A recent article in Space.com provide some interesting current estimates about the numbers of earth-like exoplanets. Based on these calculations the nearest earth-like planet may be 13 light years away. That is small be astronomic standards but I would pack a suitcase for the trip.
The data is based on that generated from the NASA Kepler space telescope, which scans more than 150,000 stars simultaneously. Planets are shown by temporary dips in brightness when they pass in front, or transit, their stars. Of 3897 red dwarfs (smaller than our sun) Kepler identified 95 exoplanets and three 'earth-like' planets (that might possibly support life.) From this data scientists estimate about 6 percent of red dwarfs in the Milky Way galaxy should host Earth-like planets. (In my view that's more like a 'guesstimate' than an estimate, more on that line later). About 75 percent of our Milky Way galaxy's 100 billion stars are red dwarfs. Doing the sums this suggests 4.5 billion "Alien Earths" spread throughout the galaxy. Within 30 light-years of our sun there are 248 red dwarfs, according to separate Research Consortium on Nearby Stars led by Georgia State University.
The closest red dwarf to Earth is Proxima Centauri, which is 4.2 light-years away in the three-star Alpha Centauri system. Although Proxima hasn't been demonstrated to have an earth-like plane, its neighbour Alpha Centauri does have an earth-sized planet, admittedly on the hot side for supporting life as we know it.The above is based on the full article by Elizabeth Howell @howellspace, SPACE.com @Spacedotcom.
It is possible to further estimate how many planets will have intelligent life. I recall Carl Sagan doing a similar calculation based on an equation of the astronomer, Frank Drake. This was some thirty years ago, when we believed planets were much rarer and in any case might be too small to ever be identifiable from the Earth.
These are my figures entirely:
Lets work with this best estimate of 4.5 billion earth like planets in the Milky Way. Of course there are almost countless other galaxies, but identifying life there is much less likely in the near future.
How may of these earth-like planets are likely to be capable of sustaining life as we know it, e.g. at least microbial? If they are earth-like I would say quit a lot, let's say 40% for argument, or 1.8 billion planets.
Of these how may will have developed life? Good question! I would suggest most would given a suitable environment. Of course there may be factors to prevent this such as radiation, but again lets say about half. That makes 0.9 billion planets with life.
Of these, how may would have developed a technological civilization? (As we have only been at this stage for a few hundred years, if they have got that far almost all in fact will be more technologically advanced than we are.)
Lets say 10% will be technologically advanced, which leaves 90 million worlds with intelligent life in advance of our own. (I am using US billions, i.e thousand millions, rather than British millions, which are million millions).
How may worlds got to about our stage and then survived the atomic age? After all our own doomsday clock on earth is edging nearer to midnight. Despite that I am optimistic that most would find a path not to end their civilizations. Lets say 80% of civilizations survive.
So we conclude there are 72 million planets with advanced intelligent species in the Milky Way.
Of course some will be space-travelers and may have colonized other worlds. Lets say 10 colonized worlds on average. May be more, may be less. But then they may have wiped out some alien life during this colonization, so perhaps let's say five new colonized worlds on average per civilisation. That gives us a final figure of 360 million planets with civilizations ahead of ours.
Why haven't any of the 360 million civilizations contacted the earth? Perhaps they have. Perhaps they will. Perhaps as Mike Oldfield said we are 'on this small planet lost in space', i.e. we are in a quiet spot on the edge of great cultures that haven't looked this way yet.
Time will tell if I am more or less accurate than in Jeff Wayne's musical version of H G Wells' War of the Worlds.
'The chances of anyone coming from Mars is a million to one, and yet they come.'
Allusions in the Text
p.s. Mathematics isn't my strongest point. Please let me know if you think I missed anything.