A planet is an object orbiting a star or stellar remnant that is large enough to be rounded by its own gravity. However, it is not large enough to cause thermonuclear fusion and has cleared its neighbouring region of planetesimals. Planets are divided into two main categories, low-density gas giants, and smaller rocky terrestrials.
In the Earth’s solar system there are eight planets. The four terrestrials are located closest to the sun. The four gas giants are the furthest from the sun. The eight planets in the solar system in order from increasing distance from the sun are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Six of the eight planets are orbited by one or more natural satellites. There are also five recognized dwarf planets in the Solar System, Ceres, Pluto, Haumea, Makemake and Eris.
Since 1992, hundreds of other planets orbiting around stars in the Milky Way have been discovered. These planets range in size and characteristics from the terrestrial planets similar to Earth and the gas giants larger than Jupiter.
Each planet has its own physical characteristics. They will each have a unique mass, internal differentiation, atmosphere and magnetosphere. This is precisely why life cannot be sustained on every planet. However, exploring other planets allows scientists to see clues on how the universe was created.