Solar System

 
The Solar System[a] 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 is in the Sun. Of the many objects that orbit the Sun, most of the mass is contained within eight relatively solitary planets[e] 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, several dozen of which are considered dwarf-planet candidates, various other small body populations including 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 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.
 
The Solar System formed from the gravitational collapse of a giant molecular cloud 4.568 billion years ago.[24] This initial cloud was likely several light-years across and probably birthed several stars.[25] As is typical of molecular clouds, this one consisted mostly of hydrogen, with some helium, and small amounts of heavier elements fused by previous generations of stars. As the region that would become the Solar System, known as the pre-solar nebula,[26] collapsed, conservation of angular momentum caused it to rotate faster. The centre, where most of the mass collected, became increasingly hotter than the surrounding disc.[25] As the contracting nebula rotated faster, it began to flatten into a protoplanetary disc with a diameter of roughly 200 AU[25] and a hot, dense protostar at the centre.[27][28] The planets formed by accretion from this disk,[29] in which dust and gas gravitationally attracted each other, coalescing to form ever larger bodies. Hundreds of protoplanets may have existed in the early Solar System, but they either merged or were destroyed, leaving eight planets, several dwarf planets, and leftover minor bodies