 in 1986. See Uranus.jpg for how Uranus would appear in visible light.
Gonggong symbol (Moskowitz, fixed width).svg
Author/Creator: Denis Moskowitz, Licence: CC BY-SA 4.0
Planetary symbol for Gonggong; common in astrology (e.g. in the default and free Astronomicon fonts used by Astrolog, the oldest and most popular shareware (free) astrology program)
Venus symbol (fixed width).svg
Author/Creator: Denis Moskowitz, Licence: CC BY-SA 4.0
Planetary symbol for Venus
Neptune symbol (fixed width).svg
Author/Creator: Denis Moskowitz, Licence: CC BY-SA 4.0
Planetary symbol for Neptune, ♆ U+2646. Approximately the alchemical symbol for quicklime/calx (the middle tine may be full length, short or absent)
Juno symbol (fixed width).svg
Author/Creator: Kwamikagami, Licence: CC BY-SA 4.0
Symbol for asteroid 3 Juno, U+26B5 ⚵. 0.8px line weight.
Uranus platinum symbol (fixed width).svg
Author/Creator: Denis Moskowitz, Licence: CC BY-SA 4.0
Planetary symbol for Uranus. U+26E2 ⛢.
Protoplanetary-disk.jpg
تصور للقرص الكوكبي الأولي
25 solar system objects smaller than Earth.jpg
Author/Creator: User:primefac, Licence: CC BY 3.0
Relative sizes of 25 solary system objects smaller than Earth. This version was created and uploaded to fix flaws in the original created by User:tony_g100. All images taken from the source image or NASA images
Jupiter interior.png
This cut-away illustrates a model of Jupiter's interior. In the upper layers the atmosphere transitions to a liquid state above a thick layer of metallic hydrogen. In the center there may be a solid core of heavier elements.
TheKuiperBelt Orbits Pluto Ecliptic.svg
Author/Creator: No machine-readable author provided. Eurocommuter~commonswiki assumed (based on copyright claims)., Licence: CC-BY-SA-3.0
Orbits of Pluto (red) and Neptune (blue); ecliptic view. Plotted by a program written by Eurocommuter.
Neptune - Voyager 2 (29347980845) flatten crop.jpg
Uploader's notes: The original NASA/Cowart PNG image has been modified by flattening (combining layers), cropping and converting to JPEG format.

Original caption released with image:
Voyager 2 Narrow Angle Camera image of Neptune taken on August 20, 1989 as the spacecraft approached the planet for a flyby on August 25. The Great Dark Spot, flanked by cirrus clouds, is at center. A smaller dark storm, Dark Spot Jr., is rotating into view at bottom left. Additionally, a patch of white cirrus clouds to its north, named "Scooter" for its rapid motion relative to other features, is visible.

This image was constructed using orange, green and synthetic violet (50/50 blend of green filter and UV filter images) taken between 626 and 643 UT.

Image Credit: NASA / JPL / Voyager-ISS / Justin Cowart
Voyager 2 - Saturn Rings - 3085 7800 2.png
Voyager 2 obtained this high-resolution picture of Saturn's rings Aug. 22, 1981, when the spacecraft was 4 million kilometers (2.5 million miles) away. Evident here are the numerous "spoke" features, in the B-ring; their very sharp, narrow appearance suggests short formation times. Scientists think electromagnetic forces are responsible in some way for these features, but no detailed theory has been worked out. Pictures such as this and analyses of Voyager 2's spoke movies may reveal more clues about the origins of these complex structures.
Saturn during Equinox (rot45).jpg
This captivating natural color view of the planet Saturn was created from images collected shortly after Cassini began its extended Equinox Mission in July 2008. (Saturn actually reached equinox on August 11, 2009.)

(This edit is a 45-degree rotation of the source image, and was done specifically for use in the w:Planet article for the purpose of fitting better visually within the montage of all 8 planets. Large triangular blank spaces in the upper left and lower right corners have been filled with a solid black background.)
Moon decrescent symbol (fixed width).svg
Author/Creator: Denis Moskowitz, Licence: CC BY-SA 4.0
Planetary symbol for the Moon
Size of Kepler Planet Candidates.jpg
Size of Kepler Planet Candidates
PIA18469-AsteroidCollision-NearStarNGC2547-ID8-2013.jpg
Building Planets Through Collisions (Artist's Concept)

http://www.jpl.nasa.gov/spaceimages/details.php?id=pia18469

http://www.jpl.nasa.gov/news/news.php?release=2014-291

http://www.nasa.gov/press/2014/august/nasas-spitzer-telescope-witnesses-asteroid-smashup/

http://www.nasa.gov/jpl/spitzer/pia18470/

Planets, including those like our own Earth, form from epic collisions between asteroids and even bigger bodies, called proto-planets. Sometimes the colliding bodies are ground to dust, and sometimes they stick together to ultimately form larger, mature planets.

This artist's conception shows one such smash-up, the evidence for which was collected by NASA's Spitzer Space Telescope. Spitzer's infrared vision detected a huge eruption around the star NGC 2547-ID8 between August 2012 and 2013. Scientists think the dust was kicked up by a massive collision between two large asteroids. They say the smashup took place in the star's "terrestrial zone," the region around stars where rocky planets like Earth take shape.

NGC 2547-ID8 is a sun-like star located about 1,200 light-years from Earth in the constellation Vela. It is about 35 million years old, the same age our young sun was when its rocky planets were finally assembled via massive collisions -- including the giant impact on proto-Earth that led to the formation of the moon. The recent impact witnessed by Spitzer may be a sign of similar terrestrial planet building. Near-real-time studies like these help astronomers understand how the chaotic process works.

NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology in Pasadena. Spacecraft operations are based at Lockheed Martin Space Systems Company, Littleton, Colorado. Data are archived at the Infrared Science Archive housed at the Infrared Processing and Analysis Center at Caltech. Caltech manages JPL for NASA.

For more information about Spitzer, visit http://spitzer.caltech.edu and http://www.nasa.gov/spitzer.
AxialTiltObliquity.png
(c) I, Dennis Nilsson, CC BY 3.0

Description of relations between Axial tilt (or Obliquity), rotation axis, plane of orbit, celestial equator and ecliptic.

Earth is shown as viewed from the Sun; the orbit direction is counter-clockwise (to the left).
Exoplanet Discovery Methods Bar.png
Bar chart of exoplanet discoveries by year, through 2015-01-01, indicating the discovery method using distinct colors:
  radial velocity (dark blue)
  transit (dark green)
  timing (dark yellow)
  direct imaging (dark red)
  microlensing (dark orange)

Exoplanet data is from the Open Exoplanet Catalogue,[1] version 298ee46

  1. Open Exoplanet Catalogue (2015-02-04). Retrieved on 2015-04-07.