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The term planet is ancient, with ties to history, astrology, science, mythology, and religion.

Planets are generally divided into two main types: large low-density giant planets, and smaller rocky terrestrials.

Under IAU definitions, there are eight planets in the Solar System.

In order of increasing distance from the Sun, they are the four terrestrials, Mercury, Venus, Earth, and Mars, then the four giant planets, Jupiter, Saturn, Uranus, and Neptune.

Although the idea that the planets orbited the Sun had been suggested many times, it was not until the 17th century that this view was supported by evidence from the first telescopic astronomical observations, performed by Galileo Galilei.

By careful analysis of the observation data, Johannes Kepler found the planets' orbits were not circular but elliptical.

As observational tools improved, astronomers saw that, like Earth, the planets rotated around tilted axes, and some shared such features as ice caps and seasons.Since the dawn of the Space Age, close observation by space probes has found that Earth and the other planets share characteristics such as volcanism, hurricanes, tectonics, and even hydrology.In 2006, the International Astronomical Union (IAU) officially adopted a resolution defining planets within the Solar System.This definition is controversial because it excludes many objects of planetary mass based on where or what they orbit.Although eight of the planetary bodies discovered before 1950 remain "planets" under the modern definition, some celestial bodies, such as Ceres, Pallas, Juno and Vesta (each an object in the solar asteroid belt), and Pluto (the first trans-Neptunian object discovered), that were once considered planets by the scientific community, are no longer viewed as such.The planets were thought by Ptolemy to orbit Earth in deferent and epicycle motions.