A planet is any celestial object, excluding the one we live on, that orbits around a solar remnant, in this case, the Sun, and has "cleared the neighborhood", a term used to describe a celestial object ridding itself of its planetesimals, or debris disks formed in the first stages of formation that soon come together to form the planet. These bodies have to have enough mass to pull itself into a shape resembling hydrospheric equilibrium, or nearly round shape. The word planet is derived from the Ancient Greek word meaning "wandering star".
Planets' classifications changed in the year of 2006 when the International Astronomical Union (IAU) issued an alternative definition of a planet. According to the IAU, a planet must: orbit the Sun, contain enough mass to contract into a shape showing hydrostatic equilibrium, or a nearly round shape, and have "cleared the neighborhood".
The planets were formed after the creation of the Sun. After the nebula cloud formed the Sun, the rest was used to create the planets. The inner planets are the innermost planets of the Solar System. They have rocky surfaces which are why they are the rocky planets, or the terrestrial planets. This is due to them being close to the Sun, which exerts a great amount of force. This force is called accretion. When accretion reached the nebula cloud, the cloud began to rotate and, due to them being exposed to high amounts of accretion, they became solid.
The outer planets, referred to as the gas giants or gaseous planets, were exposed to less accretion due to them being the outermost planets of the Solar System. Accretion did however reach the gas, causing them to become only gaseous due to low amounts of accretion. Yet, accretion was focused mainly on the center of the nebula cloud, therefore the gas giants are believed to have solid cores.
Dwarf planets are the smallest celestial bodies in the Solar System. In fact, they are so small, they are classified as asteroids. No one knows how these dwarf planets formed or where they might have originated. It is believed that they were caused when asteroids in the Asteroid belt or Kuiper belt collided with each other, creating tension. High amounts of heat fused these asteroids together and formed these dwarf planets. The dwarf planets are too far away and too small to have an accurate, focused picture of them. In fact, they only show up a few pixel lengths on computers.
Yet, not all dwarf planets are in the outer regions of the Solar System. Ceres is the only dwarf planet located in the Solar System, being located inside of the Asteroid belt. The rest are beyond the planet of Neptune. These bodies are known as Trans-Neptunian objects.
Before another classification of planets came in 2006, there were fifteen planets in the Solar System. A majority of them (Pluto, Haumea, Ceres, Eris, and Sedna) were moved to the classification of a dwarf planet. The rest of the smaller provisional dwarf planets became either asteroids or separate celestial bodies such as comets and asteroids.
Dwarf planets are extremely small compared to planets. Pluto, in fact, is smaller than Earth's moon, being able to fit in the United States. Eris is only about the size of the Mars' moon, Deimos. Because of their size, they cannot contract moons into their gravitational pulls due to them being so weak, however, they can maintain hydrostatic equilibrium. Pluto's moon, Charon, is Pluto's largest moon. Pluto, however is home to five other moons, yet they are so far away from Pluto because of Pluto's weak gravitational pull. Astronomers predict that in the next thousand years or so, these satellites will drift off into deep space.