planet

 

[Gr.,=wanderer], a large nonluminous ball of rock or gas that orbits a star. The term, once limited to any of the eight solid, nonluminous bodies (major planets) that revolve around the sun, has been extended to include similar bodies discovered revolving around other stars. The term is sometimes used to include the asteroids (or minor planets) but excludes the other members of the solar system: comets and meteoroids (see meteor; see also planetary science and planetary system, as well as the table entitled Major Planets of the Solar System).

Classification of the Major Planets

The major planets are classified either as inferior, with an orbit between the sun and the orbit of Earth (Mercury and Venus), or as superior, with an orbit beyond that of Earth (Mars, Jupiter, Saturn, and Uranus, Neptune. Pluto, long regarded after its discovery in 1930 as the ninth planet, was gradually recognized as a Kuiper belt, or transneptunian, object (see comet), and in 2006 was reclassified by astronomers as a dwarf planet.

On the basis of their physical properties the planets are further classified as terrestrial or Jovian. The terrestrial planets—Mercury, Venus, Earth, and Mars—resemble Earth in size, chemical composition, and density. Their periods of rotation range from about 24 hr for Mars to 249 days for Venus. The Jovian planets—Jupiter, Saturn, Uranus, and Neptune—are much larger in size and have thick, gaseous atmospheres and low densities. Their periods of rotation range from about 10 hr for Jupiter to 15 hr for Neptune. This rapid rotation results in polar flattening of 2% to 10%, giving the planets an elliptical appearance.

Recognition of the Planets

Identification of the Solar Planets

The ancient Greeks applied the term planet to the five major planets then known—Mercury, Venus, Mars, Jupiter, and Saturn—as well as to the sun and moon; all these bodies were observed to move back and forth against the background of the apparently fixed stars and to shine with a steady light. In the Ptolemaic system the earth was thought to lie at rest in the center of the universe while the planets moved about it in a complicated scheme of circles. The heliocentric, or sun-centered, Copernican system, introduced in the 16th cent., viewed the planets, including the earth, as revolving about the sun; the moon was viewed as a natural satellite of the earth. At the start of the 17th cent. Johannes Kepler refined the Copernican model by showing that the orbits of the planets around the sun were elliptical rather than circular.

With the development of the telescope other planets became visible. Uranus, detected in 1781 by Sir William Herschel, was the first planet discovered in modern times. Neptune was discovered in 1846 as the result of a mathematical analysis of the irregularities in the motion of Uranus, and the dwarf planet Pluto, whose existence was predicted from the perturbations of both Uranus and Neptune, was found in 1930. In addition to the major planets, the telescope has revealed thousands of minor planets, or asteroids, which orbit the sun in a bandlike cluster between Mars and Jupiter; the largest of these, the dwarf planet Ceres, was also the first discovered (1801), and was regarded as a planet for many years. Additional minor planets have been discovered since 1992 beyond the orbit of Neptune in the Kuiper belt; at least one of these transneptunian objects, Eris, has a diameter (1,500 mi/2,400 km) slightly larger than that of Pluto.

Discovery of the Extrasolar Planets

Although speculation concerning the existence of extrasolar planets and planetary systems dates back to antiquity, it was not until the last decade of the 20th cent. that astronomical tools and techniques made their detection possible. Because stars are so distant and bright and an extrasolar planet, no matter how large, is relatively small and dim, it cannot be seen or photographed directly. Its presence is usually inferred from a periodic wobble in the spectrum of a target star's frequencies. This wobble, produced by gravitational influences, causes tiny shifts in the star's frequencies that are caught by telescopes and analyzed to yield information on the body affecting the star. Another technique that proved fruitful in 1999 is the use of a telescope to record the dimming of light from a star when a planet's orbit carries it between the star and the earth.

