Ancient Greeks thought that the sun and the moon are planets as with Mercury, Venus, Mars, Jupiter, and Saturn. It was believed that Earth is the center of the universe and that the rest of the planets and celestial bodies revolve around it and this was scientifically rejected.
Astronomers in the 17th century, with the help of the telescope, proved that the sun was the celestial body around which the earth and other planets orbit around, and that the moon is not a planet.
After the failing of Copernicus hypothesis that the center of the universe is Earth, the term planet was launched only for those larger objects revolving around the sun, where the objects were discovered giant Uranus in 1781 and Neptune in 1846.

In 2006, the International Astronomical Union set out conditions that the celestial body must meet to be a planet in the solar system, to rotate in a fixed orbit around the sun.
Among the planets currently recognized in the solar system, the four planets closest to the sun are : Mercury, Venus, Earth, and Mars. The furthest planets are: Jupiter, Saturn, Uranus, and Neptune, between them locate the two main groups two which are small objects called asteroids or dwarf planets.

The habitable planets are Earth-like and have abundant liquid water and are called "water planets". The sun influences the nature of the planets and the nature of their surface, where the water froze on the planet's surface if it is away from the sun and lead to thermal retention and large evaporation of water if close to the sun.
Scientists have been able to recognize that terrestrial planets which consist of larger deserts and contain a thin water cover ,contain larger habitable areas. the solar planets are nine namely from the closet to sun:
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.