Our solar system formed about 4.6 billion years ago. The four planets closest to the Sun - Mercury, Venus, Earth, and Mars - are called the terrestrial planets because they have solid, rocky surfaces. Two of the outer planets beyond the orbit of Mars - Jupiter and Saturn - are known as gas giants; the more distant Uranus and Neptune are called ice giants.
Note: This lithograph was revised in September 2013.
For the most recent planetary science developments, visit our Planets section.