The place where one might look for life first are terrestrial planets. But, as we see in our solar system, not every terrestrial world offers good prospects for life. We have five in our solar system (if we count the Moon as well), and only one of them -- Earth -- abounds with life in manyfold forms, including sentient beings. There might be primitive microscopic life on Mars, but it is as good as certain that there is no life at all on Mercury, Venus and the Moon.

These worlds have all evolved differently. The evolutionary path a planet takes depends on two factors: surface temperature and surface gravity. Earth has exactly the right temperature, and it has enough gravity to hold an atmosphere dense enough to allow for liquid water. The Moon lies in the same optimal temperature zone -- but it is too small. With one sixth of Earth's gravity, it could not hold any significant atmosphere.

Mars is farther away from the sun than Earth, and it is colder. But the question remains if its problem really is its distance from the sun. It rather seems to be its size, similar to the case of the Moon. Regarding size, Mars lies in the middle between Earth and Moon. And this impression of Mars standing halfways between Earth and Moon is backed up by the comparison of other properties of the three bodies as well. It has an atmosphere, but it is tenuous: too thin to allow for significant amounts of liquid water. But it seems to have had a denser atmosphere in prehistoric times. Old river beds have been found on Mars, and this means that there must have been liquid water once. But the planet was too small to retain the dense atmosphere needed for that for long.

Now look at Venus. Venus is not too small -- it is only a bit smaller than Earth, the difference shouldn't matter. Indeed, it has a very dense atmosphere. The problem with Venus is that it is too close to the sun, and therefore too hot. It might have had liquid water once, but it was very warm nevertheless, and a run-away greenhouse effect set in, spoiling the thing.