Solar system. Computer artwork of the planets of the solar system, seen from the surface of the Earth's Moon (upper right). The planets are (clockwise from upper left) Saturn, Uranus, Earth, Jupiter, Mars and Neptune. The planet at upper centre is Venus and the small planet at centre is Mercury. The small planet at bottom left is Pluto. Mercury, Venus, Earth and Mars (the inner planets) are dense and rocky. Jupiter, Saturn, Uranus and Neptune are gas giants composed mainly of hydrogen and helium. Pluto, the smallest and most distant planet from the Sun, is also thought to have a rocky core. The Moon is the Earth's only natural satellite.