Planet

The currently accepted eight planets with the three newly designated dwarf planets. (Object sizes are to scale, although distances are not.)

The International Astronomical Union (IAU), the official scientific body for astronomical nomenclature, defines "planet" as a celestial body that:^[1]^[2]

(a) is in orbit around a star or stellar remnants;

(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape;

(c) is not massive enough to initiate thermonuclear fusion of deuterium in its core; and,

(d) has cleared the neighbourhood around its orbit.

Under this definition, the solar system is considered to have eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Three bodies which fulfill the first three conditions but not the fourth are now classified as dwarf planets: Ceres, Pluto and 2003 UB313.

Before the adoption of a recent resolution, there was no scientifically specified definition of "planet", although various notable astronomers had made proposals as part of the ongoing debate. Without a definition, the solar system had been traditionally seen as having various numbers of accepted planets over the years. It remains to be seen whether the new definition will become universally accepted both within the astronomical community, where there has been considerable opposition to the recent adoption, and in the eyes of the greater public.

Etymology

The name "planet" comes from the Greek term πλανήτης, planētēs, meaning "wanderer", as ancient astronomers noted how certain lights moved across the sky in relation to the other stars. These objects were believed to orbit the Earth, which was considered stationary.

In ancient times, there were seven known planets: Moon, Mercury, Venus, Sun, Mars, Jupiter, and Saturn, in orbit order outwards according to the Ptolemaic system.

Some Romans believed that the seven gods, after whom the planets were named, took hourly shifts in looking after affairs on Earth, in Ptolemaic orbit order listed inwards (the reverse of the above list). As a result, a list of which god has charge of the first hour in each day, comes out as Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn, i.e. the usual weekday name order: Sunday, Mo(o)nday, French mardi, mercredi, jeudi, vendredi, Satur(n)day.^[3]

nineplanets.org notes about days of the week: "The seven-day system we use is based on the ancient astrological notion that the seven known celestial bodies influence what happens on Earth and that each of these celestial bodies controls the first hour of the day named after it. This system was brought into Hellenistic Egypt from Mesopotamia".

Definition

Developments in astronomy, in particular the acceptance of the heliocentric model, removed the Sun and the Moon, and gradually added the currently accepted planetary members of the Solar System. Although a "planet" was commonly considered to be a large object orbiting the Sun (or, later, other stars), there was no scientifically specified definition of "planet". Thus the solar system has been traditionally seen as having various numbers of accepted planets over the years, based on culture and scientific consensus:

six (Mercury through Saturn)
seven again (with Uranus)
eleven (with Ceres, Pallas, Juno and Vesta)
eight (with Neptune but without the asteroids)
nine (with Pluto)

During the late 20th century, with the discovery of more smaller objects in the solar system, the consensus began to break down. There has been dispute over the precise size/mass limits an object needed to qualify as a "planet". There was particular dispute with objects in our solar system over whether round objects that existed in belts should qualify. However, in 2006, the general assembly of the International Astronomical Union voted to pass a resolution that redefined planets within our solar system as:

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 2003 UB313) are classified as dwarf planets, providing they are not also natural satellites of other planets.

This definition is based in modern theories of planetary formation, in which planetary embryos initially clear their orbital neighborhood of other smaller objects.

A paper by Steven Soter describes it this way:

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. ^[4]

Planetary discriminants defined in that paper very clearly separate the 8 IAU-recognized planets from all other small solar system objects.

Originally an IAU committee had proposed a definition that would have included a much larger number of planets because it did not include (c) as a criterion. Negotiations resulted in a definition that classified those objects that only satisfy criteria (a) and (b) as dwarf planets.

This resolution adds to a proposal on extrasolar planets that was released in 2003, and has been used as a working definition by the IAU since.^[2]

Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System.

Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed nor where they are located.

Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).

In the aftermath of the IAU's 2006 vote, there has been some criticism of the new definition from astronomers. 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 amongst the general public, given that it had been known as our solar system's ninth planet for over seventy years priorly. More recently, the discovery of 2003 UB313 (widely publicised as the tenth planet), and the possible reclassification of Ceres and Charon as planets, has attracted public attention.

