General Astronomy/The Solar System
The Solar System may be broadly defined as that portion of the universe under the gravitational influence of the Sun. This includes the Sun itself as well as all planets, moons, asteroids, comets, dust, and ice orbiting the Sun.
The Solar System is an example of a star system, which is similarly defined as that portion of the universe under the gravitational influence of one or more co-orbiting stars. The Solar System is a unitary star system, as it has only one star (Sol, our Sun).
Components of the Solar System
The largest, most massive, and most prominent element of the Solar System is, of course, the Sun. The Sun makes up 99.8% of the mass of the Solar System. It is literally the point around which the entire Solar System turns. The Sun is virtually at the center of the Solar System; although gravity tugs by the planets may move the center of the System slightly away from the center of the Sun, it always resides deep within the Sun's core.
The next largest objects in the Solar System are the planets. There are generally considered to be eight planets in the Solar System. They can be divided into two types: (1) the gas giant planets,which include Jupiter, Saturn, Uranus, and Neptune and (2) the terrestrial planets Mercury, Venus, Earth, and Mars. All eight planets orbit the Sun in elliptical, roughly circular orbits, in approximately the same plane. However, no planet orbits in exactly a circular orbit or exactly in the plane of the Sun's rotation. The orbit of Jupiter is the closest to the plane and circularity; the orbit of Pluto (a dwarf planet) deviates the most from both the plane and from circularity.
After the eight major planets are the minor planets, asteroids and comets. Asteroids and comets are smaller objects than planets, but also orbit the Sun. Asteroids and comets are distinguished by their content: asteroids are primarily made up of rock, while comets are primarily made of ices and volatile compounds.
Minor planets may be found anywhere in the Solar System, in orbits varying from circular to highly elliptical. Most, however, are found in three belts. The main asteroid belt is found between the planets Mars and Jupiter. As the name implies, it is made almost entirely of asteroids. The Kuiper Belt is found outside the orbit of Neptune, and encompasses the area from 30 to 100 astronomical units from the Sun. The Kuiper belt contains mainly comets, including very large comet-like objects called cubewanos or plutinos. Some astronomers also consider Pluto to be part of the Kuiper belt. The Oort Cloud is another belt of comets, and is believed to extend out to approximately one light-year from the Sun. Its existence is deduced from the frequent visitation of long-period comets, comets with extremely elliptical or even hyperbolic orbits.
Arrangement of the Solar System
The Solar System may be divided by its components into three major regions: the inner system, the near outer system, and the far outer system. The near outer system might also be referred to as the middle system. The general term outer system refers to both the near and far outer systems.
The inner system is composed of the Sun (the largest mass), the terrestrial planets (rocky and closer to the sun) and their moons(moons are drawn to planets because of their gravitational force), close-orbiting asteroids and comets, and the main asteroid belt. Objects in the inner system are almost exclusively composed of rock, with either no atmosphere or an atmosphere that composes little of the object's mass. The inner system's boundary is defined by the main asteroid belt, which separates it from the near outer system.
The near outer system is composed of the gas giant planets and their moons, and asteroids and comets that orbit between the main asteroid belt and the Kuiper belt. Objects in the near outer system may have rock, liquid, gas, and ice as significant components. The near outer system's boundary is defined by the orbit of Neptune.
The far outer system is composed of the ice planet Pluto, the Kuiper belt, the Oort Cloud, and comets that orbit between the belt and the cloud. Objects in the far outer system may have some rock components, but are mainly composed of ices.
Boundary of the Solar System
The boundary of the Solar System is defined in two ways. The gravitational boundary may be described as the point at which objects no longer orbit the Sun. This boundary includes the Oort Cloud, but is poorly defined, as an object is not compelled to orbit the Sun at any point. Another definition is to declare the heliopause as the boundary of the Solar System. This boundary is more easily detectable and definable, but resides well within the Oort Cloud.