Spurred on by the discovery of three bodies orbiting a pulsar by radio astronomers in 1992, the first extrasolar planet orbiting a sunlike star was detected in 1995. Located in the constellation Pegasus, about 40 light-years from earth, the planet—called 51 Pegasi—has about half the mass of Jupiter and is so close to the star that it has a surface temperature of about 1,000°C and completes its orbit in only four days. By the end of the decade, more than two dozen extrasolar planets were detected, including three orbiting the star Upsilon Andromedae—the first multiplanet extrasolar planetary system—that were discovered in 1999. Within six years the number of known extrasolar planets had surpassed 150, a number approached by the known planetary systems. The most earthlike of the extrasolar planets discovered to date orbits the star Gliese 876 about 15 light-years from earth. It is about 7.5 times as massive as earth, with a radius twice as large and a circular orbit slightly less than 2 million miles from its star, and is believed to be composed of nickel-iron rock. Like the other two known planets circling Gliese 876, most of the known extrasolar planets are giant gas planets with masses ranging from one half to five times that of Jupiter, the largest of the solar planets. Many have orbits that are highly elliptical rather than only slightly so, are closer to their star than the earth is to the sun, and have orbital periods ranging from three days to more than four years. In addition, the ages of the extrasolar planets differ from one another and from that of the solar planets; the oldest planet, discovered in the globular cluster M4 in 2003, is believed to have been formed 12.7 billion years ago, within a billion years of the origin of the universe and 8 billion years before the earth. Because these data are so different from that of the solar planets, planetary scientists are rethinking the accepted theories of planetary formation.

Bibliography

See P. Halpern, The Quest for Alien Planets: Exploring Worlds Outside the Solar System (1997); J. R. Gribbin and S. Goodwin, Empire of the Sun: Planets and Moons of the Solar System (1998).

A planet, 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, not massive enough to cause thermonuclear fusion in its core, and has cleared its neighbouring region of planetesimals.^[1][2]

The term planet is an ancient one, with ties to history, science, myth and religion. The planets were originally seen as a divine presence; as emissaries of the gods. Even today, many people continue to believe the movement of the planets affects their lives, although such a causation is rejected by the scientific community. As scientific knowledge improved, the human perception of the planets changed over time, 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 criticised, and remains disputed by some scientists.

The planets were initially thought to orbit the Earth in circular motions; after the development of the telescope, the planets were determined to orbit the Sun, and their orbits were found to be elliptical. As observational tools improved, astronomers saw that, like Earth, the planets rotated around tilted axes and shared 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, and the discovery of hundreds of extrasolar planets, scientists are beginning to observe similar features across the galaxy.

Under IAU definitions, there are eight planets in the Solar System (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune) and also at least three dwarf planets (Ceres, Pluto, and Eris). Many of these planets are orbited by one or more moons, which can be larger than small planets. There have also been more than two hundred planets discovered orbiting other stars.^[3] Planets are generally divided into two main types: large, low-density gas giants and smaller, rocky terrestrials. Dwarf planets, a separate category, can either be terrestrials or frozen ice dwarfs.

Etymology

In ancient times, astronomers noted how certain lights moved across the sky in relation to the other stars. The lights were first called "πλανήται" (planētai),^[4] meaning "wanderers", by the ancient Greeks, and it is from this that the word "planet" was derived.^[5][6]

The Greeks gave the planets names: the farthest was called Phainon, the shiner, while below it was Phaethon, the bright one. The red planet was known as Pyroeis, "fiery", while the brightest was known as Phosphoros, the light bringer, and the fleeting final planet was called Stilbon, the gleamer. However, the Greeks also made each planet sacred to one of their pantheon of gods, the Olympians: Phainon was sacred to Kronos, the Titan who fathered the Olympians, while Phaethon was sacred to Zeus, his son who deposed him as king. Ares, son of Zeus and god of war, was given dominion over Pyroeis, while Aphrodite, goddess of love, ruled over bright Phosphoros, and Hermes ruled over Stilbon.^[7]

The Greek practice of grafting of their gods' names onto the planets was almost certainly borrowed from the Babylonians, a contemporary civilisation in what is now Iraq, from whom they had begun to absorb astronomical learning, including constellations and the zodiac, by 600 BCE.^[8] The Babylonians named Phosphoros after their goddess of love, Ishtar, Pyroeis after their god of war, Nergal, and Phaethon after their chief god, Marduk.^[9] There are too many concordances between Greek and Babylonian naming conventions for them to have arisen separately.^[7] There does, however, appear to have been some confusion in translation. For instance, the Babylonian Nergal was a god of war, and the Greeks, seeing this aspect of Nergal's persona, identified him with Ares, their god of war. However, Nergal, unlike Ares, was also a god of the dead and a god of pestilence.^[9]

