<P> The Sun, which comprises nearly all the matter in the Solar System, is composed of roughly 98% hydrogen and helium . Jupiter and Saturn, which comprise nearly all the remaining matter, are also primarily composed of hydrogen and helium . A composition gradient exists in the Solar System, created by heat and light pressure from the Sun; those objects closer to the Sun, which are more affected by heat and light pressure, are composed of elements with high melting points . Objects farther from the Sun are composed largely of materials with lower melting points . The boundary in the Solar System beyond which those volatile substances could condense is known as the frost line, and it lies at roughly 5 AU from the Sun . </P> <P> The objects of the inner Solar System are composed mostly of rock, the collective name for compounds with high melting points, such as silicates, iron or nickel, that remained solid under almost all conditions in the protoplanetary nebula . Jupiter and Saturn are composed mainly of gases, the astronomical term for materials with extremely low melting points and high vapour pressure, such as hydrogen, helium, and neon, which were always in the gaseous phase in the nebula . Ices, like water, methane, ammonia, hydrogen sulfide, and carbon dioxide, have melting points up to a few hundred kelvins . They can be found as ices, liquids, or gases in various places in the Solar System, whereas in the nebula they were either in the solid or gaseous phase . Icy substances comprise the majority of the satellites of the giant planets, as well as most of Uranus and Neptune (the so - called "ice giants") and the numerous small objects that lie beyond Neptune's orbit . Together, gases and ices are referred to as volatiles . </P> <P> The distance from Earth to the Sun is 1 astronomical unit (150,000,000 km), or AU . For comparison, the radius of the Sun is 0.0047 AU (700,000 km). Thus, the Sun occupies 0.00001% (10%) of the volume of a sphere with a radius the size of Earth's orbit, whereas Earth's volume is roughly one millionth (10) that of the Sun . Jupiter, the largest planet, is 5.2 astronomical units (780,000,000 km) from the Sun and has a radius of 71,000 km (0.00047 AU), whereas the most distant planet, Neptune, is 30 AU (4.5 × 10 km) from the Sun . </P> <P> With a few exceptions, the farther a planet or belt is from the Sun, the larger the distance between its orbit and the orbit of the next nearer object to the Sun . For example, Venus is approximately 0.33 AU farther out from the Sun than Mercury, whereas Saturn is 4.3 AU out from Jupiter, and Neptune lies 10.5 AU out from Uranus . Attempts have been made to determine a relationship between these orbital distances (for example, the Titius--Bode law), but no such theory has been accepted . The images at the beginning of this section show the orbits of the various constituents of the Solar System on different scales . </P>

Earth is what number planet from the sun