A planet, as defined by the International Astronomical Union (IAU), is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, not massive enough to cause thermonuclear fusion in its core, and has cleared its neighbouring region of planetesimals.
After stars and stellar remnants, planets are some of the most massive objects known to man. They play an important part in the structure of planetary systems, and are also considered, along with large moons, the most feasible environment for life. Thus planetary science is essential not only to comprehend the structure of the universe, but also to better understand the development of life, and to aid the search for extraterrestrial intelligence. Additionally, the planets visible from Earth have played a vital role in the shaping of human culture, religion and philosophy in numerous civilisations. Even today, many people continue to believe the movement of the planets affects their lives, although such a causation is rejected by the scientific community.
In the absence of a formal scientific definition of "planet", the number of objects described as such has varied throughout history. This changed in 2006, when the IAU officially adopted a resolution defining planets within the Solar System. This definition has been both praised and criticised, and remains disputed by some scientists.
Under IAU definitions, there are eight planets in the Solar System (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune) and also at least three dwarf planets (Ceres, Pluto, and Eris). Many of these planets are orbited by one or more moons, which can be larger than small planets. There have also been more than two hundred planets discovered orbiting other stars. Planets are generally divided into two main types: large, low-density gas giants and smaller, rocky terrestrials. Dwarf planets, a separate category, can either be terrestrials or frozen ice dwarfs.