the term planet is ancient with ties to history astrology science mythology and religion several planets in the solar system can be seen with the naked eye these were regarded by many early cultures as divine or as emissaries of deities as scientific knowledge advanced human perception of the planets changed incorporating a number of disparate objects 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 transneptunian 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 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  at about the same time by careful analysis of pretelescopic observation data collected by tycho brahe 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  planets are generally divided into two main types large lowdensity 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 six of the planets are orbited by one or more natural satellites 