We know that these so-called "inner planets" were fiercely bombarded by cometary objects for a full half billion years after they were formed. Life began when the bombardment abated, about 4 billion years ago. The discovery of complex organic molecules in interstellar dust, as well as in dust from comets, pointed to the correctness of this general picture of the dispersal of life that was first advocated by Sir Fred Hoyle and myself in the 1970s.
The building blocks of life, which may be as complex as bacteria and viruses, appear now to be widespread, for if bacteria are found on Mars, they would surely have been dispersed throughout the entire planetary system.
The recent discoveries of planets around nearby stars adds credence to the idea that many billions of inhabited Earth-like planets exist in our galaxy alone. Of the 100 billion Sun-like stars in our galaxy it would seem reasonable to expect that one per cent of these have planetary systems like our own. At the very least, the universe must be teeming with microbial life.
It would of course be naive to suggest that all life evolves as it has done on Earth. One would expect rather to find parallel evolutionary processes, starting from the same cosmic building blocks, to occur on every habitable planet in the universe. Intelligent life cannot be much of a rarity. As the International Search for Extraterrestrial Intelligence (SETI) gathers momentum, we may confidently expect to hear our first cosmic "hello" in the not too distant future.
The writer is Professor of Applied Mathematics and Astronomy at the University of Wales, Cardiff.Reuse content