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# Solar system

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The solar system consists of the Sun and its associated objects and phenomena, including the Earth, the planet on which we live. It is the best-known example of a planetary system, which more generally is composed of one or more stars and whatever matter (planets, rocks, dust) is gravitationally bound to them.

Composition and Structure of the Solar System

The principal component of the solar system is the Sun, a comparatively small star which nevertheless contains 99.9 percent of the mass of the solar system and dominates it gravitationally. In addition, because of its large mass, the interior of the Sun achieves the density necessary to sustain nuclear fusion, releasing enormous amounts of energy, some of which is radiated into space in the form of electromagnetic radiation, mostly visible light, and a flow of charged particles called the solar wind, which profoundly affects the environment of the solar system. The remaining small amount of mass outside the Sun includes the nine planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto), and their moons and [ring system]?s. In addition, there are asteroids, comets, [trans-Neptunian objects]? and [Kuiper Belt]? objects, a theorized Oort cloud, and interplanetary dust? and gas.

Origin and Evolution of the Solar System

The solar system as a whole is currently believed to have formed through the normal process of star formation which produced the Sun itself, rather than through some special process (such as a stellar near-collision) as was once thought. The process is believed to have begun with the gravitational collapse of a cloud of interstellar gas and dust to produce what is called the solar nebula.

Discovery and Exploration of the Solar System

Because of the geocentric perspective from which humans viewed the solar system, its nature and structure were long misperceived. The apparent motions of solar system objects as viewed from a moving Earth were believed to be their actual motions about a stationary Earth. In addition, many solar system objects and phenomena are not directly sensible by humans without technical aids. Thus both conceptual and technological advances were required in order for the solar system to be correctly understood.

The first and most fundamental of these advances was the Copernican Revolution, which adopted a heliocentric model for the motions of the planets. Indeed, the term "solar system" itself derives from this perspective. But the most important consequences of this new perception came not from the central position of the Sun, but from the orbital position of the Earth, which suggested that the Earth was itself a planet, and the planets other Earths. This was the first indication of the true nature of the planets. Also, the lack of perceptible stellar parallax despite the Earth's orbital motion indicated the extreme remoteness of the fixed stars, which prompted the speculation that they could be objects similar to the Sun, perhaps with planets of their own.

The Solar System and Other Planetary Systems

Until recently, the solar system was the only known example of a planetary system, although it was widely believed that other comparable systems did exist. A number of such systems have now been detected, although the information available about them is very limited. The technique employed involves the detection through the Doppler effect of periodic variations in the motion of parent stars which is attributed to the presence of planets. This allows the mass and orbital characteristics of the unseen planets to be determined. Unfortunately the sensitivity of these techniques currently does not permit the detection of planets of mass and orbit comparable to the Earth.

Major Planets of the Solar System - Equatorial diameter (miles)

- Mercury 3,031
- Venus 7,520
- Earth 7,926
- Mars 4,200
- Jupiter 88,700
- Saturn 74,600
- Uranus 31,570
- Neptune 30,200
- Pluto 1,900

Major Planets of the Solar System - Mass in relation to the Earth's mass

- Mercury 0.06
- Venus 0.82
- Earth 1.00
- Mars 0.11
- Jupiter 318.00
- Saturn 95.00
- Uranus 14.60
- Neptune 17.20
- Pluto 0.0017

/Talk

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