Original: The two largest , Jupiter and Saturn , are composed mainly of hydrogen and helium ; the two outermost planets , Uranus and Neptune , are composed largely of ices , such as water , ammonia and methane , and are often referred to separately as '' ice giants '' .
Simplified: But with more recent discoveries of objects in the Kuiper Belt , and the Scattered Disk , and with more dwarf planet discoveries , the Outer solar system is now usually defined as above .