From Wikipedia, the free encyclopedia

Jump to: navigation, search
Top row: Uranus, Neptune; Centre row: Earth, white dwarf star Sirius B, Venus (to scale) (see inset below for Mars and Mercury)
Top row: Mars and Mercury; bottom row: the Moon, dwarf planets Pluto and Haumea (to scale)

A planet (from the Greek πλανήτης, from the verb πλανώμαι /pla΄nome/ I wander), as defined by the International Astronomical Union (IAU), is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.[a][1][2]

The term planet is ancient, with ties to history, science, myth, and religion. The planets were originally seen by many early cultures as divine, or as emissaries of the gods. Even today, many people believe in astrology, which holds that the movement of the planets affects people's lives, although such a causation is rejected by the scientific community. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects. Even now there is no uncontested definition of what a planet is. In 2006, the IAU officially adopted a resolution defining planets within the Solar System. This definition has been both praised and criticized, and remains disputed by some scientists.

The planets were thought by Ptolemy to orbit the Earth in deferent and epicycle motions. Though the idea that the planets orbited the Sun had been suggested many times, it was not until the 17th century that this view was supported by evidence from the first telescopic astronomical observations, performed by Galileo Galilei. By careful analysis of the observation data, Johannes Kepler found the planets' orbits to be not circular, but elliptical. As observational tools improved, astronomers saw that, like Earth, the planets rotated around tilted axes, and some share such features as ice-caps and seasons. Since the dawn of the Space Age, close observation by probes has found that Earth and the other planets share characteristics such as volcanism, hurricanes, tectonics, and even hydrology. Since 1992, through the discovery of hundreds of extrasolar planets (planets around other stars), scientists are beginning to understand that planets throughout the Milky Way Galaxy share characteristics in common with our own.

Planets are generally divided into two main types: large, low-density gas giants, and smaller, rocky terrestrials. Under IAU definitions, there are eight planets in the Solar System. In order from the Sun, they are the four terrestrials, Mercury, Venus, Earth, and Mars, then the four gas giants, Jupiter, Saturn, Uranus, and Neptune. The Solar System also contains at least five dwarf planets: Ceres, Pluto (originally classified as the Solar System's ninth planet), Makemake, Haumea and Eris. With the exception of Mercury, Venus, Ceres and Makemake, all of these are orbited by one or more natural satellites.

As of March 2009, there are 344 known extrasolar planets, ranging from the size of gas giants to that of terrestrial planets.[3]



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

The idea of planets has evolved over its history, from the divine wandering stars of antiquity to the earthly objects of the scientific age. The concept has also now 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.

In ancient times, astronomers noted how certain lights moved across the sky in relation to the other stars. Ancient Greeks called these lights "πλάνητες ἀστέρες" (planetes asteres: wandering stars) or simply "πλανήτοι" (planētoi: wanderers),[4] from which the today's word "planet" was derived.[5][6] In ancient Greece, China, Babylon and indeed all pre-modern civilisations,[7][8] it was almost universally believed that Earth was in the centre of the Universe and that all the "planets" circled the Earth. The reasons for this perception were that stars and planets appeared to revolve around the Earth each day,[9] and the apparently common sense perception that the Earth was solid and stable, and that it is not moving but at rest.


The first Western civilisation known to possess 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.[10] The Babylonians also laid the foundations of what would eventually become Western astrology.[11] The Enuma anu enlil, written during the Neo-Assyrian period in the 7th century BC,[12] comprises a list of omens and their relationships with various celestial phenomena including the motions of the planets.[13] The Sumerians, predecessors of the Babylonians who are considered as one of the first civilizations and are credited with the invention of writing, had identified at least Venus by 1500 BC.[14]

Ancient Greece to Medaeval Europe

Ptolemy's "planetary spheres"
Modern Moon Mercury Venus the Sun Mars Jupiter Saturn

The ancient Greek cosmological system was taken from that of the Babylonians,[14] from whom they began to acquire astronomical learning from around 600 BC, including the constellations and the zodiac.[16] In the 6th century BC, the Babylonians' astronomical knowledge at the time was far in advance of the Greeks. The earliest known Greek sources, such as the Iliad and the Odyssey, do not mention the planets.[11]

By the first century BC, the Greeks had begun to develop their own mathematical schemes for predicting the positions of the planets. These schemes, which were based on geometry rather than the arithmetic of the Babylonians, would eventually eclipse the Babylonians' theories in complexity and comprehensiveness, and account for 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 AD. 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.[17][10]

To the Greeks and Romans there were seven known planets, each presumed to be circling the Earth according to the complex laws laid out by Ptolemy. They were, in increasing order from Earth (in Ptolemy's order): the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn.[17][18][6]

European Renaissance

Renaissance planets
Mercury Venus Earth Mars Jupiter Saturn

The five naked-eye planets may have been known since ancient times, and have had a significant impact on mythology, religious cosmology, and ancient astronomy. As scientific knowledge progressed, however, understanding of the term "planet" changed from something that moved across the sky (in relation to the star field); to a body that orbited the Earth (or that were believed to do so at the time); and in the 16th century to something that directly orbited the Sun when the heliocentric model of Copernicus, Galileo and Kepler gained sway.

Thus the Earth became included in the list of planets,[19] while the Sun and Moon were excluded. At first, when the first satellites of Jupiter and Saturn were discovered in the 17th century, the terms "planet" and "satellite" were used interchangeably – although the latter would gradually become more prevalent in the following century.[20] Until the mid-19th century, the number of "planets" rose rapidly since any newly discovered object directly orbiting the Sun was listed as a planet by the scientific community.

19th Century

Planets in early 1800s
Mercury Venus Earth Mars Vesta Juno Ceres Pallas Jupiter Saturn Uranus

In the 19th century astronomers began to realize that recently discovered bodies that had been classified as planets for almost half a century (such as Ceres, Pallas, and Vesta), were very different from the traditional ones. These bodies shared the same region of space between Mars and Jupiter (the Asteroid belt), and had a much smaller mass; as a result they were reclassified as "asteroids". In the absence of any formal definition, a "planet" came to be understood as any "large" body that orbited the Sun. Since there was a dramatic size gap between the asteroids and the planets, and the spate of new discoveries seemed to have ended after the discovery of Neptune in 1846, there was no apparent need to have a formal definition.[21]

20th Century

Planets from late 1800s to 1930
Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune

However, in the 20th century, Pluto was discovered. After initial observations led to the belief it was larger than Earth,[22] the object was immediately accepted as the ninth planet. Further monitoring found the body was actually much smaller: in 1936, Raymond Lyttleton suggested that Pluto may be an escaped satellite of Neptune,[23] and Fred Whipple suggested in 1964 that Pluto may be a comet.[24] However, as it was still larger than all known asteroids and seemingly did not exist within a larger population,[25] it kept its status until 2006.

