Although the idea that the planets orbited the Sun had been suggested many times and had much scientific backing, it wasn’t until the 17th century that this view was supported by the first concrete evidence, which came in the form of the first telescopic astronomical observations, performed by Galileo Galilei. Ptolemy thought that the planets orbited Earth in deferent and epicycle motions. Some of the excluded objects include Ceres, Pallas, Juno, Vesta (all of which are objects in the solar asteroid belt), and Pluto (the first trans-Neptunian object discovered). Although eight of the planetary bodies discovered before 1950 remain planets under the current definition, some celestial bodies that were once considered planets by the scientific community are no longer viewed as such under the current definition. This definition is controversial because it excludes many objects of planetary mass based on where or what they orbit. In 2006, the International Astronomical Union (IAU) officially adopted a resolution defining planets within the Solar System. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects that ranged in size, shape, orbital plane, and other characteristics. These planets were regarded by many early cultures as divine or as emissaries of deities. Apart from Earth itself, five planets in the Solar System are often visible to the naked eye. The term planet is ancient, with ties to history, astrology, science, mythology, and religion. Sizes are not to scale.Ī planet is an astronomical body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and – according to the International Astronomical Union but not all planetary scientists – has cleared its neighbouring region of planetesimals. Shown in order from the Sun and in true color. Jupiter and Saturn ( gas giants) Uranus and Neptune ( ice giants) The eight known planets of the Solar System: Class of astronomical body directly orbiting a star or stellar remnant