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INTRODUCTION
INTRODUCTION
The Solar System consists of the Sun and the objects that orbit it. The name comes from Sōl, the Latin name for the Sun. It formed about 4.6 billion years ago when a dense region of a molecular cloud collapsed, creating the Sun and a protoplanetary disc from which the orbiting bodies assembled. The fusion of hydrogen into helium inside the Sun's core releases energy, which is primarily emitted through its outer photosphere. This creates a decreasing temperature gradient across the system. Over 99.86% of the Solar System's mass is located within the Sun.
Definition
The Solar System includes the Sun and all objects that are bound to it by gravity and orbit it.
The International Astronomical Union describes the Solar System as all objects that are bound by the gravity of the Sun, the Sun itself, its eight planets, and the other celestial bodies which orbit it. NASA describes the Solar System as a planetary system, including the Sun and all objects that orbit it.
When not used as a proper noun and written without capitalization, "solar system" may refer to either the Solar System itself or any system reminiscent of the Solar System.
The Sun
The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light and infrared radiation with 10% at ultraviolet energies. It is the main source of energy for life on Earth. The Sun has been an object of veneration in many cultures and a central subject for astronomical research since antiquity.
The most massive objects that orbit the Sun are the eight planets.
Between the bodies of the Solar System is an interplanetary medium of dust and particles. The Solar System is constantly flooded by outflowing charged particles from the solar wind, forming the heliosphere.
Inner Solar System
The inner Solar System is the region comprising the terrestrial planets and the asteroids. Composed mainly of silicates and metals, the objects of the inner Solar System are relatively close to the Sun; the radius of this entire region is less than the distance between the orbits of Jupiter and Saturn. This region is within the frost line, which is a little less than 5 AU from the Sun.
Comets
Comets are small Solar System bodies, typically only a few kilometers across, composed largely of volatile ices. They have highly eccentric orbits, generally a perihelion within the orbits of the inner planets and an aphelion far beyond Pluto. When a comet enters the inner Solar System, its proximity to the Sun causes its icy surface to sublimate and ionise, creating a coma: a long tail of gas and dust often visible to the naked eye.
Short-period comets have orbits lasting less than two hundred years. Long-period comets have orbits lasting thousands of years. Short-period comets are thought to originate in the Kuiper belt, whereas long-period comets, such as Hale–Bopp, are thought to originate in the Oort cloud. Many comet groups, such as the Kreutz sungrazers, formed from the breakup of a single parent. Some comets with hyperbolic orbits may originate outside the Solar System, but determining their precise orbits is difficult. Old comets whose volatiles have mostly been driven out by solar warming are often categorized as asteroids.
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