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The Solar System consists of the Sun and the astronomical objects bound to it by gravity, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. 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 two 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 trans-Neptunian objects composed mostly of ices such as water, ammonia and methane. Within these two regions, 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 planetsIn addition to thousands of small bodie in those two regions, various other small body populations, such as comets, centaurs and interplanetary dust, freely travel between regions.

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.

Six of the planets and three of the dwarf planets 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 particles.* 1 Discovery and exploration
* 2 Structure
* 3 Sun
o 3.1 Interplanetary medium
* 4 Inner Solar System
o 4.1 Inner planets
+ 4.1.1 Mercury
+ 4.1.2 Venus
+ 4.1.3 Earth
+ 4.1.4 Mars
o 4.2 Asteroid belt
+ 4.2.1 Ceres
+ 4.2.2 Asteroid groups
* 5 Outer Solar System
o 5.1 Outer planets
+ 5.1.1 Jupiter
+ 5.1.2 Saturn
+ 5.1.3 Uranus
+ 5.1.4 Neptune
o 5.2 Comets
+ 5.2.1 Centaurs
* 6 Trans-Neptunian region
o 6.1 Kuiper belt
+ 6.1.1 Pluto and Charon
+ 6.1.2 Haumea and Makemake
o 6.2 Scattered disc
+ 6.2.1 Eris
* 7 Farthest regions
o 7.1 Heliopause
o 7.2 Oort cloud
+ 7.2.1 Sedna
o 7.3 Boundaries
* 8 Galactic context
o 8.1 Neighbourhood
* 9 Formation and evolution
* 10 See also
* 11 Notes
* 12 References
* 13 External links
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 center 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. 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.
Solar System
Solar System
Solar System
Solar System
Solar System
Solar System
Solar System
Solar System
Solar System