Planet

The planets were anticipation by Ptolemy to apogee the Earth in deferent and epicycle motions. Although the abstraction that the planets orbited the Sun had been appropriate abounding times, it was not until the 17th aeon that this appearance was accurate by affirmation from the aboriginal scope ample observations, performed by Galileo Galilei. By accurate assay of the ascertainment data, Johannes Kepler begin the planets' orbits to be not circular, but elliptical. As empiric accoutrement improved, astronomers saw that, like Earth, the planets rotated about agee axes, and some aggregate such appearance as ice caps and seasons. Since the aurora of the Space Age, abutting ascertainment by probes has begin that Earth and the added planets allotment characteristics such as volcanism, hurricanes, tectonics, and alike hydrology.

Planets are about disconnected into two capital types: large, low-density gas giants, and smaller, bouldered terrestrials. Under IAU definitions, there are eight planets in the Solar System. In adjustment of accretion ambit from the Sun, they are the four terrestrials, Mercury, Venus, Earth, and Mars, again the four gas giants, Jupiter, Saturn, Uranus, and Neptune. Six of the planets are orbited by one or added accustomed satellites. Additionally, the Solar System additionally contains at atomic bristles dwarf planets and hundreds of bags of baby Solar System bodies.

Since 1992, hundreds of planets about added stars ("extrasolar planets" or "exoplanets") in the Milky Way Galaxy accept been discovered. As of July 15, 2012, 777 accepted extrasolar planets (in 623 all-embracing systems and 105 assorted all-embracing systems) are listed in the Extrasolar Planets Encyclopaedia, alignment in admeasurement from that of earthbound planets agnate to Earth to that of gas giants beyond than Jupiter. On December 20, 2011, the Kepler Space Telescope aggregation appear the analysis of the aboriginal Earth-sized extrasolar planets, Kepler-20e and Kepler-20f, orbiting a Sun-like star, Kepler-20. A 2012 study, allegory gravitational microlensing data, estimates an boilerplate of at atomic 1.6 apprenticed planets for every brilliant in the Milky Way.