The nine planets that orbit the Sun can be divided into two principal groups – the terrestrial planets and the giant planets. Mercury, Venus, the Earth, and Mars are known as the terrestrial (Earth-like) planets. Like the Earth itself, they are all relatively small, dense bodies composed of rocks and metals such as iron and nickel; they all have solid surfaces on which a spacecraft could land. Jupiter, Saturn, Uranus, and Neptune are called the giant, or jovian (Jupiter-like), planets because they are much larger than the Earth and are similar in a number of respects to the planet Jupiter. There are significant differences, though, between the larger giants (Jupiter and Saturn), which, like the Sun, are composed mainly of hydrogen and helium, and the smaller giants (Uranus and Neptune), which contain a much higher proportion of “icy” materials. Pluto fits into neither category. It is a tiny world of rock and ice on the outer fringes of the Solar System.

THE TERRESTRIAL PLANETS

Mercury

Innermost member of the Sun's system of planets, Mercury, with a diameter of 4,878 km, is the smallest of the terrestrial planets. At its mean distance of 57.9 million km (0.387 AU), it travels around the Sun in just under 88 days. Its orbit is markedly elongated; the planet's distance from the Sun ranges from 45.9 million km (0.306 AU) at perihelion to 69.8 million km (0.467 AU) at aphelion.