October 19, 2021
The Solar System (with capital letter), or solar system (without capital letter), is the planetary system of the Sun, to which the Earth belongs. It is made up of this star and the celestial objects revolving around it: the eight confirmed planets and their 214 known natural satellites (usually called "moons"), the five dwarf planets and their nine known satellites, as well as billions of small bodies (almost all asteroids and other minor planets, comets, cosmic dust, etc.). The Solar System is part of the galaxy called the Milky Way, where it resides in the arm of Orion. It is located about 8 kpc (∼26 100 a.l.) from the galactic center, around which it revolves in 225 to 250 million years. It formed just under 4.6 billion years ago from the gravitational collapse of a molecular cloud, followed by the formation of a protoplanetary disk according to the nebula hypothesis. Schematically, the Solar System is made up of the Sun, which dominates it gravitationally - it comprises 99.85% of its mass - and provides energy by nuclear fusion of hydrogen into helium. In order of increasing distance from the star, the Inner Solar System includes four inner terrestrial planets, mainly composed of rocks and metals (Mercury, Venus, Earth and Mars) then an asteroid belt of small rocky bodies, including the dwarf planet Ceres. Further on, the four giant planets of the Outer Solar System orbit: successively two gaseous giants consisting mainly of hydrogen and helium that are Jupiter and Saturn - which also contain the vast majority of the total mass orbiting the Sun - and two ice giants Uranus and Neptune, containing more volatile substances such as water, ammonia and methane. All of them have an orbit close to the circle and are concentrated near the plane of the ecliptic, the plane of rotation of the Earth. The objects located beyond the orbit of Neptune, known as transneptunians, include in particular the Kuiper belt and the disc of scattered objects, formed of icy objects. Four icy dwarf planets are found in the Transneptunian region and are also called plutoids: Pluto - previously classified as a planet -, Hauméa, Makemake and Eris. The heliopause, the magnetic limit of the Solar System, is defined by the stopping of the solar winds against the winds of the interstellar medium at a hundred astronomical units, while the gravitational limit of the Solar System is located much further, up to one or two light years from the Sun, towards which a hypothetical spherical area, the Oort Cloud, could exist and be the source of long-lived comets. All of the planets in the Solar System starting from Earth have orbiting satellites - some, such as Ganymede and Titan, are larger than Mercury - while each of the four outer planets is further surrounded by a ring system. dust and other particles, the most prominent of which is that of Saturn. All the planets except Earth are named after gods and goddesses from Roman mythology. The Earth, with a thick atmosphere and 71% liquid water, is the only planet in the Solar System to harbor life and a thinking species that acts on its evolution. In the state of human knowledge, this planet in the Solar System has no equivalent in the Universe.