In the 1990s and early 2000s, there was a flood of discoveries of similar objects in the same region of the Solar System (the Kuiper belt). Like Ceres and the asteroids before it, Pluto was found to be just one small body in a population of thousands. A growing number of astronomers argued for it to be declassified as a planet, since many similar objects approaching its size were found. The discovery of Eris, a more massive object widely publicised as the "tenth planet", brought things to a head. The IAU set about creating the definition of planet, and eventually produced one in 2006. The number of planets dropped to the eight significantly larger bodies that had cleared their orbit (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), and a new class of dwarf planets was created, initially containing three objects (Ceres, Pluto and Eris).[26]

Planets 1930-2006
Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto

In 1992, astronomers Aleksander Wolszczan and Dale Frail announced the discovery of planets around a pulsar, PSR B1257+12.[27] This discovery is generally considered to be the first definitive detection of a planetary system around another star. Then, on October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[28]

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".[29] Brown dwarfs are generally considered stars due to their ability to fuse deuterium, a heavier isotope of hydrogen. While stars more massive than 75 times that of Jupiter fuse hydrogen, stars of only 13 Jupiter masses can fuse deuterium. However, deuterium is quite rare, and most brown dwarfs would have ceased fusing deuterium long before their discovery, making them effectively indistinguishable from supermassive planets.[30]

As large Kuiper belt and scattered disc objects were discovered in the late 1990s and early years of the twenty-first century, a number including Quaoar, Sedna and Eris were heralded in the popular press as the 'tenth planet', however none of these received widespread scientific recognition as such, although Eris has now been classified as a Dwarf Planet.

21st Century

Planets 2006-
Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune

With the discovery during the latter half of the 20th century of more objects within the Solar System and large objects around other stars, disputes arose over what should constitute a planet. There was particular disagreement over whether an object should be considered a planet if it was part of a distinct population such as a belt, or if it was large enough to generate energy by the thermonuclear fusion of deuterium. A number including Quaoar, Sedna and Eris were heralded in the popular press as the 'tenth planet', however none of these received widespread scientific recognition as such.

Extrasolar planet definition

In 2003, The International Astronomical Union (IAU) Working Group on Extrasolar Planets made a position statement on the definition of a planet that incorporated the following working definition, mostly focused upon the boundary between planets and brown dwarves:[2]

Dwarf Planets 2006-
Ceres Pluto Makemake Haumea Eris
The Earth Dysnomia (136199) Eris Charon (134340) Pluto (136472) Makemake (136108) Haumea (90377) Sedna (90482) Orcus (50000) Quaoar (20000) Varuna File:EightTNOs.png
The IAU's 2006 decision was prompted by discovery of the largest trans-Neptunian objects.
  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[31]) 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. 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.
  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).

This definition has since been widely used by astronomers when publishing discoveries of exoplanets in academic journals.[32] Although temporary, it remains an effective working definition until a more permanent one is formally adopted. However, it does not address the dispute over the lower mass limit,[33] and so it steered clear of the controversy regarding objects within the Solar System.

2006 definition

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, the assembly voted to pass a resolution that defined planets within the Solar System as:[1]

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

Under this definition, the Solar System is considered to have eight planets. Bodies which fulfill the first two conditions but not the third (such as Pluto, Makemake 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.[34] After much discussion, it was decided via a vote that those bodies should instead be classified as dwarf planets.[35]

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:[36]

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. Asteroids and comets, including KBOs(definition needed), differ from planets in that they can collide with each other and with planets.

In the aftermath of the IAU's 2006 vote, there has been controversy and debate about the definition,[37][38] and many astronomers have stated that they will not use it.[39] Part of the dispute centres around the belief that point (c) (clearing its orbit) should not have been listed, and that those objects now categorised as dwarf planets should actually be part of a broader planetary definition. The next IAU conference is not until 2009, when modifications could be made to the IAU definition, also possibly including extrasolar planets.

Beyond the scientific community, Pluto has held a strong cultural significance for many in the general public considering its planetary status since its discovery in 1930. The discovery of Eris was widely reported in the media as the tenth planet and therefore the reclassification of all three objects as dwarf planets has attracted a lot of media and public attention as well.[40]

Former classifications

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

Bodies Notes
Sun, Moon Classified as planets in antiquity, in accordance with the definition then used.
Io, Europa, Ganymede, and Callisto The four largest moons of Jupiter, known as the Galilean moons after their discoverer Galileo Galilei. He referred to them as the "Medicean Planets" in honor of his patron, the Medici family.
Titan,[b] Iapetus,[c] Rhea,[c] Tethys,[d] and Dione[d] Five of Saturn's larger moons, discovered by Christiaan Huygens and Giovanni Domenico Cassini.
Ceres,[e] Pallas, Juno, and Vesta The first known asteroids, from their discoveries between 1801 and 1807 until their reclassification as asteroids during the 1850s.[41]

Ceres has subsequently been classified as a dwarf planet.

Astrea, Hebe, Iris, Flora, Metis, Hygeia, Parthenope, Victoria, Egeria, Irene, Eunomia More asteroids, discovered between 1845 and 1851. The rapidly expanding list of planets prompted their reclassification as asteroids by astronomers, and this was widely accepted by 1854.[42]
Pluto[f] Trans-Neptunian object with a semi-major axis beyond Neptune. In 2006, Pluto was reclassified as a dwarf planet.


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

The names for the planets in the Western world are derived from the naming practices of the Romans, which ultimately derive from those of the Greeks and the Babylonians. In ancient Greece, the two great luminaries the Sun and the Moon were called Helios and Selene; the farthest planet was called Phainon, the shiner; followed by Phaethon, "bright"; the red planet was known as Pyroeis, the "fiery"; the brightest was known as Phosphoros, the light bringer; and the fleeting final planet was called Stilbon, the gleamer. The Greeks also made each planet sacred to one of their pantheon of gods, the Olympians: Helios and Selene were the names of both planets and gods; Phainon was sacred to Kronos, the Titan who fathered the Olympians; Phaethon was sacred to Zeús, Kronos's son who deposed him as king; Pyroeis was given to Ares, son of Zeus and god of war; Phosphorus was ruled by Aphrodite, the goddess of love; and Hermes, messenger of the gods and god of learning and wit, ruled over Stilbon.[10]

The Greek practice of grafting of their gods' names onto the planets was almost certainly borrowed from the Babylonians. The Babylonians named Phosphorus after their goddess of love, Ishtar; Pyroeis after their god of war, Nergal, Stilbon after their god of wisdom Nabu, and Phaethon after their chief god, Marduk.[43] There are too many concordances between Greek and Babylonian naming conventions for them to have arisen separately.[10] The translation was not perfect. For instance, the Babylonian Nergal was a god of war, and thus the Greeks identified him with Ares. However, unlike Ares, Nergal was also god of pestilence and the underworld.[44]

Today, most people in the western world know the planets by names derived from the Olympian pantheon of gods. While modern Greeks still use their ancient names for the planets, other European languages, because of the influence of the Roman Empire and, later, the Catholic Church, use the Roman (or Latin) names rather than the Greek ones. The 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.[45] When the Romans studied Greek astronomy, they gave the planets their own gods' names: Mercurius (for Hermes), Venus (Aphrodite), Mars (Ares), Iuppiter (Zeus) and Saturnus (Kronos). When subsequent planets were discovered in the 18th and 19th centuries, the naming practice was retained: Uranus (Ouranos) and Neptūnus (Poseidon).

