Some of the theories that attempt to explain the formation of planetary systems include the Nebular Cloud Theory, Passing Star Theory and Solar Nebular Disk Model. The Nebular Cloud Theory suggests that a cloud of gas and dust condensed to form the planets and the sun. The Passing Star Theory states that a star moved next to the sun, removed its materials and the materials combined to form planets. Astronomers argue, however, that the likelihood of such a close proximity to the sun by a star is very low. The Solar Nebular Disk Model, perhaps the most accepted theory by astronomers, involves rotation of the accumulated dust, matter, gas, planetesimals and asteroids that form by the gravitational collapse of a molecular cloud. Nebular Cloud Theory contends that clumps of galaxies, planets and matter are the resultant effect of the gravitational force of gaseous clouds.
A planet is a celestial body with a hydrostatic equilibrium shape, and it orbits around a sun. Earth's solar system consists of eight planets, four of which are small, rocky terrestrials and four of which are large, low-density gas giants. The latter are Neptune, Uranus, Saturn and Jupiter, in order of decreasing distance from the sun. The four terrestrials, in order of increasing distance from the sun, are Mercury, Venus, Earth and Mars. With the discovery of large objects around other stars, Pluto, which had been classified as the ninth planet from the sun, was declassified in August 2006.
A dwarf planet is not a satellite. It is a celestial, spherical body that attained a hydrostatic equilibrium shape and orbits around a star. It is spherical due to its gravitational force with the sun. The International Astronomical Union classifies only five objects as dwarf planets: Pluto, Makemake, Ceres, Eris and Haumea. Astronomers who studied Pluto and Ceres extensively revealed that those dwarf planets have a diameter of 838km and an absolute magnitude of more than plus one (+1). Astronomers believe, however, that more than 40 objects in the planetary system's Kuiper Belt can be classified as dwarf planets.
Small solar system bodies orbit around the sun but are not planets, dwarf planets or satellites. They consist of comets, asteroids, the Kuiper Belt, Trans Neptune objects and unknown objects. Usually, asteroids orbit Jupiter and Mars, but some asteroids can orbit the sun. Comets have elliptical orbits and are usually far from the sun. Nonetheless, some comets pass next to the sun.