Today, most people in the western world know the planets by names derived from the Olympian pantheon of gods; however, because of the influence of the Roman Empire and, later, the Catholic Church, they are known by their Roman (or Latin) names, rather than the Greek. 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.^[10] When the Romans studied Greek astronomy, they gave the planets their own gods' names. To the Greeks and Romans, there were five known planets; each presumed to be circling the Earth according to the complex laws laid out by Claudius Ptolemy in the 2nd century. They were, in increasing order from Earth (according to Ptolemy): Mercury (Hermes), Venus (Aphrodite), Mars (Ares), Jupiter (Zeus), and Saturn (Kronos). Although strictly the term "planetai" referred only to those five objects, the term was often expanded to include the Sun and the Moon.^[11] When subsequent planets were discovered in the 18th and 19th centuries, the naming practice was retained: Uranus (Ouranos) and Neptune (Poseidon). The Greeks still use their original names for the planets.

Some Romans, following a belief imported from Mesopotamia into Hellenistic Egypt,^[12] believed that the seven gods after whom the planets were named took hourly shifts in looking after affairs on Earth. The order of shifts began with Jupiter and worked inwards; as a result, a list of which god had charge of the first hour in each day became Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn, i.e. the usual weekday name order.^[13] 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), 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, there is no tradition of naming it after a god. Many of the Romance languages (including French, Italian, Spanish and Portuguese), which are descended from Latin, retain the old Roman name of Terra or some variation thereof. However, the non-Romance languages use their own respective native words. Again, 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,"^[14] as can be seen in the English Earth, the German Erde, the Dutch Aarde, and the Scandinavian Jorde. The same is true for the Sun and the Moon, though they are no longer considered planets.

Some non-European cultures use their own planetary naming systems. India uses a naming system based on the Navagraha, which incorporates the seven traditional planets (Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn) and the ascending and descending lunar nodes Rahu and Ketu. China, and the countries of eastern Asia subject to Chinese cultural influence, such as Japan, Korea and Vietnam, use a naming system based on the five Chinese elements.^[13]

History

See also: List of Solar System bodies formerly regarded as planets

As scientific knowledge progressed, understanding of the term "planet" changed from something that moved across the sky (in relation to the starfield), to a body that orbited the Earth (or that were believed to do so at the time). When the heliocentric model gained sway in the 16th century, it became accepted that a planet was actually something that directly orbited the Sun. Thus the Earth was itself a planet,^[15] while the Sun and Moon were not. At the end of the 17th century, when the first satellites of Saturn were discovered, the terms "planet" and "satellite" were at first used interchangeably, although "satellite" would gradually become more prevalent in the following century.^[16] Until the mid-19th century, any newly discovered object orbiting the Sun was listed with the planets by the scientific community, and the number of "planets" swelled rapidly towards the end of that period.

During the 1800s, astronomers began to realize most recent discoveries were unlike the traditional planets. They shared the same region of space, between Mars and Jupiter, and had a far smaller mass. Bodies such as Ceres, Pallas, and Vesta, which had been classed as planets for almost half a century, became classified with the new designation "asteroid." From this point, a "planet" came to be understood, in the absence of any formal definition, as any "large" body that orbited the Sun. There was no apparent need to create a set limit, as 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.^[17]

However, in the 20th century, Pluto was discovered. After initial observations led to the belief it was larger than Earth, the recently-created IAU accepted the object as a planet. Further monitoring found the body was actually much smaller, but, as it was still larger than all known asteroids and seemingly did not exist within a larger population, it kept its status for some seventy years.^[18]

In the 1990s and early 2000s, there was a flood of discoveries of similar objects in the same region of the Solar System. 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 & Neptune), and a new class of dwarf planets was created, initially containing three objects (Ceres, Pluto and Eris).^[19]

Definition and disputes

Main article: Definition of planet

With the discovery during the latter half of the twentieth 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.

