Planet

Planet
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This article is about the astronomical term. For "planet" as defined by astrologers, see Planets in astrology. For the related but distinct class of objects, see Dwarf planet. For other uses, see Planet (disambiguation).

Artist's depiction of the extrasolar planet HD 209458 b orbiting its star
A planet, as defined by the International Astronomical Union (IAU) for the Solar System, is a body that orbits the Sun, is massive enough to be rounded by its own gravity, and has cleared its neighbouring region of planetesimals.[1][2] No formal definition has been made for extrasolar planets.
The term planet is an ancient one having 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 advanced, 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 criticized, 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, through the discovery of hundreds of extrasolar planets (planets around other stars), scientists are beginning to observe similar features throughout the Milky Way Galaxy.
Under IAU definitions, there are eight planets in the Solar System (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune) and 277 known extrasolar ones.[3] The Solar System also contains 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. Planets are generally divided into two main types: large, low-density gas giants and smaller, rocky terrestrials.
Contents[hide]
1 History
1.1 Antiquity
1.2 Modern times
1.3 Former classifications
1.4 Modern definition
2 Mythology
3 Formation
4 Solar System
4.1 Dwarf planets
5 Extrasolar planets
6 Interstellar "planets"
7 Attributes
7.1 Dynamic characteristics
7.1.1 Orbit
7.1.2 Axial tilt
7.1.3 Rotation
7.1.4 Orbital clearance
7.2 Physical characteristics
7.2.1 Mass
7.2.2 Internal differentiation
7.2.3 Atmosphere
7.2.4 Magnetosphere
7.3 Secondary characteristics
8 See also
9 References
10 External links
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Universe

The Universe is most commonly defined as everything that physically exists: the entirety of space and time, all forms of matter, energy and momentum, and the physical laws and constants that govern them. However, the term "universe" may be used in slightly different contextual senses, denoting such concepts as the cosmos, the world or Nature.
Astronomical observations indicate that the universe is 13.73 ± 0.12 billion years old and at least 93 billion light years across. The event that started the universe is called the Big Bang. At this point in time all matter and energy of the observable universe was concentrated in one point of infinite density. After the Big Bang the universe started to expand to its present form. Since special relativity states that matter cannot exceed the speed of light in a fixed space-time, it may seem paradoxical that two galaxies can be separated by 93 billion light years in 13 billion years; however, this separation is a natural consequence of general relativity. Stated simply, space can expand with no intrinsic limit on its rate; thus, two galaxies can separate more quickly than the speed of light if the space between them grows. Experimental measurements such as the redshifts and spatial distribution of distant galaxies, the cosmic microwave background radiation, and the relative percentages of the lighter chemical elements, support this theoretical expansion and, more generally, the Big Bang theory, which proposes that space itself was created ex nihilo at a specific time in the past. Recent observations have shown that this expansion is accelerating, and that most of the matter and energy in the universe is fundamentally different from that observed on Earth and not directly observable (cf. dark energy). The imprecision of current observations has hindered predictions of the ultimate fate of the universe.
Experiments suggest that the universe has been governed by the same physical laws and constants throughout its extent and history. The dominant force at cosmological distances is gravity, and general relativity is currently the most accurate theory of gravitation. The remaining three fundamental forces and the particles on which they act are described by the Standard Model. The universe has at least three dimensions of space and one of time, although extremely small additional dimensions cannot be ruled out experimentally. Spacetime appears to be smoothly and simply connected, and space has very small mean curvature, so that Euclidean geometry is accurate on the average throughout the universe.
Physical cosmology

Universe · Big BangAge of the universeTimeline of the Big BangUltimate fate of the universe
[show]Early universe
Inflation · NucleosynthesisGWB · Neutrino BackgroundCosmic microwave background
[show]Expanding universe
Redshift · Hubble's lawMetric expansion of spaceFriedmann equationsFLRW metric
[show]Structure formation
Shape of the universeStructure formationGalaxy formationLarge-scale structure
[show]Components
Lambda-CDM modelDark energy · Dark matter
[show]History
Timeline of cosmology...
[show]Experiments
Observational cosmology2dF · SDSSCOBE · BOOMERanG · WMAP
[show]Scientists
Einstein · Hawking · Friedman · Lemaître · Hubble · Penzias · Wilson · Gamow · Dicke · Zel'dovich · Mather · Smoot · others
This box: view • talk • edit
According to some speculations, this universe may be one of many disconnected universes, which are collectively denoted as the multiverse. In one theory, there is an infinite variety of universes, each with different physical constants. In another theory, new universes are spawned with every quantum measurement. By definition, these speculations cannot be tested experimentally.
Throughout recorded history, several cosmologies and cosmogonies have been proposed to account for observations of the universe. The earliest quantitative models were developed by the ancient Greeks, who proposed that the universe possesses infinite space and has existed eternally, but contains a single set of concentric spheres of finite size - corresponding to the fixed stars, the Sun and various planets - rotating about a spherical but unmoving Earth. Over the centuries, more precise observations and improved theories of gravity led to Copernicus' heliocentric model and the Newtonian model of the solar system, respectively. Further improvements in astronomy led to the characterization of the Milky Way, and the discovery of other galaxies and the microwave background radiation; careful studies of the distribution of these galaxies and their spectral lines have led to much of modern cosmology.
Contents[hide]
1 Etymology, synonyms and definitions
1.1 Broadest definition: reality and probability
1.2 Definition as reality
1.3 Definition as connected space-time
1.4 Definition as observable reality
2 Size, age, contents, structure, and laws
3 Historical models
3.1 Creation myths
3.2 Philosophical models
3.3 Astronomical models
4 Theoretical models
4.1 General relativity
4.2 Special relativity and space-time
4.3 Solving Einstein's equations
4.4 Prevailing Big Bang model
4.5 Alternative cosmologies
5 Multiverse
6 See also
7 Notes and references
8 Further reading
9 External links


Earth in the Universe
Universe
Observable universe
Large-scale structures
Virgo Supercluster
Local Group
Milky Way Galaxy
Orion Arm of the Milky Way
Gould Belt
Local Bubble
Local Interstellar Cloud
Solar System
Earth
Show expanded version with scale