The Solar System consists of the Sun and its planetary system of eight planets, their moons, and other non-stellar objects. It formed 4.6 billion years ago from the collapse of a giant molecular cloud. The vast majority of the system's mass is in the Sun, with most of the remaining mass contained in Jupiter. The four smaller inner planets, Mercury, Venus, Earth and Mars, also called the terrestrial planets, are primarily composed of rock and metal. The four outer planets, called the gas giants, are substantially more massive than the terrestrials. The two largest, Jupiter and Saturn, are composed mainly of hydrogen and helium; the two outermost planets, Uranus and Neptune, are composed largely of substances with relatively high melting points (compared with hydrogen and helium), called ices, such as water, ammonia and methane, and are often referred to separately as "ice giants". All planets have almost circular orbits that lie within a nearly flat disc called the ecliptic plane.
For many thousands of years, humanity, with a few notable exceptions, did not recognize the existence of the Solar System. People believed the Earth to be stationary at the centre of the universe and categorically different from the divine or ethereal objects that moved through the sky. Although the Greek philosopher Aristarchus of Samos had speculated on a heliocentric reordering of the cosmos, Nicolaus Copernicus was the first to develop a mathematically predictive heliocentric system. His 17th-century successors, Galileo Galilei, Johannes Kepler and Isaac Newton, developed an understanding of physics that led to the gradual acceptance of the idea that the Earth moves around the Sun and that the planets are governed by the same physical laws that governed the Earth. Additionally, the invention of the telescope led to the discovery of further planets and moons. In more recent times, improvements in the telescope and the use of unmanned spacecraft have enabled the investigation of geological phenomena such as mountains and craters, and seasonal meteorological phenomena such as clouds, dust storms and ice caps on the other planets.
[youtubeHD=640,420]BT49AiYFV98[/youtubeHD] The Solar System also contains a number of regions populated by smaller objects.The asteroid belt, which lies between Mars and Jupiter, is similar to the terrestrial planets as it mostly contains objects composed of rock and metal. Beyond Neptune's orbit lie the Kuiper belt and scattered disc; linked populations of trans-Neptunian objects composed mostly of ices. Within these populations, several dozen to more than ten thousand objects may be large enough to have been rounded by their own gravity. Such objects are referred to as dwarf planets. Identified dwarf planets include the asteroid Ceres and the trans-Neptunian objects Pluto, Eris, Haumea, and Makemake. In addition to these two regions, various other small-body populations including comets, centaurs and 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 Earth's Moon. Each of the outer planets is encircled by planetary rings of dust and other small objects.
The solar wind, a flow of plasma from the Sun, creates a bubble in the interstellar medium known as the heliosphere, which 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 heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of interstellar wind. The Solar System is located within one of the outer arms of Milky Way galaxy, which contains about 200 billion stars.
樓主: 趙雲[子龍]
在線雷達
發表於 2013-1-26 13:01:08
發表於 2013-1-26 17:34:25