In 2003, The International Astronomical Union (IAU) Working Group on Extrasolar Planets made a position statement on the definition of a planet that incorporated a working definition:^[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)^[20] 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 our Solar System.
2. Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed nor where they are located.
3. Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).

This definition has since been widely used by astronomers when publishing discoveries in journals,^[21] although it remains a temporary yet effective, working definition until a more permanent one is formally adopted. It also did not address the dispute over the lower mass limit and steered clear of the controversy regarding objects within the Solar System.

This matter was finally 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 and Eris) are classified as dwarf planets, providing they are not also natural satellites of other planets. Originally an IAU committee had proposed a definition that would have included a much larger number of planets as it did not include (c) as a criterion. After much discussion, it was decided via a vote that those bodies should instead be classified as dwarf planets.

This definition is based in modern 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. Asteroids and comets, including KBOs, differ from planets in that they can collide with each other and with planets."^[22]

In the aftermath of the IAU's 2006 vote, there has been criticism of the new definition,^[23] and some astronomers have even stated that they will not use it.^[24] 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 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 during most of the 20th century, in a similar way to Ceres and its kin in the 1800s. More recently, the discovery of Eris was widely reported in the media as the "tenth planet". The reclassification of all three objects as dwarf planets has attracted much media and public attention.^[25]

Formation

Main article: Planetary formation

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 more dense until they collapse inward under gravity to form protoplanets.^[26] 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.^[27]

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.^[28][29] 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.^[30][31] 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. 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.^[32] (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—a astronomical term describing the abundance of isotopes with an atomic number greater than 2 (Helium)—is now believed to determine the likelihood that a star will have planets.^[33] 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.

Within the Solar System

Main article: Solar System

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

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

• Terrestrials: Planets (and possibly dwarf planets) that are similar to Earth — with bodies largely composed of rock: Mercury, Venus, Earth and Mars. If including dwarf planets, Ceres would also be counted, with as many as three other asteroids that might be added.
• Gas giants: Planets with a composition largely made up of gaseous material and are significantly more massive than terrestrials: Jupiter, Saturn, Uranus, Neptune. Ice giants are a sub-class of gas giants, distinguished from gas giants by their depletion in hydrogen and helium, and a significant composition of rock and ice: Uranus and Neptune.
• Ice dwarfs: Objects that are composed mainly of ice, and do not have planetary mass. The dwarf planets Pluto and Eris are ice dwarfs, and several dwarf planetary candidates also qualify.

Planetary attributes
	Name	Equatorial diameter[a]	Mass[a]	Orbital radius (AU)	Orbital period (years)	Inclination to Sun's equator (°)	Orbital eccentricity	Rotation period (days)	Moons	Rings	Atmosphere
Terrestrials	Mercury	0.39	0.06	0.39	0.24	3.38	0.206	58.64	—	no	minimal
	Venus	0.95	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	0.53	0.11	1.52	1.88	5.65	0.093	1.03	2	no	CO2, N2
Gas giants	Jupiter	11.21	317.8	5.20	11.86	6.09	0.048	0.41	63	yes	H2, He
	Saturn	9.41	95.2	9.54	29.46	5.51	0.054	0.43	56	yes	H2, He
	Uranus	3.98	14.6	19.22	84.01	6.48	0.047	-0.72	27	yes	H2, He
	Neptune	3.81	17.2	30.06	164.8	6.43	0.009	0.67	13	yes	H2, He
Measured relative to the Earth.   See Earth article for absolute values.

Dwarf planets

Main article: Dwarf planet

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 three dwarf planets in the Solar System are recognized by the IAU: Ceres, Pluto 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 diameter[c]	Mass[c]	Orbital radius (AU)	Orbital period (years)	Inclination to ecliptic (°)	Orbital eccentricity	Rotation period (days)	Moons	Rings	Atmosphere
Terrestrials	Ceres	0.08	0.0002	2.76	4.60	10.59	0.080	0.38	—	no	none
Ice dwarfs	Pluto	0.18	0.0022	39.48	248.09	17.14	0.249	-6.39	3	no	temporary
	Eris	0.19	0.0025	67.67