Around 1677, Edmond Halley observed a transit of Mercury across the Sun, leading him to realise that observations of the solar parallax of a planet (more ideally using the transit of Venus) could be used to trigonometrically determine the distances between Earth, Venus, and the Sun. Christiaan Huygens followed on from Galileo's discoveries by discovering Saturn's moon Titan and the shape of the rings of Saturn. In the 17th century, Galileo discovered that the Sun was marked with sunspots, and that Jupiter had four satellites in orbit around it. 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. 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. The nearest stars are within the so-called Local Bubble, with the closest, Proxima Centauri, at 4.25 light-years.Īndreas Cellarius's illustration of the Copernican system, from the Harmonia Macrocosmica (1660)įor most of history, humanity did not recognize or understand the concept of the Solar System. The Solar System is located 26,000 light-years from the center of the Milky Way galaxy in the Orion Arm, which contains most of the visible stars in the night sky. The Oort cloud, which is thought 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 the interstellar medium it extends out to the edge of the scattered disc. The solar wind, a stream of charged particles flowing outwards from the Sun, creates a bubble-like region of interplanetary medium in the interstellar medium known as the heliosphere.
Each of the giant planets and some smaller bodies are encircled by planetary rings of ice, dust and moonlets. Six of the major planets, the six largest possible dwarf planets, and many of the smaller bodies are orbited by natural satellites, commonly called "moons" after the Moon. Various small-body populations, including comets, centaurs and interplanetary dust clouds, freely travel between the regions of the Solar System. Astronomers generally accept about nine objects as dwarf planets: the asteroid Ceres, the Kuiper-belt objects Pluto, Orcus, Haumea, Quaoar and Makemake, the scattered-disk objects Gonggong and Eris, and Sedna. Such objects are categorized as dwarf planets. Within these populations, some objects are large enough to have rounded under their own gravity and thus to be planets under some definitions, though there is considerable debate as to how many such objects there will prove to be. Beyond Neptune's orbit lie the Kuiper belt and scattered disc, which are populations of objects composed mostly of ice and rock, and beyond them a newly discovered population of sednoids. The asteroid belt, which lies between the orbits of Mars and Jupiter, contains objects composed of rock, metal and ice. The Solar System also contains smaller objects. All eight have nearly circular orbits that lie close to the plane of the Earth's orbit, called the ecliptic.
The two largest planets, Jupiter and Saturn, are gas giants, being composed mainly of hydrogen and helium the next two, Uranus and Neptune, are ice giants, being composed mostly of substances with relatively high melting points compared with hydrogen and helium, called volatiles, such as water, ammonia and methane. The four giant planets of the outer system are substantially more massive than the terrestrials.
The four inner system planets – Mercury, Venus, Earth and Mars – are terrestrial planets, being composed primarily of rock and metal. The vast majority of the system's mass is in the Sun, with the majority of the remaining mass contained in Jupiter. The Solar System formed 4.6 billion years ago from the gravitational collapse of a giant interstellar molecular cloud. Of the bodies that orbit the Sun indirectly-the natural satellites-two are larger than Mercury and one is nearly as large. Of the bodies that orbit the Sun directly, the largest are the four gas and ice giants and the four terrestrial planets, followed by an unknown number of dwarf planets and innumerable small Solar System bodies. The Solar System is the gravitationally bound system of the Sun and the objects that orbit it.