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).[46] Therefore, the first day was started by Saturn (1st hour), second day by Sun (25th hour), followed by Moon (49th hour), Mars, Mercury, Jupiter and Venus. Since each day was named by the god that started it, this is also the order of the days of the week in the Roman calendar after the Nundinal cycle was rejected – and still preserved many modern languages.[47] Sunday, Monday, and Saturday are straightforward translations of these Roman names. In English the other days were renamed after Tiw, (Tuesday) Wóden (Wednesday), Thunor (Thursday), and Fríge (Friday), the Anglo-Saxon gods considered similar or equivalent to Mars, Mercury, Jupiter, and Venus respectively.

Since Earth was only generally accepted as a planet in the 17th century,[19] there is no tradition of naming it after a god (the same is true, in English at least, of the Sun and the Moon, though they are no longer considered planets). The name originates from the 8th century Anglo-Saxon word erda, which means ground or soil and was first used in writing as the name of the sphere of the Earth perhaps around 1300.[48][49] It is the only planet whose name in English is not derived from Greco-Roman mythology. 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").[50] However, the non-Romance languages use their own respective native words. The Greeks retain their original name, Γή (Ge or Yi); the Germanic languages, including English, use a variation of an ancient Germanic word ertho, "ground,"[49] as can be seen in the English Earth, the German Erde, the Dutch Aarde, and the Scandinavian Jorde.

Non-European cultures use other planetary naming systems. India uses a naming system based on the Navagraha, which incorporates the seven traditional planets (Surya for the Sun, Chandra for the Moon, and Budha, Shukra, Mangala, Bṛhaspati and Shani for the traditional planets Mercury, Venus, Mars, Jupiter and Saturn) and the ascending and descending lunar nodes Rahu and Ketu. China and the countries of eastern Asia influenced by it (such as Japan, Korea and Vietnam) use a naming system based on the five Chinese elements: water (Mercury), metal (Venus), fire (Mars), wood (Jupiter) and earth (Saturn).[47]


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.[51] After a planet reaches a diameter larger than the Earth's moon, it begins to accumulate an extended atmosphere, greatly increasing the capture rate of the planetesimals by means of atmospheric drag.[52]

An artist's impression of protoplanetary disk

When the protostar has grown such that it ignites to form a star, the surviving disk is removed from the inside outward by photoevaporation, the solar wind, Poynting-Robertson drag and other effects.[53][54] 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.[55] Those objects that have become massive enough will capture most matter in their orbital neighbourhoods to become planets. Meanwhile, protoplanets that have avoided collisions may become natural satellites of planets through a process of gravitational capture, or remain in belts of other objects to become either dwarf planets or small Solar System bodies.

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.[56] 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.[57] (Smaller planets will lose any atmosphere they gain through various escape mechanisms.)

With the discovery and observation of planetary systems around stars other than our own, it is becoming possible to elaborate, revise or even replace this account. The level of metallicity – an astronomical term describing the abundance of chemical elements with an atomic number greater than 2 (helium) – is now believed to determine the likelihood that a star will have planets.[58] Hence it is thought less likely that a metal-poor, population II star will possess a more substantial planetary system than a metal-rich population I star.

Solar System

The terrestrial planets: Mercury, Venus, Earth, Mars (Sizes to scale)
The four gas giants against the Sun: Jupiter, Saturn, Uranus, Neptune (Sizes to scale)

According to the IAU's current definitions, there are eight planets in the Solar System. In increasing distance from the Sun, they are:

  1. ☿ Mercury
  2. ♀ Venus
  3. ⊕ Earth
  4. ♂ Mars
  5. ♃ Jupiter
  6. ♄ Saturn
  7. ♅ Uranus
  8. ♆ Neptune

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

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

  • Terrestrials: Planets that are similar to Earth, with bodies largely composed of rock: Mercury, Venus, Earth and Mars.
  • Gas giants (Jovians): Planets with a composition largely made up of gaseous material and are significantly more massive than terrestrials: Jupiter, Saturn, Uranus, Neptune. Ice giants, comprising Uranus and Neptune, are a sub-class of gas giants, distinguished from gas giants by their significantly lower mass, and by depletion in hydrogen and helium in their atmospheres together with a significantly higher proportion of rock and ice.
Planetary attributes
Name Equatorial
Mass[a] Orbital
radius (AU)
Orbital period
to Sun's equator
Rotation period
Rings Atmosphere
Terrestrials Mercury 0.382 0.06 0.39 0.24 3.38 0.206 58.64 no minimal
Venus 0.949 0.82 0.72 0.62 3.86 0.007 -243.02 no CO2, N2
Earth[b] 1.00 1.00 1.00 1.00 7.25 0.017 1.00 1 no N2, O2
Mars 0.532 0.11 1.52 1.88 5.65 0.093 1.03 2 no CO2, N2
Gas giants Jupiter 11.209 317.8 5.20 11.86 6.09 0.048 0.41 49 yes H2, He
Saturn 9.449 95.2 9.54 29.46 5.51 0.054 0.43 52 yes H2, He
Uranus 4.007 14.6 19.22 84.01 6.48 0.047 -0.72 27 yes H2, He
Neptune 3.883 17.2 30.06 164.8 6.43 0.009 0.67 13 yes H2, He
a  Measured relative to the Earth.
b  See Earth article for absolute values.

Dwarf planets

Before the August 2006 decision, several objects were proposed by astronomers, including at one stage by the IAU, as planets. However in 2006 several of these objects were reclassified as dwarf planets, objects distinct from planets. Currently five dwarf planets in the Solar System are recognized by the IAU: Ceres, Pluto, Haumea, Makemake and Eris. Several other objects in both the Asteroid belt and the Kuiper belt are under consideration, with as many as 50 that could eventually qualify. There may be as many as 200 that could be discovered once the Kuiper belt has been fully explored. Dwarf planets share many of the same characteristics as planets, although notable differences remain – namely that they are not dominant in their orbits. Their attributes are:

Dwarf planetary attributes
Name Equatorial
Mass[c] Orbital
radius (AU)
Orbital period
to ecliptic
Rotation period
Moons Rings Atmosphere
Ceres 0.08 0.000 2 2.5–3.0 4.60 10.59 0.080 0.38 0 no none
Pluto 0.19 0.002 2 29.7–49.3 248.09 17.14 0.249 −6.39 3 no temporary
Haumea 0.37×0.16 0.000 7 35.2–51.5 285.38 28.19 0.189 0.16 2
Makemake ~0.12 0.000 7 38.5–53.1 309.88 28.96 0.159  ? 0  ?  ? [d]
Eris 0.19 0.002 5 37.8–97.6 ~557 44.19 0.442 ~0.3 1  ?  ? [d]
c Measured relative to the Earth. d A temporary atmosphere is suspected but has not yet been directly observed by stellar occultation.

