WHAT IS SOLAR SYSTEM?

THE MEANING AND DEFINITION OF

SOLAR SYSTEM

The Solar System comprises the Sun and the objects that orbit it, either directly or indirectly. Of those objects that orbit the Sun directly, the largest eight are the planets that form the planetary systemaround it, while the remainder are significantly smaller objects, such as dwarf planetsand small Solar System bodiessuch as cometsand asteroids. Of those that orbit the Sun indirectly, two are larger than the smallest planet.


The Solar System formed 4.6 billion years agofrom the gravitational collapse of a giant interstellar molecular cloud. The vast majority of the system's massis in the Sun, with most of the remaining mass contained in Jupiter. The four smaller inner planets, Mercury, Venus, Earthand Mars, are terrestrial planets, being primarily composed of rock and metal. The four outer planets are giant planets, being substantially more massive than the terrestrials. The two largest, Jupiter and Saturn, are gas giants, being composed mainly of hydrogenand helium;the two outermost planets, Uranus and Neptune, are ice giants, being composed largely of substances with relatively high melting points compared with hydrogen and helium, called ices, such as water, ammonia and methane.

All planets have almost circular orbits that lie within a nearly flat disc called the ecliptic.

The Solar System also contains smaller objects. The asteroid belt, which lies between Mars and Jupiter, mostly contains objects composed, like the terrestrial planets, of rock and metal. Beyond Neptune's orbit lie the Kuiper beltand scattered disc, populations of trans-Neptunian objectscomposed mostly of ices, and beyond them a newly discovered population of sednoids. Within these populations are several dozen to possibly tens of thousands of objects large enough to have been rounded by their own gravity. Such objects are categorized as dwarf planets. Identified dwarf planets include the asteroid Ceresand the trans-Neptunian objects Plutoand Eris.

In addition to these two regions, various other small-body populations, including comets, centaursand interplanetary dust, freely travel between regions. Six of the planets, at least three of the dwarf planets, and many of the smaller bodies are orbited by natural satellites, usually termed "moons" after the Moon. Each of the outer planets is encircled by planetary ringsof dust and other small objects.
The solar wind, plasmaflowing outwards from the Sun, creates a bubblein the interstellar mediumknown as the heliosphere. The heliopauseis the point at which pressure from the solar wind is equal to the opposing pressure of interstellar wind; it extends out to the edge of the scattered disc. The Oort cloud, which is believed to be the source for long-period comets, may also exist at a distance roughly a thousand times further than the heliosphere. The Solar System is located in the Orion Arm, 26,000 light-years from the center of the Milky Way.

Main article: Discovery and exploration of the Solar System
For many thousands of years, humanity, with a few notable exceptions, did not recognize the existence of the Solar System.

STRUCTURE AND COMPOSITION

The orbitsof the bodies in the Solar System to scale (clockwise from top left)
The principal component of the Solar System is the Sun, a G2 main-sequence starthat contains 99.86% of the system's known mass and dominates it gravitationally.The Sun's four largest orbiting bodies, the giant planets, account for 99% of the remaining mass, with Jupiter and Saturn together comprising more than 90%. The remaining objects of the Solar System (including the four terrestrial planets, the dwarf planets, moons, asteroids, and comets) together comprise less than 0.002% of the Solar System's total mass.
Most large objects in orbit around the Sun lie near the plane of Earth's orbit, known as the ecliptic. The planets are very close to the ecliptic, whereas comets and Kuiper beltobjects are frequently at significantly greater angles to it. All the planets and most other objects orbit the Sun in the same direction that the Sun is rotating (counter-clockwise, as viewed from above Earth's north pole). There are exceptions, such as Halley's Comet.
The overall structure of the charted regions of the Solar System consists of the Sun, four relatively small inner planets surrounded by a next belt of mostly rocky asteroids, and four giant planets surrounded by the Kuiper belt of mostly icy objects. Astronomers sometimes informally divide this structure into separate regions. The inner Solar System includes the four terrestrial planets and the asteroid belt. The outer Solar System is beyond the asteroids, including the four giant planets. [ 20 ]Since the discovery of the Kuiper belt, the outermost parts of the Solar System are considered a distinct region consisting of the objects beyond Neptune.

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