A planet is a general term given to a relatively large mass of accreted matter in orbit around a star. A mass that becomes massive enough for nuclear reactions to start is considered a star, instead of a planet.
It is unknown 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. 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.
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. Meanwhile, protoplanets that have avoided collisions may become natural satellites of larger planets through a process of gravitational capture.
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. (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.
This video shows 13 planets (mercury, venus, earth, mars, ceres, jupiter, saturn, uranus, neptune, pluto, haumea, makemake, and eris: http://www.youtube.com/watch?v=brnnUd4TaYI
- Main article: Extrasolar planet.
Of the 193 extrasolar planets (those outside our solar system) discovered to date (28 July 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, the planets orbiting the stars Mu Arae, 55 Cancri and GJ 436 which are approximately Neptune-sized, 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 occurance 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 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 these planets form at "normal" distances and then migrate inward through some debatable mechanism. This could not have occurred here, the outer star pair disrupting outer planet formation.
Interstellar planets are rogues in interstellar space, not gravitationally linked to any given solar system. No interstellar planet is known to date, but their existence is considered a likely hypothesis based on computer simulations of the origin and evolution of planetary systems, which often include the ejection of bodies of significant mass.
Definition and classification of planets
- Main article: Definition of planet
Much like "continent", "planet" is a word without a precise definition, with history and culture playing as much of a role as geology and astrophysics. Recent definitions have been vague and imprecise; The American Heritage Dictionary, for instance, formerly defined a planet as:
- A nonluminous celestial body larger than an asteroid or comet, illuminated by light from a star, such as the Sun, around which it revolves. In the solar system there are nine known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.'
However, for some time that definition has been viewed by many as inadequate. The eight largest planets are universally recognised as such (although Jupiter, Saturn and Neptune are not strictly nonlumious since they emit more radiation than they receive from the Sun), and for this reason are often universally referred to as "major planets", but there is controversy over Pluto and other smaller objects.
Suggested wide definitions
Since the discoveries of many of the objects in the Kuiper belt and around other stars, there has been a concerted push amongst scientists to come up with a precise definition of what constitutes a planet. In 1999, the IAU set up a working group to develop a scientifically plausible recommendation, but as of August 2005 they had not reached a conclusion. After the discovery of 2003 UB313 (informally called "Obi-Wan"), a member of the committee, Alan Stern, has said that the group wanted "to get something done, pronto." He also informed journalists that a "consensus" in the group was moving towards the following definition:
- A planet is a body that directly orbits a star, is large enough to be round because of self gravity, and is not so large that it triggers nuclear fusion in its interior.
Note that this definition also covers disputes at the upper end of a planet's size, which provides the extra benefit of forming a barrier between planets and brown dwarfs. Many consider this definition the best option as it sets up divisions based on physical characteristics rather than an arbitrary size limit. It is also somewhat universal in its application where other definitions have been crafted mainly to sort our own solar system into simple categories (such as placing the size limit as just under Mars, Mercury or Pluto). Depending how it is interpreted, objects counted as planets under such a new system would include some or all of the objects listed above, with potentially many more yet to be found.
Gibor Basri, professor of astronomy at the University of California, Berkeley, has suggested a similar definition and has also proposed the terms "fusor" (any object that achieves fusion in its core) and "planemo" (an object that is round from self-gravity but not a fusor) to help improve the astronomical nomenclature. Under Basri's definition:
- A planet is a planemo orbiting a fusor
These definitions have the advantage of creating a group including larger moons (which share many characteristics with the smaller planets) and also covering large free-roaming objects, which some astronomers think should be included in the definition of a planet. Basri has also suggested 'liberal use of adjectives' such as "major", "beltway", "dwarf", "giant", "super" and "historical". Others have suggested categories of planet/planemo based on composition such as "rock" (composed mainly of silicate), "gas" (composed mainly of hydrogen and helium), and "ice" (composed mainly of oxygen and carbon).
