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A planet is generally considered to be a relatively large mass of accreted
matter 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. Based on
historical consensus, the International Astronomical Union (IAU) lists nine
planets in our solar system. Since the term "planet" has no precise scientific
definition, however, many astronomers contest that figure. Some say it should be
lowered to eight by removing Pluto from the list, whilst others claim it should
be raised to ten, fifteen, twenty, or even higher.
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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 don't condense
under gravity to form a protostar. Instead, these remnants become a thin disc of
dust and gas revolving around the protostar and begin to condense about local
concentrations of mass within the disc. These concentrations become ever more
dense until they collapse inward under gravity to form protoplanets. 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 moons of larger planets.
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 process of naming planets and their features is known as planetary
nomenclature. All the currently accepted planets in the solar system are named
after Roman gods, except for Uranus (named after a Greek god) and the Earth,
which was not seen as a planet by the ancients but rather the centre of the
universe. The original number of planets was seven: Sun, Moon, Mercury, Venus,
Mars, Jupiter, and Saturn. These were all seen as regular "wanderers" in the
sky. Developments in Astronomy removed the Sun and the Moon, and added the
currently accepted planetary members of the Solar System.
The designated planetary names are near-universal in the Western world, but some
non-European languages, such as Chinese, use their own. Moons are also named
after gods and characters from classical mythology, or, in the case of Uranus,
after characters from English literature. Asteroids can be named after anybody
or anything at the discretion of their discoverers, subject to approval by the
IAU's nomenclature panel.
Planets in approximate scale of size, but not distance. A portion of the solar disc is shown at the top.According to the authority of the IAU, there are nine planets in our solar system. In increasing distance from the Sun they are (with the astronomical symbol in brackets):
Click on a link for more information on the planet
However, there is some pressure for Pluto to be reclassified as a Kuiper Belt object, especially in light of the discovery of 2003 UB313. This object, however, has not yet received a definitive classification from the IAU.
When Ceres was found orbiting between Mars and Jupiter in 1801, it was
initially touted as a planet, but after many smaller objects were found with a
similar orbit, it was classified as an asteroid. However, due to its large size
(relative to the other asteroids), and its roughly spherical shape, Ceres would
be considered a planet by some astronomers' definitions.
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 order of increasing distance from the Sun)
Orcus, 2003 EL61, Quaoar, 2005 FY9, 2003 UB313 and Sedna. (However, it should be
noted that Sedna is often considered to be beyond the Kuiper Belt; being either
a member of the scattered disc or 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 2003 UB313 (nicknamed Xena),
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 likely
either see 2003 UB313 become a planet, or have Pluto stripped of its status.
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.
The discovery of a planet-sized satellite of a brown dwarf has blurred the
distinction between "planet" and "moon." A brown dwarf, though a star in theory,
in practice is often described as in between a planet and a star. It is formally
defined by the IAU by its official statement that "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."
To the IAU, 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."
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.
Such objects are not formally called planets, however, since the IAU has not
defined the term "planet".
