From Infogalactic: the planetary knowledge core
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Space (or outer space) describes the vast empty regions between and around planets and stars. The study of these, and other, astronomical objects is called astronomy, one of the oldest sciences. It is often said that space exploration began with the launch of Sputnik 1, the first man-made object to orbit the Earth. Then, in an almost unbelievable feat of human achievement, in 1969 Neil Armstrong and Buzz Aldrin travelled to the Moon and set foot on the surface during the Apollo 11 mission. Recently, it has become clear that the possibility of space colonization may no longer be exclusively reserved for science-fiction stories, and many controversial issues surrounding space have come to light, including commercial spaceflight, space laws and space weapons.
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A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals. The planets were originally seen as a divine presence; as emissaries of the gods. As scientific knowledge advanced, the human perception of the planets changed over time, incorporating a number of disparate objects. On 24 August 2006, the IAU officially adopted a resolution defining planets within the Solar System. Copernicus suggested that the planets orbited the Sun, and this view was supported by Galileo after the development of the telescope. By careful analysis of the observation data, Johannes Kepler found their orbits to be not circular, but elliptical. Since 1992, through the discovery of hundreds of extrasolar planets, scientists are beginning to observe similar features throughout the Milky Way Galaxy. Planets are generally divided into two main types: large, low-density gas giants and smaller, rocky terrestrials. As of 1 June 2016, 3422 known extrasolar planets (in 2560 planetary systems and 582 multiple planetary systems) are listed in the Extrasolar Planets Encyclopaedia, ranging from the size of gas giants to that of terrestrial planets. Additionally, the IAU accepts five dwarf planets: Ceres, Pluto (originally classified as the Solar System's ninth planet), Makemake, Haumea and Eris. No extrasolar dwarf planets have been detected.
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An illustration of relative astronomical orders of magnitude, starting with the terrestrial planets of the Solar System in image 1 (top left) and ending with the largest known star, VY Canis Majoris, at the bottom right. The biggest celestial body in each image is shown on the left of the next frame.
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