Suggested narrow definitions
There are alternate suggestions which would instead reduce the number of planets in the system. Upon his discovery of Sedna, Mike Brown of Caltech suggested a definition which would exclude both Sedna and Pluto from being classified as planets, proposing the following:
- A planet is any body in the solar system that is more massive than the total mass of all of the other bodies in a similar orbit
This definition generally plays down the importance of size, but instead focuses on the formation of the proposed planet. Under this definition, no Kuiper Belt objects (including Pluto) would be considered planets.
Brown's wish to "demote" Pluto prompted many to criticize him for setting out to create a purely scientific definition for a term which had an existing popular (albeit 'flawed') application. Upon his discovery of 2003 UB313, Brown indicated he had become a convert to this way of thinking, and proposed that whatever definition of planet be adopted, it should include both Pluto and any Kuiper Belt object found to be larger than Pluto. 
Planets within Earth's solar system can be divided into categories according to composition.
- Terrestrial or rocky: Planets that are similar to Earth — with bodies largely composed of rock: Mercury, Venus, Earth, Mars
- Jovian or gas giant: Those with a composition largely made up of gaseous material: Jupiter, Saturn, Uranus, Neptune. Uranian planets, or ice giants, are a sub-class of gas giants, distinguished from true Jovians by their depletion in hydrogen and helium and a significant composition of rock and ice.
- Icy: Sometimes a third category is added to include bodies like Pluto, whose composition is primarily ice; this category of "icy" bodies also includes many non-planetary bodies such as the icy moons of the outer planets of our solar system (e.g. Triton).
Many consider the Earth and its Moon to be a double planet, also called binary planet, for several reasons:
- The Moon, as measured by its diameter, is 1.5 times larger than Pluto.
- The gravitational force of the Sun on the Moon is larger than the gravitational force of the Earth on the Moon by a factor of approx. 2.2. (This is not a unique situation in the solar system. The Sun's gravity is also stronger than the primary's on Jupiter's moon S/2003 J 2; Uranus' moon S/2001 U 2; Neptune's moons S/2002 N 4 and Psamathe; and several asteroid moons. However, Luna is the sole case of this phenomenon affecting an object of planetary mass.)
The Pluto-Charon system has been considered to be a double planet because the mass ratio between these two bodies is small (about 8.6). So much so, that they both orbit a common centre of mass which lies beyond Pluto's surface, and are therefore said to orbit each other. Whereas, the centre of gravity of the Moon and Earth is within Earth itself, so the Moon is said to orbit the Earth, thereby, for some, ruling it out as a planet.
- "Scientists reveal smallest extra-solar planet yet found", SpaceFlight Now, 2005-02-11. Retrieved on 2006-07-28.
- "Fourteen Times the Earth", ESO, 2004-08-25. Retrieved on 2006-07-22.
- "NASA Scientist Finds World With Triple Sunsets", ESO, 2005-07-13. Retrieved on 2006-07-22.
- S. A. Stern, H. F. Levison (2000). "Regarding the Criteria for Planethood And Proposed Planetary Classification Schemes". 2000-11-11. Retrieved on 2006-07-28.
- Basri, Gibor (2003-03-20). Defining "Planet" (in English). Retrieved on 2006-07-22.
- Brown, Michael E. (2003-03-20). Sedna (in English). California Institute of Technology. Retrieved on 2006-07-22.
- Brown, Michael E. (2004-03-15). The discovery of 2003 UB313, the 10th planet. (in English). California Institute of Technology. Retrieved on 2006-07-22.
- NinePlanets.org - tour of the solar system
- MySolarSystem Educational Solar System Website.
- International Astronomical Union
- Solar System Live (an interactive orrery)
- Solar System Viewer (animation)
- Pictures of the solar system
- Renderings of the planets
- NASA Planet Quest
- 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
- On the status of Pluto; IAU, February 3, 1999
- Planets A website on Planets aimed for kids.