By definition, all dwarf planets are members of larger populations. Ceres is the largest body in the asteroid belt, while Pluto and Makemake are members of the Kuiper belt and Eris is a member of the scattered disc. Scientists such as Mike Brown believe that there may soon be over forty trans-Neptunian objects that qualify as dwarf planets under the IAU's recent definition.[59]

Extrasolar planets

HR 8799, the first extrasolar planetary system to be directly imaged

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. Of the 342 extrasolar planets discovered by February 2009, most have masses which are comparable to or larger than Jupiter's, though masses ranging from just below that of Mercury to many times Jupiter's mass have been observed.[60] The smallest extrasolar planets found to date have been discovered orbiting burned-out star remnants called pulsars, such as PSR B1257+12.[61] There have been roughly a dozen extrasolar planets found of between 10 and 20 Earth masses,[60] such as those orbiting the stars Mu Arae, 55 Cancri and GJ 436.[62] These planets have been nicknamed "Neptunes" because they roughly approximate that planet's mass (17 Earths).[63] Another new category are the so-called "super-Earths", possibly terrestrial planets far larger than Earth but smaller than Neptune or Uranus. To date, six possible super-Earths have been found: Gliese 876 d, which is roughly six times Earth's mass,[64] OGLE-2005-BLG-390Lb and MOA-2007-BLG-192Lb, frigid icy worlds discovered through gravitational microlensing,[65][66] COROT-Exo-7b, a planet with a diameter estimated at around 1.7 times that of Earth, (making it the smallest super-Earth yet measured), but with an orbital distance of only 0.02 AU, which means it probably has a molten surface at a temperature of 1000–1500 °C,[67]and two planets orbiting the nearby red dwarf Gliese 581. Gliese 581 d is roughly 7.7 times Earth's mass,[68] while Gliese 581 c is five times Earth's mass and was initially thought to be the first terrestrial planet found within a star's habitable zone.[69] However, more detailed studies revealed that it was slightly too close to its star to be habitable, and that the farther planet in the system, Gliese 581 d, though it is much colder than Earth, could potentially be habitable if its atmosphere contained sufficient greenhouse gases.[70]

It is far from clear if the newly discovered large planets would resemble the gas giants in the Solar System or if they are of an entirely different type as yet unknown, like ammonia giants or carbon planets. In particular, some of the newly-discovered planets, known as hot Jupiters, orbit extremely close to their parent stars, in nearly circular orbits. They therefore receive much more stellar radiation than the gas giants in the Solar System, which makes it questionable whether they are the same type of planet at all. There may also exist a class of hot Jupiters, called Chthonian planets, that orbit so close to their star that their atmospheres have been blown away completely by stellar radiation. While many hot Jupiters have been found in the process of losing their atmospheres, as of 2008, no genuine Chthonian planets have been discovered.[71]

More detailed observation of extrasolar planets will require a new generation of instruments, including space telescopes. Currently the COROT spacecraft is searching for stellar luminosity variations due to transiting planets. Several projects have also been proposed to create an array of space telescopes to search for extrasolar planets with masses comparable to the Earth. These include the proposed NASA's Kepler Mission, Terrestrial Planet Finder, and Space Interferometry Mission programs, the ESA's Darwin, and the CNES' PEGASE.[72] The New Worlds Mission is an occulting device that may work in conjunction with the James Webb Space Telescope. However, funding for some of these projects remains uncertain. The first spectra of extrasolar planets were reported in February 2007 (HD 209458 b and HD 189733 b).[73][74] The frequency of occurrence of such terrestrial planets is one of the variables in the Drake equation which estimates the number of intelligent, communicating civilizations that exist in our galaxy.[75]

Interstellar "planets"

Several computer simulations of stellar and planetary system formation have suggested that some objects of planetary mass would be ejected into interstellar space.[76] Some scientists have argued that such objects found roaming in deep space should be classed as "planets". However, others have suggested that they could be low-mass stars.[77] The IAU's working definition on extrasolar planets takes no position on the issue.

In 2005, astronomers announced the discovery of Cha 110913-773444, the smallest brown dwarf found to date, at only seven times Jupiter's mass. Since it was not found in orbit around a fusing star, it is a sub-brown dwarf according to the IAU's working definition. However, some astronomers believe it should be referred to as a planet.[77] For a brief time in 2006, astronomers believed they had found a binary system of such objects, Oph 162225-240515, which the discoverers described as "planemos", or "planetary mass objects". However, recent analysis of the objects has determined that their masses are probably each greater than 13 Jupiter-masses, making the pair brown dwarfs.[78][79][80]


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

Dynamic characteristics


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).

All planets revolve around stars. In the Solar System, all the planets orbit in the same direction as the Sun rotates. It is not yet known whether all extrasolar planets follow this pattern. The period of one revolution of a planet's orbit is known as its sidereal period or year.[81] A planet's year depends on its distance from its star; the farther a planet is from its star, not only the longer the distance it must travel, but also the slower its speed, as it is less affected by the star's gravity. Because no planet's orbit is perfectly circular, the distance of each varies over the course of its year. The closest approach to its star is called its periastron (perihelion in the Solar System), while its farthest separation from the star is called its apastron (aphelion). 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.[82]

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, while planets with high eccentricities have more elliptical orbits. The planets in the Solar System have very low eccentricities, and thus nearly circular orbits.[81] Comets and Kuiper belt objects (as well as several extrasolar planets) have very high eccentricities, and thus exceedingly elliptical orbits.[83][84]
  • 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, as no planet's orbit has its star at its exact centre.[81]
  • 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 of the observer's line of sight from Earth.[85] 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.[86] The points at which a planet crosses above and below its reference plane are called its ascending and descending nodes.[81] 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.[81]

Axial tilt

Earth's axial tilt is about 23°.

