In the original definition of the habitable zone, its boundaries first encompassed the orbits of Venus to Mars, planets close enough to the Sun for solar energy to drive the chemistry of life – but not so close as to boil off water or break down the organic molecules on which life depends. These planets and their neighbourhood have been observed from the ground since the earliest times and explored in situ since the beginning of the space age. What have we found so far?

Although they are all members of the terrestrial planet family, Mercury, Venus, the Earth and Mars have very distinct properties. These differences are basically the result of two factors: their heliocentric distance (and hence their temperature) and their mass. Mercury, Venus and Mars, all easily visible with the naked eye, have been known since antiquity and celebrated in every mythology. But their analysis as physical objects has mostly developed since the beginning of the twentieth century, with the advent of spectroscopy and photography. A new era started in the 1960s with the advent of space exploration. The space adventure, marked with many failures, continues today. Two orbiters, Messenger and Venus Express, are currently observing Mercury and Venus, respectively; several orbiters are operating around Mars and a sophisticated rover, Curiosity, is exploring the nature of the Martian surface.