The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass (well over 99%) is in the Sun. Of the many objects that orbit the Sun, most of the mass is contained within eight relatively solitary planets whose orbits are almost circular and lie within a nearly flat disc called the ecliptic plane. 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, 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 ices, such as water, ammonia and methane, and are often referred to separately as "ice giants". The Solar System is also home to 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 is composed mainly 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 such as water, ammonia and methane. Within these populations, five individual objects, Ceres, Pluto, Haumea, Makemake and Eris, are recognized to be large enough to have been rounded by their own gravity, and are thus termed dwarf planets.[e] In addition to thousands of small bodies[e] in those two regions, various other small body populations, such as comets, centaurs and interplanetary dust, freely travel between regions. Six of the planets and three of the dwarf planets are orbited by natural satellites,[b] usually termed "moons" after Earth's Moon. Each of the outer planets is encircled by planetary rings of dust and other particles. 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 hypothetical Oort cloud, which acts as 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.
Age of the Solar System
The discovery of "deep time", the idea that time in our universe is measured in billions of years, goes back at least to the speculations of the German philosopher Immanuel Kant (1724-1804), the French mathematician and astronomer Pierre Simon Laplace (1749-1827) and the Scottish naturalist James Hutton (1726-1797). Kant and Laplace proposed the "nebular hypothesis" of the origin of the solar system, whereby the planets formed from condensation out of matter orbiting the early Sun. Obviously such an origin implies enormous amounts of time. Hutton proposed immensely long time spans to explain how the observable rates of erosion and deposition, and the ongoing volcanic activity, can be made responsible for the origin of great valleys, thick sediment sequences and mountain ranges, and all other features of Earth's surface. Early estimates of actual time spans, in the 19th century, ran to 100 million years and more. Of course, such estimates did not sit well with those who reckoned geologic time by studying the genealogies given in the Old Testament, which summarizes the history of the Jewish people. (In the 1650s Archbishop James Ussher of Ireland, one of the more careful and distinguished scholars of time, put the beginning of Earth at 4004 B.C., a surprisingly “precise” estimate.) An estimate of 100 million years or more proved unacceptable to the famous British physicist William...