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 possesses 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.[87] Among extrasolar planets, axial tilts are not known for certain, though most hot Jupiters are believed to possess negligible to no axial tilt, as a result of their proximity to their stars.[88]


The planets also rotate around invisible axes through their centres. A planet's rotation period is known as its day. All planets in the Solar System rotate in a counter-clockwise direction, except for Venus, which rotates clockwise[89] (Uranus is generally said to be rotating clockwise as well[90] though because of its extreme axial tilt, it can be said to be rotating either clockwise or anti-clockwise, depending on whether one states it to be inclined 82° from the ecliptic in one direction, or 98° in the opposite direction).[91] There is great variation in the length of day between the planets, with Venus taking 243 Earth days to rotate, and the gas giants only a few hours.[92] The rotational periods of extrasolar planets are not known; however their proximity to their stars means that hot Jupiters are tidally locked (their orbits are in sync with their rotations). This means they only ever show one face to their stars, with one side in perpetual day, the other in perpetual night.[93]

Orbital clearance

The defining dynamic characteristic of a planet is that it has cleared its neighborhood. A planet that has cleared its neighborhood has accumulated enough mass to gather up or sweep away all the planetesimals in its orbit. 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.[94]

Physical characteristics


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.[95]

Mass is also the prime attribute by which planets are distinguished from stars. The upper mass limit for planethood is roughly 13 times Jupiter's mass, beyond which it achieves conditions suitable for nuclear fusion. Other than the Sun, no objects of such mass exist in the Solar System; however a number of extrasolar planets lie at that threshold. The Extrasolar Planets Encyclopedia lists several planets that are close to this limit: HD 38529c, AB Pictorisb, HD 162020b, and HD 13189b. A number of objects of higher mass are also listed, but since they lie above the fusion threshold, they would be better described as brown dwarfs.[60]

The smallest known planet, excluding dwarf planets and satellites, is PSR B1257+12 a, one of the first extrasolar planets discovered, which was found in 1992 in orbit around a pulsar. Its mass is roughly half that of the planet Mercury.[60]

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 which either is or was a fluid. The terrestrial planets are sealed within hard crusts,[96] but in the gas giants the mantle simply dissolves into the upper cloud layers. The terrestrial planets possess cores of magnetic elements such as iron and nickel, and mantles of silicates. Jupiter and Saturn are believed to possess cores of rock and metal surrounded by mantles of metallic hydrogen.[97] Uranus and Neptune, which are smaller, possess rocky cores surrounded by mantles of water, ammonia, methane and other ices.[98] The fluid action within these planets' cores creates a geodynamo that generates a magnetic field.[96]


Earth's atmosphere

All of the Solar System planets have atmospheres as their large masses mean gravity is strong enough to keep gaseous particles close to the surface. The larger gas giants are massive enough to keep large amounts of the light gases hydrogen and helium close by, while the smaller planets lose these gases into space.[99] The composition of the Earth's atmosphere is different from the other planets because the various life processes that have transpired on the planet have introduced free molecular oxygen.[100] The only solar planet without a substantial atmosphere is Mercury which had it mostly, although not entirely, blasted away by the solar wind.[101]

Planetary atmospheres are affected by the varying degrees of energy received from either the Sun or their interiors, leading to the formation of dynamic weather systems such as hurricanes, (on Earth), planet-wide dust storms (on Mars), an Earth-sized anticyclone on Jupiter (called the Great Red Spot), and holes in the atmosphere (on Neptune).[87] At least one extrasolar planet, HD 189733 b, has been claimed to possess such a weather system, similar to the Great Red Spot but twice as large.[102]

Hot Jupiters have been shown to be losing their atmospheres into space due to stellar radiation, much like the tails of comets.[103][104] These planets may have vast differences in temperature between their day and night sides which produce supersonic winds,[105] although the day and night sides of HD 189733b appear to have very similar temperatures, indicating that that planet's atmosphere effectively redistributes the star's energy around the planet.[102]


Schematic of Earth's 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 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 can't effectively protect the planet.[106]

Of the eight planets in the Solar System, only Venus and Mars lack such a magnetic field.[106] In addition, the moon of Jupiter Ganymede also has one. Of the magnetized planets the magnetic field of Mercury is the weakest, and is barely able to deflect the solar wind. Ganymede's magnetic field is several times larger, and Jupiter's is the strongest in the Solar System (so strong in fact that it poses a serious health risk to future manned missions to its moons). The magnetic fields of the other giant planets are roughly similar in strength to that of Earth, but their magnetic moments are significantly larger. The magnetic fields of Uranus and Neptune are strongly tilted relative the rotational axis and displaced from the centre of the planet.[106]

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 hypothesised that the planet's magnetosphere was transferring energy onto the star's surface, increasing its already high 14,000 degree temperature by an additional 750 degrees.[107]

Secondary characteristics

Several planets or dwarf planets in the Solar System (such as Neptune and Pluto) have orbital periods that are in resonance with each other or with smaller bodies (this is also common in satellite systems). All except Mercury and Venus have natural satellites, often called "moons." Earth has one, Mars has two, and the gas giants have numerous moons in complex planetary-type systems. Many gas giant moons have similar features to the terrestrial planets and dwarf planets, and some have been studied as possible abodes of life (especially Europa).[108][109][110]

The four gas giants 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.[111][112]

No secondary characteristics have been observed around extrasolar planets. However the sub-brown dwarf Cha 110913-773444, which has been described as a rogue planet, is believed to be orbited by a tiny protoplanetary disc.[77]

"Planet"-derived terms

Terms containing and related to the modern astronomical term 'planet', that are also a term for a type of celestial object.

See also


  1. ^ This definition is drawn from two separate IAU declarations; a formal definition agreed by the Union in 2006, and an informal working definition established by the Union in 2003. The 2006 definition, while official, applies only to the Solar System, while the 2003 definition applies to planets around other stars. The extrasolar planet issue was deemed too complex to resolve at the 2006 IAU conference.
  2. ^ Referred to by Huygens as a Planetes novus ("new planet") in his Systema Saturnium
  3. ^ Both labelled nouvelles planètes (new planets) by Cassini in his Découverte de deux nouvelles planetes autour de Saturne
  4. ^ Both once referred to as "planets" by Cassini in his An Extract of the Journal Des Scavans.... The term "satellite", however, had already begun to be used to distinguish such bodies from those around which they orbited ("primary planets").
  5. ^ Recently reclassified as a dwarf planet in 2006.
  6. ^ Regarded as a planet from its discovery in 1930 until redesignated as a trans-Neptunian dwarf planet in August 2006.