Within the solar system

According to the IAU definition, there are eight planets in our solar system. In increasing distance from the Sun they are (with the astronomical symbol in brackets):

Mercury ( ) with no confirmed natural satellites
Venus ( ) with no confirmed natural satellites
Earth ( ) with one confirmed natural satellite, the Moon
Mars ( ) with two confirmed natural satellites, Phobos and Deimos
Jupiter ( ) with sixty-three confirmed natural satellites
Saturn ( ) with fifty-six confirmed natural satellites
Uranus ( ) with twenty-seven confirmed natural satellites
Neptune ( ) with thirteen confirmed natural satellites

Planetary names

The process of naming planets and their features is known as planetary nomenclature. In near-universal practice in the Western world, the planets in the solar system are named after Greek gods, as, in Europe, it was the Greeks who originally named them. The Romans had adopted many Greek god figures and gave them equivalent names in Latin. Today, most names are known by their Roman (Latin) equivalents: Mercury (called Hermes by the Greeks), Venus (Aphrodite), Mars (Ares), Jupiter (Zeus), Saturn (Kronos), Uranus (Ouranos) and Neptune (Poseidon). The Greeks still use their original names for the planets.

Earth is the exception to this rule; 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; again, the Greeks retain their original name, Γή {Ge or Yi; derived from Gaia). However, the non-Romance languages use their own respective native words. The Germanic languages, including English, use a variation of an ancient Germanic word ertho, "ground," as can be seen in the English Earth, the German Erde, the Dutch Aarde, and the Scandinavian Jorden.

Some non-European cultures use their own planetary naming systems. 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.^[3]

Attributes of planets


Planetary attributes
	Name	Equator^* diam.	Mass^*	Orbital radius^* (AU)	Orbital period^* (years)	Inclination to Sun's equator (°)	Orbital eccentricity	Day^* (days)	Moons
Terrestrials	Mercury	0.382	0.06	0.387	0.241	3.38	0.206	58.6	none
	Venus	0.949	0.82	0.72	0.615	3.86	0.0068	-243	none
	Earth^**	1.00	1.00	1.00	1.00	7.25	0.0167	1.00	1
	Mars	0.53	0.11	1.52	1.88	5.65	0.0934	1.03	2
Gas giants	Jupiter	11.2	318	5.20	11.86	6.09	0.0484	0.414	63
	Saturn	9.41	95	9.54	29.46	5.51	0.0542	0.426	56
	Uranus	3.98	14.6	19.22	84.01	6.48	0.0472	-0.718	27
	Neptune	3.81	17.2	30.06	164.8	6.43	0.0086	0.671	13

*Measured relative to the Earth. **See Earth article for absolute values.

Dwarf planets

Main article: Dwarf planet

In recent years there has been much debate over the definition of planet and thus many 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. Currently three dwarf planets in the Solar System are recognized by IAU, those are Ceres, Pluto and 2003 UB313. Dwarf planets share many of the same characteristics as planets, although notable differences remain. Their attributes are:


Dwarf planetary attributes
Name	Equator^* diam.	Mass^*	Orbital radius^* (AU)	Orbital period^* (years)	Orbital inclination (°)	Orbital eccentricity	Day^* (days)	Moons
Terrestrials	Ceres	0.15		2.5-2.9	4.6	10.587	0.080		none
Ice dwarfs	Pluto	0.18	0.002	39.5	248.5	17.1	0.249	-6.5	3
Ice dwarfs	2003 UB313	0.19		37.77-97.56	557	44.187	0.44177		1

*Measured relative to the Earth.

By definition, all dwarf planets are members of larger populations. Ceres is the largest body in the asteroid belt, while Pluto and 2003 UB313 are members of the Kuiper Belt. According to Mike Brown there may soon be over forty trans-Neptunian objects that qualify as dwarf planets under the IAU's recent definition. Before this, Ceres was considered an asteroid, a term which has now been replaced by small solar system bodies, and Pluto and 2003 UB₃₁₃ have been frequently called ice dwarfs by scientists studying the outer reaches of the solar system.[1]

Historical planets

When the objects Ceres, Pallas, Juno and Vesta were found orbiting between Mars and Jupiter in the early 1800s, they were declared and accepted as planets, and this remained the case for many years. However, as more and more objects began to be found in the same region of the solar system, they became classified as asteroids, along with their orbital kin.

Ceres' relative large size and degree of roundness meant that some have recently argued for it to qualify as a planet once again, but in 2006 it was placed by the International Astronomical Union in the new category of dwarf planets.

A similar scenario has occurred with Pluto. It was first discovered beyond Neptune in 1930 and was accepted by the International Astronomical Union as a planet after it was initially believed to be larger than the Earth. However, after further observation it was found that Pluto was actually much smaller, being less massive than the Moon.

After many similar new bodies were found in a similar region during the 1990s and the early 2000s, the IAU decided to reclassify Pluto, along with one other object in its belt, as dwarf planets. This decision was made on August 24, 2006.

Extrasolar planets

Main article: Extrasolar planet.

