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.
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.
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.