  1. ^ a b c "IAU 2006 General Assembly: Result of the IAU Resolution votes". International Astronomical Union. 2006. Archived from the original on 2007-10-25. Retrieved on 2008-08-23. 
  2. ^ a b "Working Group on Extrasolar Planets (WGESP) of the International Astronomical Union". IAU. 2001. Retrieved on 2008-08-23. 
  3. ^ Schneider, Jean (2008-06-13). "Interactive Extra-solar Planets Catalog". The Extrasolar Planets Encyclopaedia. Retrieved on 2008-08-23. 
  4. ^ H. G. Liddell and R. Scott, A Greek–English Lexicon, ninth edition, (Oxford: Clarendon Press, 1940).
  5. ^ "Definition of planet". Merriam-Webster OnLine. Retrieved on 2007-07-23. 
  6. ^ a b "planet, n.". Oxford English Dictionary. December 2007. Retrieved on 2008-02-07.  Note: select the Etymology tab
  7. ^ Neugebauer, Otto E. (1945). "The History of Ancient Astronomy Problems and Methods". Journal of Near Eastern Studies 4 (1): 1–38. doi:10.1086/370729. 
  8. ^ Ronan, Colin. "Astronomy Before the Telescope". Astronomy in China, Korea and Japan (Walker ed.). pp. 264–265. 
  9. ^ Kuhn, Thomas S. (1957). The Copernican Revolution. Harvard University Press. pp. 5–20. 
  10. ^ a b c d Evans, James (1998). "The History and Practice of Ancient Astronomy". Oxford University Press. 296–7.,M1. Retrieved on 2008-02-04. 
  11. ^ a b Holden, James Herschel (1996). A History of Horoscopic Astrology. AFA. pp. 1. ISBN 978-0866904636. 
  12. ^ Hermann Hunger, ed (1992). Astrological reports to Assyrian kings. State Archives of Assyria. 8. Helsinki University Press. ISBN 951-570-130-9. 
  13. ^ Lambert, W. G. (1987). "Babylonian Planetary Omens. Part One. Enuma Anu Enlil, Tablet 63: The Venus Tablet of Ammisaduqa". Journal of the American Oriental Society 107: 93. doi:10.2307/602955. Retrieved on 2008-02-04. 
  14. ^ a b Kasak, Enn; Veede, Raul (2001). Mare Kõiva and Andres Kuperjanov. ed. "Understanding Planets in Ancient Mesopotamia (PDF)" (PDF). Electronic Journal of Folklore (Estonian Literary Museum) 16: 7–35. ISSN 1406-0957. Retrieved on 2008-02-06. 
  15. ^ Cosmographia, Antwerp, 1539 "Celestial Orbs in the Latin Middle Ages", Isis, Vol. 78, No. 2. (Jun., 1987), pp. 152-173.
  16. ^ Burnet, John (1950). Greek philosophy: Thales to Plato. Macmillan and Co.. pp. 7–11.,M1. Retrieved on 2008-02-07. 
  17. ^ a b Goldstein, Bernard R. (1997). "Saving the phenomena : the background to Ptolemy's planetary theory". Journal for the History of Astronomy (Cambridge (UK)) 28 (1): 1–12. Retrieved on 2008-02-06. 
  18. ^ Ptolemy; Toomer, G. J. (1998). Ptolemy's Almagest. Princeton University Press. ISBN 9780691002606. 
  19. ^ a b Van Helden, Al (1995). "Copernican System". The Galileo Project. Retrieved on 2008-01-28. 
  20. ^ Cassini, Signor (1673). "A Discovery of two New Planets about Saturn, made in the Royal Parisian Observatory by Signor Cassini, Fellow of both the Royal Societys, of England and France; English't out of French.". Philosophical Transactions (1665–1678) 8: 5178–85. doi:10.1098/rstl.1673.0003.  Note: This journal became the Philosophical Transactions of the Royal Society of London in 1775. There may just be earlier publications within the Journal des sçavans.
  21. ^ Hilton, James L. (2001-09-17). "When Did the Asteroids Become Minor Planets?". U. S. Naval Observatory. Retrieved on 2007-04-08. 
  22. ^ Croswell, K. (1997). Planet Quest: The Epic Discovery of Alien Solar Systems. The Free Press. pp. 57. ISBN 978-0684832524. 
  23. ^ Lyttleton, Raymond A. (1936). "On the possible results of an encounter of Pluto with the Neptunian system". Monthly Notices of the Royal Astronomical Society 97: 108. 
  24. ^ Whipple, Fred (1964). "The History of the Solar System". Proceedings of the National Academy of Sciences of the United States of America 52: 565–594. doi:10.1073/pnas.52.2.565. PMID 16591209. 
  25. ^ Luu, Jane X.; Jewitt, David C. (May 1996). "The Kuiper Belt". Scientific American 274 (5): 46–52. 
  26. ^ Green, D. W. E. (2006-09-13) (PDF). (134340) Pluto, (136199) Eris, and (136199) Eris I (Dysnomia). Circular No. 8747. Central Bureau for Astronomical Telegrams, International Astronomical Union. Retrieved on 2008-08-23. 
  27. ^ Wolszczan, A.; Frail, D. A. (1992). "A planetary system around the millisecond pulsar PSR1257+12". Nature 355: 145–147. doi:10.1038/355145a0. 
  28. ^ Mayor, Michel; Queloz, Didier (1995). "A Jupiter-mass companion to a solar-type star". Nature 378: 355–359. doi:10.1038/355145a0. 
  29. ^ "IAU General Assembly: Definition of Planet debate" (.wmv). 2006. Retrieved on 2008-08-23. 
  30. ^ Basri, Gibor (2000). "Observations of Brown Dwarfs". Annual Review of Astronomy and Astrophysics 38: 485. doi:10.1146/annurev.astro.38.1.485. 
  31. ^ Saumon, D.; Hubbard, W. B.; Burrows, A.; Guillot, T.; Lunine, J. I.; Chabrier, G. (1996). "A Theory of Extrasolar Giant Planets". Astrophysical Journal 460: 993–1018. doi:10.1086/177027. 
  32. ^ See for example the list of references for: Butler, R. P. et al (2006). "Catalog of Nearby Exoplanets". University of California and the Carnegie Institution. Retrieved on 2008-08-23. 
  33. ^ Stern, S. Alan (2004-03-22). "Gravity Rules: The Nature and Meaning of Planethood". SpaceDaily. Retrieved on 2008-08-23. 
  34. ^ Rincon, Paul (2006-08-16). "Planets plan boosts tally 12". BBC. Retrieved on 2008-08-23. 
  35. ^ "Pluto loses status as a planet". BBC. 2006-08-24. Retrieved on 2008-08-23. 
  36. ^ Soter, Steven (2006). "What is a Planet". Astronomical Journal 132 (6): 2513–19. doi:10.1086/508861. arΧiv:astro-ph/0608359. 
  37. ^ Rincon, Paul (2006-08-25). "Pluto vote 'hijacked' in revolt". BBC. Retrieved on 2008-08-23. 
  38. ^ Britt, Robert Roy (2006-08-24). "Pluto Demoted: No Longer a Planet in Highly Controversial Definition". Retrieved on 2008-08-23. 