Of the 202 extrasolar planets (those outside our solar system) discovered to date (August 2006) most have masses which are about the same as or larger than Jupiter's.

Exceptions include a number of planets discovered orbiting burned-out star remnants called pulsars, such as PSR B1257+12,^[5] the planets orbiting the stars Mu Arae, 55 Cancri and GJ 436 which are approximately Neptune-sized,^[6] and a planet orbiting Gliese 876 that is estimated to be about 6 to 8 times as massive as the Earth and is probably rocky in composition.

It is far from clear if the newly discovered large planets would resemble the gas giants in our 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 our solar system, which makes it questionable whether they are the same type of planet at all. There is also a class of hot Jupiters that orbit so close to their star that their atmospheres are slowly blown away in a comet-like tail: the Chthonian planets.

Several projects have been proposed to create an array of space telescopes to search for extrasolar planets with masses comparable to the Earth. The NASA Terrestrial Planet Finder was one such program, but (as of 2006-02-06) this program has been put on indefinite hold. The ESA is considering a comparable mission called Darwin. 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.

In 2005, astronomers^[7] detected a planet in a triple star system, a finding that challenges current theories of planetary formation. The planet, a gas giant slightly larger than Jupiter, orbits the main star of the HD 188753 system, in the constellation Cygnus, and is hence known as HD 188753 Ab. The stellar trio (yellow, orange, and red) is about 149 light-years from Earth. The planet, which is at least 14% larger than Jupiter, orbits the main star (HD 188753 A) once every 80 hours or so (3.3 days), at a distance of about 8 Gm, a twentieth of the distance between Earth and the Sun. The other two stars whirl tightly around each other in 156 days, and circle the main star every 25.7 years at a distance from the main star that would put them between Saturn and Uranus in our own Solar System. The latter stars invalidate the leading hot Jupiter formation theory, which holds that these planets form at "normal" distances and then migrate inward through some debatable mechanism. This could not have occurred here; the outer star pair would have disrupted outer planet formation.

Interstellar planets

Interstellar planets are rogues in interstellar space, not gravitationally linked to any given solar system. For a brief time in 2006, astronomers beleived they had found a binary system of interstellar planets, which they termed Oph 162225-240515. However, recent analysis of the objects has determined that their masses are each greater than 13 Jupiter-masses, making the pair, once considered to be planemos, brown dwarfs. The existence of actual interstellar planets is considered likely based on computer simulations of the origin and evolution of planetary systems, which often include the ejection of bodies of significant mass.

The term is a controversial one. A prominent school of astronomers, including the IAU, argue that only objects that orbit stars qualify as planets and thus an "interstellar planet" is an oxymoron. The discoverers of the bodies mentioned above decided to avoid the debate over what constitutes a planet by referring to the objects as planemos. See also definition of planet.

Planetary formation

It is not known with certainty how planets are formed. The prevailing theory is that they are formed from those remnants of a nebula that do not condense under gravity to form a protostar. Instead, these remnants become a thin, protoplanetary disk of dust and gas revolving around the protostar and begin to condense about local concentrations of mass within the disc known as planetesimals. These concentrations become ever more dense until they collapse inward under gravity to form protoplanets.^[8] After a planet reaches a diameter larger than the Earth's moon, it begins to accumulate an extended atmosphere. This serves to increase the capture rate of the planetesimals by a factor of ten. ^[9]

When the protostar has grown such that it ignites to form a star, its solar wind blows away most of the disc's remaining material. 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.^[10]^[11] 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 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 due to this accretion, but the lost gases can be replaced by outgassing from the mantle and from the subsequent impact of comets.^[12] (Note that 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 is now believed to determine the likelihood that a star will have planets.^[13] Hence it is thought less likely that a metal-poor, population II star will possess a substantial planetary system than a metal-rich population I star.

