Solar System

**Solar System** is a star system consists of eight planets in order: Sun, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune, along with The Asteroid Belt between Mars and Jupiter, and the Kuiper belt, Oort cloud, and other Trans-Neptune objects such as Ceres, Pluto, Eris and Sedna. All of which are celestial bodies bound to The Sun by gravity, which formed from the collapse of a giant nebula cloud approximately 4.6 billion years ago. The Sun's retina of objects circle it in a nearly flat disc called the ecliptic plane, most of the mass of which is contained within eight relatively solitary planets whose orbits are almost circular. 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, Jupiter, Saturn, Uranus and Neptune, also called the gas giants, are composed largely of hydrogen and helium and are far more massive than the terrestrials.

The Solar System is also home to two main belts of small bodies. The asteroid belt, which lies between Mars and Jupiter, is a belt consisting of Asteroids, and a dwarf planet. The asteroid belt might be the rests of the creation of the Solar System, or might have been a planet in the beginning that was destroyed, which can explain all the numerous impact craters on Mars, its closest terrestrial planet neighbour. The Kuiper belt, which lies beyond Neptune's orbit, is composed mostly of ice such as water, ammonia and methane. Within these belts, five individual objects, Ceres, Pluto, Haumea, Makemake and Eris, are recognised to be large enough to have been rounded by their own gravity, and the dwarf planets. The hypothetical Oort cloud, which acts as the source for long-period comets, may also exist at a distance roughly a thousand times beyond these regions.

Within the Solar System, various populations of small bodies, such as comets and interplanetary dust, freely travel between these regions, while the solar wind, a flow of light and energy from the Sun, creates a bubble in the interstellar medium known as the heliosphere, which extends out to the edge of the scattered disc. 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.

## Discovery and exploration

For most of history, humanity did not recognize or understand the concept of the Solar System. Most people up to the Late Middle Ages–Renaissance believed 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.

In the 17th century, Galileo discovered that the Sun was marked with sunspots, and that Jupiter had four satellites in orbit around it. Christiaan Huygens followed on from Galileo's discoveries by discovering Saturn's moon Titan and the shape of the rings of Saturn. Edmond Halley realised in 1705 that repeated sightings of a comet were recording the same object, returning regularly once every 75–76 years. This was the first evidence that anything other than the planets orbited the Sun. Around this time (1704), the term "Solar System" first appeared in English. In 1838, Friedrich Bessel successfully measured a stellar parallax, an apparent shift in the position of a star created by Earth's motion around the Sun, providing the first direct, experimental proof of heliocentrism. Improvements in observational astronomy and the use of uncrewed spacecraft have since enabled the detailed investigation of other bodies orbiting the Sun.

## Location

The Solar System is located in the Local Interstellar Cloud, which is located in the Orion-Cygnus Arm of the Milky Way Galaxy. It is around 27,000 light years away from the Milky Way Galaxy's galactic center and has an orbital period around the galaxy every 225 million years. The solar apex, the direction of the Sun's path through interstellar space, is near the constellation Hercules in the direction of the current location of the bright star Vega. The plane of the ecliptic lies at an angle of about 60° to the galactic plane.

Within ten light-years of the Sun there are relatively few stars, the closest being the triple star system Alpha Centauri, which is about 4.4 light-years away and in the Local Bubble's G-Cloud. The next closest known fusors and rogue planets to the Sun are the red dwarf Barnard's Star (at 5.9 ly), the nearest brown dwarfs of the binary Luhman 16 system (6.6 ly), the closest known rogue or free-floating planetary-mass object at less than 10 Jupiter masses the sub-brown dwarf WISE 0855−0714, (7 ly), as well as the red dwarfs Wolf 359 (7.8 ly) and Lalande 21185 (8.3 ly).

The next closest at 8.6 ly is Sirius, the brightest star in Earth's night sky, with roughly twice the Sun's mass, orbited by the closest white dwarf to Earth, Sirius B. Other stars within ten light-years are the binary red-dwarf system Luyten 726-8 (8.7 ly) and the solitary red dwarf Ross 154 (9.7 ly). The closest solitary Sun-like star to the Solar System is Tau Ceti at 11.9 light-years. It has roughly 80% of the Sun's mass but only about half of its luminosity.

## See also

* Astronomical Symbols
* List of Solar System objects
* List of Solar System objects by size
* Planetary system
* Planetary orbit