  39. ^ Britt, Robert Roy (2006-08-31). "Pluto: Down But Maybe Not Out". Retrieved on 2008-08-23. 
  40. ^ Moskowitz, Clara (2006-10-18). "Scientist who found '10th planet' discusses downgrading of Pluto". Stanford news. Retrieved on 2008-08-23. 
  41. ^ "The Planet Hygea". 1849. Retrieved on 2008-04-18. 
  42. ^ Hilton, James L.. "When did asteroids become minor planets?". U.S. Naval Observatory. Retrieved on 2008-05-08. 
  43. ^ Ross, Kelley L. (2005). "The Days of the Week". The Friesian School. Retrieved on 2008-08-23. 
  44. ^ Cochrane, Ev (1997). Martian Metamorphoses: The Planet Mars in Ancient Myth and Tradition. Aeon Press. ISBN 0965622908.,M1. Retrieved on 2008-02-07. 
  45. ^ Cameron, Alan (2005). Greek Mythography in the Roman World. Oxford University Press. ISBN 0195171217. 
  46. ^ Zerubavel, Eviatar (1989). The Seven Day Circle: The History and Meaning of the Week. University of Chicago Press. pp. 14. ISBN 0226981657.,M1. Retrieved on 2008-02-07. 
  47. ^ a b Falk, Michael (1999). "Astronomical Names for the Days of the Week". Journal of the Royal Astronomical Society of Canada 93: 122–133. 
  48. ^ "earth, n.". Oxford English Dictionary. 1989. Retrieved on 2008-02-06. 
  49. ^ a b Harper, Douglas (2001-09). "Earth". Online Etymology Dictionary. Retrieved on 2008-08-23. 
  50. ^ Harper, Douglas (2001-09). "Etymology of "terrain"". Online Etymology Dictionary. Retrieved on 2008-01-30. 
  51. ^ Wetherill, G. W. (1980). "Formation of the Terrestrial Planets". Annual Review of Astronomy and Astrophysics 18: 77–113. doi:10.1146/annurev.aa.18.090180.000453. Retrieved on 2008-08-23. 
  52. ^ Inaba, S.; Ikoma, M. (2003). "Enhanced Collisional Growth of a Protoplanet that has an Atmosphere". Astronomy and Astrophysics 410: 711–723. doi:10.1051/0004-6361:20031248. Retrieved on 2008-08-23. 
  53. ^ Dutkevitch, Diane (1995). "The Evolution of Dust in the Terrestrial Planet Region of Circumstellar Disks Around Young Stars". Ph. D. thesis, University of Massachusetts Amherst. Archived from the original on 2007-11-25. Retrieved on 2008-08-23.  (Astrophysics Data System entry)
  54. ^ Matsuyama, I.; Johnstone, D.; Murray, N. (2005). "Halting Planet Migration by Photoevaporation from the Central Source". The Astrophysical Journal 585 (2): L143–L146. doi:10.1086/374406. Retrieved on 2008-08-23. 
  55. ^ Kenyon, Scott J.; Bromley, Benjamin C. (2006). "Terrestrial Planet Formation. I. The Transition from Oligarchic Growth to Chaotic Growth". Astronomical Journal 131: 1837. doi:10.1086/499807. Lay summary – Kenyon, Scott J. Personal web page. 
  56. ^ Ida, Shigeru; Nakagawa, Yoshitsugu; Nakazawa, Kiyoshi (1987). "The Earth's core formation due to the Rayleigh-Taylor instability". Icarus 69: 239. doi:10.1016/0019-1035(87)90103-5. 
  57. ^ Kasting, James F. (1993). "Earth's early atmosphere". Science 259: 920. doi:10.1126/science.11536547. PMID 11536547. 
  58. ^ Aguilar, D.; Pulliam, C. (2004-01-06). "Lifeless Suns Dominated The Early Universe". Harvard-Smithsonian Center for Astrophysics (Press release). Retrieved on 2006-08-26. 
  59. ^ Amburn, Brad (2006-02-28). "Behind the Pluto Mission: An Interview with Project Leader Alan Stern". Retrieved on 2008-08-23. 
  60. ^ a b c d Schneider, Jean (2006-12-11). "Interactive Extra-solar Planets Catalog". The Extrasolar Planets Encyclopedia. Retrieved on 2008-08-23. 
  61. ^ Kennedy, Barbara (2005-02-11). "Scientists reveal smallest extra-solar planet yet found". SpaceFlight Now. Retrieved on 2008-08-23. 
  62. ^ Santos, N.; Bouchy, F.; Vauclair, S.; Queloz, D.; Mayor, M. (2004-08-25). "Fourteen Times the Earth". European Southern Observatory (Press Release). Retrieved on 2008-08-23. 
  63. ^ "Trio of Neptunes". Astrobiology Magazine. May 21, 2006. Retrieved on 2008-08-23. 
  64. ^ "Star: Gliese 876". Extrasolar planet Encyclopedia. Retrieved on 2008-02-01. 
  65. ^ "Small Planet Discovered Orbiting Small Star". ScienceDaily. 2008. Retrieved on 2008-06-06. 
  66. ^ Beaulieu, J.-P.; D. P. Bennett; P. Fouqué; A. Williams; et al. (2006-01-26). "Discovery of a Cool Planet of 5.5 Earth Masses Through Gravitational Microlensing". Nature 439: 437–440. doi:10.1038/nature04441. Retrieved on 2008-08-23. 
  67. ^
  68. ^ "Gliese 581 d". The Extrasolar Planets Encyclopedia. Retrieved on 2008-09-13. 
  69. ^ "New 'super-Earth' found in space". BBC News. 25 April 2007. Retrieved on 2008-08-23. 
  70. ^ von Bloh et al. (2007). "The Habitability of Super-Earths in Gliese 581". Astronomy and Astrophysics 476 (3): 1365–1371. doi:10.1051/0004-6361:20077939. Retrieved on 2008-08-20. 
  71. ^ Lecavelier des Etangs, A.; Vidal-Madjar, A.; McConnell, J. C.; Hébrard, G. (2004). "Atmospheric escape from hot Jupiters". Astronomy and Astrophysics 418: L1–L4. doi:10.1051/0004-6361:20040106. Retrieved on 2008-08-23. 
  72. ^ "Future American and European Planet Finding Missions". Space Today Online. Retrieved on 2008-02-06. 
  73. ^ Thompson, Tabatha; Clavin, Whitney (2007-02-21). "NASA's Spitzer First To Crack Open Light of Faraway Worlds". Jet Propulsion Laboratory, California Institute of Technology (Press Release). Retrieved on 2008-08-23. 
  74. ^ Richardson, L. Jeremy; Deming, Drake; Horning, Karen; Seager, Sara; Harrington, Joseph (2007). "A spectrum of an extrasolar planet". Nature 445: 892. doi:10.1038/nature05636. 
  75. ^ Drake, Frank (2003-09-29). "The Drake Equation Revisited". Astrobiology Magazine. Retrieved on 2008-08-23. 
  76. ^ Lissauer, J. J. (1987). "Timescales for Planetary Accretion and the Structure of the Protoplanetary disk". Icarus 69: 249–265. doi:10.1016/0019-1035(87)90104-7. 
  77. ^ a b c Luhman, K. L.; Adame, Lucía; D'Alessio, Paola; Calvet, Nuria (2005). "Discovery of a Planetary-Mass Brown Dwarf with a Circumstellar Disk". Astrophysical Journal 635: L93. doi:10.1086/498868. Lay summary – NASA Press Release (2005-11-29). 
