Jovian planets are a group of four planets in the solar system that are also known as gas giants. The four Jovian planets are Jupiter, Saturn, Uranus, and Neptune. The term "Jovian" means "Jupiter-like" and is derived from Jupiter, the largest of the outer planets and the first to be observed using a telescope. The general structures of the Jovian planets are opposite those of the terrestrial planets. Rather than having thin atmospheres around relatively large rocky bodies, the Jovian planets have relatively small, dense cores surrounded by massive layers of gas. Made almost entirely of hydrogen and helium, these planets do not have solid surfaces.