Due to unplanned maintenance of the back-end systems supporting article purchase on Cambridge Core, we have taken the decision to temporarily suspend article purchase for the foreseeable future. We apologise for any inconvenience caused whilst we work with the relevant teams to restore this service.
Nothing drives astronomy like that oldest of all philosophical questions: Are we alone in the universe? Spacecraft missions concentrate on Mars because of the Red Planet's relative similarity to Earth and the existence of water there, leading to the possibility of microbial life. The rapidly growing cottage industry of finding and studying extrasolar planets looks forward to detecting Earth analogs that may also reveal atmospheric signatures of living beings. The discovery of life elsewhere in the cosmos would certainly mark one of the most incredible moments in human history, a milestone at which we would understand we are not unique in the universe.
Of course, we know of only one example of life in the universe, right here on Earth. In the minds of some, that means the odds of life being an extremely rare thing in the cosmos are high. At least intelligent life; civilizations that could communicate. They point back to the idea that Italian physicist Enrico Fermi (1901–1954) raised in 1950: “If the universe contains life, then where is it? Why hasn't life showed up on our doorstep?” The so-called Fermi Paradox still stands as a fair question. But the odds of life in the universe are staggeringly large, overwhelmingly so, in the minds of the majority of astronomers and cosmologists.
Recall that the universe contains at least 100 billion galaxies, and probably considerably more because inflation theory means we are not seeing the whole universe that exists. And let's consider the number of stars in a galaxy like the Milky Way, about 400 billion. Let's set inflation aside. From what we see of star systems near the Sun, planetary systems appear to be common, and we are seeing the first glimpses of planets within the habitable zones of their suns – the areas in which water would be a liquid. From what we can see, water is essential for life.
Astronomers currently believe that something like 70 or 80 percent of stars have planets. With 100 billion galaxies in the universe and to play it conservatively, let's assume roughly 100 billion stars on average for each galaxy. That works out to 10,000 billion billion stars in the universe, and roughly 1022 planetary systems, or 8,000 billion billion.