  78. ^ Close, Laird M. et al (2007). "The Wide Brown Dwarf Binary Oph 1622-2405 and Discovery of A Wide, Low Mass Binary in Ophiuchus (Oph 1623-2402): A New Class of Young Evaporating Wide Binaries?". Astrophysical Journal 660: 1492. doi:10.1086/513417. arΧiv:astro-ph/0608574. 
  79. ^ Luhman, K. L. (April 2007). "Ophiuchus 1622-2405: Not a Planetary-Mass Binary". The Astrophysical Journal 659 (2): 1629–36. doi:10.1086/512539. 
  80. ^ Britt, Robert Roy (2004-09-10). "Likely First Photo of Planet Beyond the Solar System". Retrieved on 2008-08-23. 
  81. ^ a b c d e Young, Charles Augustus (1902). Manual of Astronomy: A Text Book. Ginn & company. pp. 324–7. 
  82. ^ Dvorak, R.; Kurths, J.; Freistetter, F. (2005). Chaos And Stability in Planetary Systems. New York: Springer. ISBN 3540282084. 
  83. ^ Moorhead, Althea V.; Adams, Fred C. (2008). "Eccentricity evolution of giant planet orbits due to circumstellar disk torques". Icarus 193: 475. doi:10.1016/j.icarus.2007.07.009. arΧiv:0708.0335. 
  84. ^ "Planets – Kuiper Belt Objects". The Astrophysics Spectator. 2004-12-15. Retrieved on 2008-08-23. 
  85. ^ Tatum, J. B. (2007). "17. Visual binary stars". Celestial Mechanics. Personal web page. Retrieved on 2008-02-02. 
  86. ^ Trujillo, Chadwick A.; Brown, Michael E. (2002). "A Correlation between Inclination and Color in the Classical Kuiper Belt". Astrophysical Journal 566: L125. doi:10.1086/339437. 
  87. ^ a b Harvey, Samantha (2006-05-01). "Weather, Weather, Everywhere?". NASA. Retrieved on 2008-08-23. 
  88. ^ Winn, Joshua N.; Holman, Matthew J. (2005). "Obliquity Tides on Hot Jupiters". The Astrophysical Journal 628: L159. doi:10.1086/432834. 
  89. ^ Goldstein, R. M.; Carpenter, R. L. (1963). "Rotation of Venus: Period Estimated from Radar Measurements". Science 139: 910. doi:10.1126/science.139.3558.910. PMID 17743054. 
  90. ^ Belton, M. J. S.; Terrile R. J. (1984). "Uranus and Neptune". 327. Retrieved on 2008-02-02. 
  91. ^ Borgia, Michael P. (2006). The Outer Worlds; Uranus, Neptune, Pluto, and Beyond. Springer New York. pp. 195–206. 
  92. ^ Strobel, Nick. "Planet tables". Retrieved on 2008-02-01. 
  93. ^ Zarka, Philippe; Treumann, Rudolf A.; Ryabov, Boris P.; Ryabov, Vladimir B. (2001). "Magnetically-Driven Planetary Radio Emissions and Application to Extrasolar Planets". Astrophysics & Space Science 277: 293. doi:10.1023/A:1012221527425. 
  94. ^ Faber, Peter; Quillen, Alice C. (2007-07-12). "The Total Number of Giant Planets in Debris Disks with Central Clearings". Department of Physics and Astronomy, University of Rochester. Retrieved on 2008-08-23. 
  95. ^ Brown, Michael E. (2006). "The Dwarf Planets". California Institute of Technology. Retrieved on 2008-02-01. 
  96. ^ a b "Planetary Interiors". Department of Physics, University of Oregon. Retrieved on 2008-08-23. 
  97. ^ Elkins-Tanton, Linda T. (2006). Jupiter and Saturn. New York: Chelsea House. ISBN 0-8160-5196-8. 
  98. ^ Podolak, M.; Weizman, A.; Marley, M. (1995). "Comparative model of Uranus and Neptune". Planet. Space Sci. 43 (12): 1517–1522. doi:10.1016/0032-0633(95)00061-5. 
  99. ^ Sheppard, Scott S.; Jewitt, David; Kleyna, Jan (2005). "An Ultradeep Survey for Irregular Satellites of Uranus: Limits to Completeness". The Astronomical Journal 129: 518–525. doi:10.1086/426329. arΧiv:astro-ph/0410059v1. 
  100. ^ Zeilik, Michael A.; Gregory, Stephan A. (1998). Introductory Astronomy & Astrophysics (4th ed. ed.). Saunders College Publishing. pp. 67. ISBN 0030062284. 
  101. ^ Hunten D. M., Shemansky D. E., Morgan T. H. (1988), The Mercury atmosphere, In: Mercury (A89-43751 19–91). University of Arizona Press, pp. 562–612
  102. ^ a b Knutson, Heather A.; Charbonneau, David; Allen, Lori E.; Fortney, Jonathan J. (2007). "A map of the day-night contrast of the extrasolar planet HD 189733b". Nature 447: 183. doi:10.1038/nature05782. Lay summary – Center for Astrophysics press release (2007-05-09). 
  103. ^ Weaver, D.; Villard, R. (2007-01-31). "Hubble Probes Layer-cake Structure of Alien World's Atmosphere". University of Arizona, Lunar and Planetary Laboratory (Press Release). Retrieved on 2008-08-23. 
  104. ^ Ballester, Gilda E.; Sing, David K.; Herbert, Floyd (2007). "The signature of hot hydrogen in the atmosphere of the extrasolar planet HD 209458b". Nature 445: 511. doi:10.1038/nature05525. 
  105. ^ Harrington, Jason; Hansen, Brad M.; Luszcz, Statia H.; Seager, Sara (2006). "The phase-dependent infrared brightness of the extrasolar planet Andromeda b". Science 314: 623. doi:10.1126/science.1133904. PMID 17038587. Lay summary – NASA press release (2006-10-12). 
  106. ^ a b c Kivelson, Margaret Galland; Bagenal, Fran (2007). "Planetary Magnetospheres". in Lucyann Mcfadden, Paul Weissman, Torrence Johnson. Encyclopedia of the Solar System. Academic Press. p. 519. ISBN 9780120885893. 
  107. ^ Gefter, Amanda (2004-01-17). "Magnetic planet". Astronomy. Retrieved on 2008-01-29. 
  108. ^ Grasset, O.; Sotin C.; Deschamps F. (2000). "On the internal structure and dynamic of Titan". Planetary and Space Science 48: 617–636. doi:10.1016/S0032-0633(00)00039-8. 
  109. ^ Fortes, A. D. (2000). "Exobiological implications of a possible ammonia-water ocean inside Titan". Icarus 146 (2): 444–452. doi:10.1006/icar.2000.6400. 
  110. ^ Jones, Nicola (2001-12-11). "Bacterial explanation for Europa's rosy glow". New Scientist Print Edition. Retrieved on 2008-08-23. 
  111. ^ Molnar, L. A.; Dunn, D. E. (1996). "On the Formation of Planetary Rings". Bulletin of the American Astronomical Society 28: 77–115. 
  112. ^ Thérèse, Encrenaz (2004). The Solar System (Third edition ed.). Springer. pp. 388–390. ISBN 3540002413. 

External links

Personal tools