The answer is 361. And that's just the low-ball figure, according to a new study by Duncan Forgan of the University of Edinburgh.
Forgan's work builds on the Drake Equation, a formula from the 1960s that attempted to pinpoint the number of alien civilization in the Milky Way. The equation considers the number of stars formed each year, how many will have planets orbiting them, and the chance of life developing on those planets. Dr. Drake's problem was that he didn't have clear answers to many of these questions.
Now that science has progressed almost 50 years, Forgan has added in modern figures and (he hopes) calculated a more accurate number.
The paper considers three different models for the odds of alien life:
i. panspermia: if life forms on one planet, it can spread to others in a system
ii. the rare-life hypothesis: Earth-like planets are rare but life progresses pretty well on them when they occur
iii. the tortoise and hare hypothesis: Earth-like plants are common but the steps towards civilisation are hard
Each model came out with a different number: 37,964.97 for panspermia, 361.2 for rare life, and 31,573.52 for tortoise and hare.
But, as several people have pointed out since Forgan released this paper, just because these numbers are really precise (down to the hundredth digit) doesn't mean they are really accurate. After all, "the results of simulations like this are no better than than the assumptions you make in developing them," writes the Physics arXiv Blog. As science advances over the next 50 years, our knowledge of this galaxy will grow – and maybe by then we'll actually met one of the 360 other civilizations.