In my novel, “A Face on Cydonia”, an alien message was finally intercepted. That raises the question, what is the probability of alien life? Frank Drake answered that question with the Drake equation, which involved the product of the number of potentially suitable stars, the probability such a star has a suitable planet, the probability that life will evolve on such a planet, and the probability that it will develop to a civilization. (There is a little more to it, relating to communications, but we leave that.)
In my ebook, “Planetary Formation and Biogenesis” I tried to put some numbers on these, or at least the conditions that have to be met. I should add that what I put forward is NOT in accord with most astronomical thinking. Most astronomers and physicists believe that planets form through gravitational attraction of planetesimals (Bodies of the 100 km size) into embryos (bodies about Mars size) then these accrete into planets by gravitational collisions. While this theory has been around for 60 years, nobody has any real idea how planetesimals form. My concept is that the initial bodies accrete through chemistry that differs at different temperatures, and that means you do not get a uniform distribution of planetesimals. Unfortunately, if I am correct, there are a number of different types of solar system that can evolve.
For life to evolve, it is usually considered the planet must be in what is called the “habitable zone”, which is usually defined by a zone in which planets have liquid water. Venus is usually considered to be too hot, and Mars too cold. The distance from the star for the habitable zone depends on the luminosity of the star, which in turn depends on the stellar mass to a power of approximately four. Thus if we require the planet to be in the habitable zone, for very small stars the planet has to be very close to the star. The smaller the star, the more common it is. If the star is very big, it burns so much faster and does not last. For these reasons, it is usually thought that stars have to be roughly the same size as the sun, i.e. G-type stars (our sun is a G-type, but one of the smaller ones) or K-type (the next size range down). The next problem for a planet is whether the star is a single star, and if so, do they come close enough to gravitationally throw the planets away. Double stars are more common than single stars. Further, stars have to have sufficient elements heavier than helium. You cannot have rocky planets without silicon! Finally, for life to evolve very far, the star has to be old enough.
None of the closest stars to Earth seem particularly promising. The most promising is Alpha Centauri, which also happens to be the closest, at a little over 4 light years, and has two stars that approach about as close as the Sun-Saturn distance. One star is slightly bigger than Sol, and the other is a smaller star. Neither star could hold a gas giant, but rocky planets might be possible, and the smaller star appears to have a small planet. A star like Sirius or Procyon is simply too big and will not last long enough to let animal-type life evolve. The two closest single stars that seem big enough have their problems. Epsilon Eridani is known to have a Jupiter-type planet, but is only 900 million years old, so any planets will not have had time to evolve advanced life. Tau ceti is probably old enough, but it has a low fraction of heavy elements, and may not be able to form rocky planets.
There are only 2 G-type stars (our sun is a G-type star) within ten light years, and about 18 within thirty light years, however K-type stars might also be adequate, and there are about 38 of them within 30 light years. Unfortunately, the heavier G-type and the lighter K-type are probably not suitable, so we may have a lot of space to ourselves. On the other hand, our galaxy is huge, and by my count it probably contains something like a hundred billion suitably sized stars. Those near the centre of the galaxy probably have to be discounted (the region is too violent) and we may have to eliminate about half of the rest for various reasons, nevertheless, it is almost certain that there are plenty of suitable stars. It is just that they are rather far away both from us and from each other. How many will have planets? That is for a later post.