The Marvelous Solar System
Our celestial neighborhood in the vast expanse of the Milky Way galaxy
The Solar System is a gravitationally bound system of the Sun and the objects that orbit it, either directly or indirectly. Of the objects that orbit the Sun directly, the largest are the eight planets, with the remainder being smaller objects, such as the five dwarf planets and small Solar System bodies like asteroids and comets. The Solar System formed 4.6 billion years ago from the gravitational collapse of a giant interstellar molecular cloud.
At the center of the Solar System is the Sun, a G-type main-sequence star that contains 99.86% of the system's known mass. The four inner terrestrial planets—Mercury, Venus, Earth, and Mars—are primarily composed of rock and metal. The four giant planets of the outer system are substantially more massive than the terrestrials. The two largest, Jupiter and Saturn, are gas giants, being composed mainly of hydrogen and helium; the next two, Uranus and Neptune, are ice giants, composed mostly of substances with relatively high melting points compared with hydrogen and helium, called volatiles, such as water, ammonia, and methane.
The Solar System is also home to regions populated by smaller objects. The asteroid belt, which lies between Mars and Jupiter, mostly contains objects composed of rock and metal. Beyond Neptune's orbit lie the Kuiper belt and scattered disc, which are populations of trans-Neptunian objects composed mostly of ices. 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. The best-known dwarf planet is Pluto. In addition to these two regions, various other small-body populations, including comets, centaurs, and interplanetary dust clouds, freely travel between regions.
The solar wind, a stream of charged particles flowing outwards from the Sun, creates a bubble-like region in the interstellar medium known as the heliosphere. The heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of the interstellar medium; it extends out to the edge of the scattered disc. The Oort cloud, which is thought to be the source for long-period comets, may also exist at a distance roughly a thousand times further than the heliosphere.
