The Solar System is the gravitationally bound system consisting of the Sun and all natural and artificial objects that orbit it. The Sun is a star located at the center of the Solar System and contains more than 99% of the system’s total mass. Its immense gravitational force holds the planets, dwarf planets, moons, asteroids, comets, meteoroids, dust, and gas in orbit around it. The Solar System formed approximately 4.6 billion years ago from the gravitational collapse of a large cloud of gas and dust known as a solar nebula. As this cloud contracted, most of the material accumulated at the center to form the Sun, while the remaining material flattened into a rotating disk from which the planets and other bodies developed. The Solar System has a rotational motion around the Suns axis, the planets orbit the Sun, and the entire Solar System orbits the center of the Milky Way.
The eight recognized planets of the Solar System, listed in order of increasing distance from the Sun, are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The four inner planets, Mercury, Venus, Earth, and Mars are primarily composed of rock and metal and are known as terrestrial planets. The four outer planets, Jupiter, Saturn, Uranus, and Neptune are much larger and consist mainly of gases, ices, and fluids. Jupiter and Saturn are commonly classified as gas giants, while Uranus and Neptune are classified as ice giants.
In addition to the planets, the Solar System contains dwarf planets, including Pluto, Ceres, Eris, Haumea, and Makemake. Many planets and dwarf planets possess natural satellites, commonly called moons. The Solar System also contains millions of smaller objects. Most asteroids are concentrated in the asteroid belt located between Mars and Jupiter, while many comets originate from distant reservoirs known as the Kuiper Belt and the Oort Cloud. The Kuiper Belt is a region beyond Neptune containing numerous icy bodies, whereas the Oort Cloud is a vast, distant spherical region believed to surround the Solar System and serve as a source of many long-period comets.
The distances within the Solar System are immense. Astronomers commonly use the astronomical unit (AU), which is defined as the average distance between Earth and the Sun, approximately 149.6 million kilometers (93 million miles). Neptune, the outermost planet, orbits at an average distance of about 30 AU from the Sun, while the Oort Cloud may extend thousands to tens of thousands of AU farther outward.
The Solar System can be defined as the Sun and every object that is gravitationally bound to it, including planets, dwarf planets, moons, asteroids, comets, meteoroids, dust, gas, and other small bodies, all moving through space under the influence of the Sun’s gravity.