References

^ IAU 2006 General Assembly: Result of the IAU Resolution votes
^ ^a ^b "Working Group on Extrasolar Planets (WGESP) of the International Astronomical Union". IAU. 2001. Retrieved 2006-05-25.
^ ^a ^b Falk, Michael (1999). "Astronomical Names for the Days of the Week". Journal of the Royal Astronomical Society of Canada. 93: 122–133.
^ Soter, Steven (2006-08-31). "What is a Planet" (PDF). Retrieved 2006-09-01. {{cite web}}: Check date values in: |date= (help)
^ "Scientists reveal smallest extra-solar planet yet found". SpaceFlight Now. 2005-02-11. Retrieved 2006-07-28.
^ "Fourteen Times the Earth". ESO. 2004-08-25. Retrieved 2006-07-22.
^ "NASA Scientist Finds World With Triple Sunsets". ESO. 2005-07-13. Retrieved 2006-07-22.
^ G. W. Wetherill (1980). "Formation of the Terrestrial Planets". Annual Review of Astronomy and Astrophysics. 18: 77–113.
^ S. Inaba, M. Ikoma (2003). "Enhanced Collisional Growth of a Protoplanet that has an Atmosphere". Astronomy and Astrophysics. 410: 711–723.
^ S. Kenton, B. Bromley. "Dusty Rings & Icy Planet Formation". Smithsonian Astrophysical Observatory. Retrieved 2006-07-25.
^ Ron Cowen (2003-01-25). "Planet Formation on the Fast Track". Science News. Retrieved 2006-07-25.
^ Musgrave, Ian (1998-06-01). "The Standard Model of Planet Formation". Retrieved 2006-07-23.
^ "Lifeless Suns Dominated The Early Universe". Harvard-Smithsonian Center for Astrophysics. 2004-01-06. Retrieved 2006-08-26.

Roy, Robert

Britt, Robert Roy (2000-11-02). "What is a Planet? Debate Forces New Definition (1)". space.com. {{cite news}}: Check date values in: |date= (help)
Britt, Robert Roy (2000-11-02). "What is a Planet? Debate Forces New Definition (2)". space.com. {{cite news}}: Check date values in: |date= (help)
Britt, Robert Roy (2004-09-10). "Likely First Photo of Planet Beyond the Solar System". space.com. {{cite news}}: Check date values in: |date= (help)

Stern, S.A., & Levison, H.F. 2002.

"Regarding the criteria for planethood and proposed planetary classification schemes."

External links

International Astronomical Union

The Planets

NinePlanets.org - tour of the solar system
Solar System Live (an interactive orrery)
Solar System Viewer (animation)
Pictures of the solar system
Renderings of the planets
NASA Planet Quest
Planets A website on Planets aimed for kids.
Test your knowledge of planets Lessons and quizzes on planets for kids.

Definition and reclassification debate

Working definition of "planet" from IAU WGESP — the lower bound remained a matter of consensus in February 2003
Dan Green's page on planet classification
Gravity Rules: The Nature and Meaning of Planethood; S. Alan Stern; March 22, 2004
IAU Press Release 01/99 "The status of Pluto: A Clarification"; IAU, 1999-02-03
BBC: "Planets plan boost tally 12" 2006-08-16
BBC: "Pluto loses status as a planet" 2006-08-24
BBC: "Pluto vote 'hijacked' in revolt" 2006-08-25

This article contains special characters. Without proper rendering support, you may see question marks, boxes, or other symbols.

Template:Link FA

[IAU-1] IAU 2006 General Assembly: Result of the IAU Resolution votes

[WSGESP-2] "Working Group on Extrasolar Planets (WGESP) of the International Astronomical Union". IAU. 2001. Retrieved 2006-05-25.

[weekdays-3] Falk, Michael (1999). "Astronomical Names for the Days of the Week". Journal of the Royal Astronomical Society of Canada. 93: 122–133.

[4] Soter, Steven (2006-08-31). "What is a Planet" (PDF). Retrieved 2006-09-01. {{cite web}}: Check date values in: |date= (help)

[5] "Scientists reveal smallest extra-solar planet yet found". SpaceFlight Now. 2005-02-11. Retrieved 2006-07-28.

[6] "Fourteen Times the Earth". ESO. 2004-08-25. Retrieved 2006-07-22.

[7] "NASA Scientist Finds World With Triple Sunsets". ESO. 2005-07-13. Retrieved 2006-07-22.

[8] G. W. Wetherill (1980). "Formation of the Terrestrial Planets". Annual Review of Astronomy and Astrophysics. 18: 77–113.

[9] S. Inaba, M. Ikoma (2003). "Enhanced Collisional Growth of a Protoplanet that has an Atmosphere". Astronomy and Astrophysics. 410: 711–723.

[10] S. Kenton, B. Bromley. "Dusty Rings & Icy Planet Formation". Smithsonian Astrophysical Observatory. Retrieved 2006-07-25.

[11] Ron Cowen (2003-01-25). "Planet Formation on the Fast Track". Science News. Retrieved 2006-07-25.

[12] Musgrave, Ian (1998-06-01). "The Standard Model of Planet Formation". Retrieved 2006-07-23.

[13] "Lifeless Suns Dominated The Early Universe". Harvard-Smithsonian Center for Astrophysics. 2004-01-06. Retrieved 2006-08-26.

[1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

[10]

[11]

[12]

[13]