The sun is a G-type star that's about half-way through its life. It makes up 99 percent of the mass of the solar system. Nearly everything in the solar system orbits the sun.
An object is a planet if it has enough mass to form a sphere and is able to clear its neighborhood. This means that no other large bodies share its orbit. Mercury, Venus, Earth and Mars are the four inner, or terrestrial planets in the solar system. An asteroid belt lies between the inner and outer planets. Beyond it are the gas and ice giants Jupiter, Saturn, Uranus and Neptune. All of these planets have moons except Venus and Mercury.
Dwarf planets don't clear their neighborhood, and some of them are smaller than the moons of regular planets. Pluto is considered a dwarf planet that has moons of its own.
Asteroids are rocky, icy bits astronomers believe were left over from the formation of the solar system. They orbit the sun but are too small to be planets. Still, they are larger than meteoroids, which travel through space. Some meteoroids are no bigger than grains of sand.
