Probably because the Drake Equation is finally coming into its own. It has seven terms, and American astronomer Frank Drake could not give a value to any of them when he first wrote it in 1961. It was just a formula to estimate the number of civilisations in the Milky Way when the relevant data eventually became available.
To fill in the first three terms, we needed to know how many stars there are in the galaxy, how many of them have planets, and how many of those planets are in the "habitable zone" where liquid water can exist.
In 1961, the estimate was 100 billion stars - now it is 400 billion, of which 300 billion are essentially similar to our sun.
Until 1992, we didn't even know if other stars had planets circling them. Now we can estimate that at least 40 per cent of them do.
As for how many planets are in the "habitable" zone, not too close or too far from their parent star, the answer is probably one or two per star.
Using the data acquired in the past 20 years, Nasa estimates that there are 144 billion habitable planets in our galaxy. Not all of them will harbour life, of course, but that is an encouraging number.
There are other questions: how many "habitable" planets will actually have life on them? On how many of those planets will an intelligent species appear? How many of those intelligent species will build civilisations that use electromagnetic communications?
We don't know the answers to those questions, but we do know that organic compounds are common even in interstellar space, and that they are continuously raining on our own planet. So the standard assumption is that they somehow combined on Earth to form the first single-celled creatures, and evolution did the rest.
But if it were easy for those organic compounds to combine into complex microbes and viruses, then you would expect it to have happened here a number of times. There would be several or many unrelated genetic lineages on Earth - and there aren't. All life here has a common ancestor.
So it must be rare for life to develop spontaneously. If it actually happened here, it would mean that we are a miracle, and pretty much alone in the galaxy.
But maybe the miracle happened on another of those 144 billion planets, billions of years ago, and life has been spreading through the galaxy ever since - not as alien beings on starships, but as microbes and viruses on meteorites and comets.
This is the "panspermia" hypothesis proposed by astronomers Sir Fred Hoyle and Dr Chandra Wickramasinghe in 1974. Dissatisfied with the notion that Earth was unique, they suggested that not only organic compounds but actual microbes and viruses could travel through interstellar space, dormant but still viable in the liquid water that they suspected was present in the interior of many comets.
If it turns out that Hoyle and Wickramasinghe were right, then most of those 144 billion planets will have life on them. The history of evolution on Earth tends always to greater complexity, so a fair proportion of them would have intelligent life on them.
Gwynne Dyer is an independent journalist whose articles are published in 45 countries.
