A planet in common parlance is a large object in orbit around a Star that is not a star itself. The name 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. The International Astronomical Union lists nine planets in our Solar system, based on historical consensus. Since the term planet has no scientific definition, many astronomers contest that figure, with some saying it should be lowered to eight (removing Pluto from the list), and others claiming it should be raised to fifteen, about twenty, or even dozens, depending on the definition used.
This theory is being greatly challenged with the discovery of extrasolar systems that vary greatly from our own.
According to the authority of the International Astronomical Union, there are nine planets in our solar system (in increasing distance from the Sun):
However, there is some pressure for Pluto to be reclassified as a Kuiper belt object, especially in light of the discovery of 2003 UB313, which, however, has not yet been accepted by the IAU.
Similarly, since 1992 many objects have been found in the predicted Kuiper belt that exists beyond Neptune. Several of the largest of these have challenged the planetary status quo, as they are both spherical and larger than the bodies in the Mars-Jupiter Asteroid belt, and are similar in size, orbit and composition to Pluto. However, as yet none have been accepted as planets by the IAU. The most significant of these are (in increasing distance from the Sun) Orcus, Ixion, 2002 UX25, Varuna, 2002 TX300, 1996 TO66, 2003 EL61,Quaoar, , , , and Sedna. (It should be noted that Sedna is often considered to be beyond the Kuiper belt and actually a member of the inner Oort cloud.)
Like Ceres before it, Sedna was widely touted as a planet when it was discovered in 2003, as it was the largest object found since Pluto. However, mainly due to its size still being smaller than Pluto's, it did not achieve planetary status from the IAU. However, the discovery in 2005 of , with a size and mass larger than Pluto seems to have forced the issue. As of September 2005 it has not yet been accepted as a planet, but the IAU is expected to announce a definition of a planet by the end of the year, which will either see 2003 UB313 become a planet, or have Pluto stripped of its status.
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:
However, for some time that definition has been viewed by many as inadequate. The eight largest planets (which are also the eight nearest to the Sun) are universally recognised as such, but there is controversy over Pluto and other objects rounded by their own gravity. For this reason these eight are often universally referred to by astronomers as "major planets".
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 , 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:
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.
However, 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:
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 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 application, "flawed" as it might be. Upon his discovery of 2003 UB313, Brown indicated he had become a convert to this way of thinking, and instead proposed that whatever definition of planet be adopted 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.
Many consider the Earth and its Moon to be a double planet, for several reasons:
The latter fact is not unique in the solar system, but is unusual for such a large satellite. Other satellites for which the Sun's gravity is actually stronger than the primary's:
Of the 173 extrasolar planets (those outside our solar system) discovered to date (October 2005) most have masses which are about the same 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 origin.
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.
The National Aeronautics and Space Administration of the United States has a program underway to develop a Terrestrial Planet Finder artificial satellite, which would be capable of detecting the planets with masses comparable to terrestrial planets. The frequency of occurrence of these planets is one of the variables in the Drake equation which estimates the number of intelligent, communicating civilizations that exist in our galaxy.
Astronomers have recently 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.
To the International Astronomical Union, the question of whether an object in orbit around a brown dwarf is a "planet" or a "moon" was simply not relevant, as it does not use the term "moon," only "satellite" and as yet has no official definition for "planet."
Such objects are not formally called planets, however, since the International Astronomical Union has not defined the term "planet".