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What is Solar system?
The solar system comprises the Earth's Sun and the retinue of celestial objects gravitationally bound to it. Traditionally, this is said to consist of the Sun, nine planets and their 158 currently known moons; however, a large number of other objects, including asteroids, meteoroids, planetoids, comets, and interplanetary dust, orbit the Sun as well. Astronomers are debating over the presence of a tenth planet. Although the term "solar system" is frequently applied to other star systems and the planetary systems which may comprise them, it should strictly refer to Earth's system specifically: the word "solar" is derived from the Sun's Latin name, Sol, and thus the term sometimes appears as Solar System. 
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Structure and Layout of the Solar System
The Sun is a main sequence G2 star that contains 99.86% of the system's known mass. Its two largest orbiting bodies, Jupiter and Saturn, account for 91% of the remainder. In broad terms, the charted regions of the solar system consist of the Sun and its planetary system: the eight bodies in relatively unique orbits commonly called planets or major planets and two belts of smaller objects, which can be called minor planets, planetoids, meteoroids, or planetesimals. Pluto, the ninth planet, is also considered a member of the outer belt, and its status is currently uncertain. Most objects in orbit round the Sun all lie within the same shallow plane, called the ecliptic plane, and orbit in the same direction. Many are in turn orbited by moons, and the largest are encircled by planetary rings of dust and other particles. The major planets are, in order, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Distances within the solar system are measured most often in astronomical units, or AU. One AU is the mean distance between the Earth and the Sun, or 149 598 000 kilometres. Other units in common use include the gigametre (Gm, one million kilometres) and the terametre (Tm, one billion/milliard kilometres). Pluto is roughly 38 AU (5.9 Tm) from the Sun, while Jupiter lies at roughly 5.2 AU (778 Gm). |
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Age and Origin of the Solar System
Using radiometric dating, scientists can estimate that the solar system is 4.6 billion years old. The oldest rocks on Earth are approximately 3.9 billion years old. Rocks this old are rare, as the Earth is constantly being resurfaced. To estimate the age of the solar system scientists must use meteorites, which were formed during the early condensation of the solar nebula. The oldest meteorites are found to have an age of 4.6 billion years, hence the solar system must be at least 4.6 billion years old. |
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Discovery of the Solar System
The planets out to Saturn were known to ancient astronomers, who observed the wandering of these objects against the apparently fixed pattern of stars. Venus and Mercury were each identified as single objects despite the difficulty of connecting "evening" and "morning stars". It was also identified that the two non-pointlike objects, the sun and the Moon, moved across the same fixed background. However knowledge of the nature of these celestial drifters was entirely speculative and largely incorrect. The nature and structure of the solar system were long misperceived, for at least two reasons:
Over the last several hundred years, conceptual and technological advances have helped humans understand the solar system much better. The first and most fundamental of the conceptual advances was the Copernican Revolution, which proposed that the planets orbit the sun—models of the solar system with the sun in the center are called heliocentric; helios meaning "Sun" in Greek). Despite the name, the most striking and then-controversial Copernican realization was not that the sun was central but that the Earth was peripheral, orbital: planets had been considered merely points in the sky, but if the Earth itself was a planet, perhaps the other planets were, like Earth, huge solid spheres. Philosophically, there were a number of objections to heliocentrism:
The subsequent invention of the telescope gave the principal technological advance on discovering the solar system, with Galileo's improved version of the telescope rapidly giving benefit in terms of discovering satellites of other planets, especially Jupiter's four major satellites. This showed that all objects in the universe did not orbit the Earth. However, perhaps Galileo's most important discovery was that the planet Venus has phases like the Moon, proving that it must orbit the Sun. Then, in 1687, Isaac Newton devised his law of universal gravitation which explained the force that both kept the Earth moving through the heavens and also kept the air from flying away. Finally, in 1838, astronomer Friedrich Wilhelm Bessel successfully measured the parallax of the star 61 Cygni, proving conclusively that the Earth was in motion. |