Of the planets in our solar system, the four closest to the Sun (Mercury, Venus, Earth, Mars) are considered terrestrial planets because except for their extreme temperatures, the other three are somewhat like earth. All are of similar size and others probably have metallic cores as Earth does. The other four large planets (Jupiter, Saturn, Uranus, Neptune) are called Jovian planets or gas giants. They are composed primarily of hydrogen, helium and other gases and they are much less dense than the terrestrial planets. Saturn would in fact float if it could be placed on a giant sea. Between the orbits of Mars and Jupiter lies the asteroid belt.
Until the year 2006, Pluto was considered a planet but not anymore. The scientists call it a ‘dwarf planet’. While celestial objects classified as planets must have a cleared neighborhood around its orbit, dwarf planets have no cleared neighborhood around its orbit.
Pluto is smaller than the smallest planet and even the moon. As it is very far from the sun, the heat and light that it receives from the sun is not enough to make it warm and bright. Findings reveal that Pluto would probably be composed of frozen gases. It orbits the sun not in a circle, but in a shape like an oval. Pluto is so far from the sun that it takes about 247.7 Earth years to complete a revolution around the sun.