Where to Look for Alien Life?

One intriguing question is what is the probability of life elsewhere in the Universe? In my ebook, “Planetary Formation and Biogenesis” I argue that if you need the sort of chemistry I outline to form the appropriate precursors, then to get the appropriate planet in the habitable zone your best bet is to have a G-type or heavy K-type star. Our sun is a G-type. While that eliminates most stars such as red dwarfs, there are still plenty of possible candidates and on that criterion alone the universe should be full of life, albeit possibly well spread out, and there may be other issues. Thus, of the close stars to Earth, Alpha Centauri has two of the right stars, but being a double star, we don’t know whether it might have spat out its planets when it was getting rid of giants, as the two stars come as close as Saturn is to our sun. Epsilon Eridani and Tau Ceti are K-type, but it is not known whether the first has rocky planets, and further it is only about 900 million years old so any life would be extremely primitive. Tau Ceti has claims to about 8 planets, but only four have been confirmed, and for two of these, one gets about 1.7 times Earth’s light (Venus get about 1.9 times as much) while the other gets about 29%. They are also “super Earths”. Interestingly, if you apply the relationship I had in my ebook, the planet that gets the most light, is the more likely to be similar geologically to Earth (apart from its size) and is far more likely than Venus to have accreted plenty of water, so just maybe it is possible.

So where do we look for suitable planets? Very specifically how probable are rocky planets? One approach to address this came from Nibauer et al. (Astrophysical Journal, 906: 116, 2021). What they did was to look at the element concentration of stars and picked on 5 elements for which he had data. He then focused on the so-called refractory elements, i.e., those that make rocks, and by means of statistics he separated the stars into two groups: the “regular” stars, which have the proportion of refractory elements expected from the nebular clouds, or a “depleted” category, where the concentrations are less than expected. Our sun is in the “depleted” category, and oddly enough, only between 10 – 30% are “regular”. The concept here is the stars are depleted because these elements have been taken away to make rocky planets. Of course, there may be questions about the actual analysis of the data and the model, but if the data holds up, this might be indicative that rocky planets can form, at least around single stars. 

One of the puzzles of planetary formation is exemplified by Tau Ceti. The planet is actually rather short of the heavy elements that make up planets, yet it has so many planets that are so much bigger than Earth. How can this be? My answer in my ebook is that there are three stages of the accretion disk: the first when the star is busily accreting and there are huge inflows of matter; the second a transition when supply of matter declines, and a third period when stellar accretion slows by about four orders of magnitude. At the end of this third period, the star creates huge solar winds that clear out the accretion disk of gas and dust. However, in this third stage, planets continue accreting. This third stage can last from less than 1 million years to up to maybe forty. So, planets starting the same way will end up in a variety of sizes depending on how long the star takes to remove accretable material. The evidence is that our sun spat out its accretion disk very early, so we have smaller than average planets.

So, would the regular stars not have planets? No. If they formed giants, there would be no real selective depletion of specific elements, and a general depletion would register as the star not having as many in the first place. The amount of elements heavier than helium is called metallicity by astronomers, and this can vary by a factor of at least 40, and probably more. There may even be some first-generation stars out there with no heavy elements. It would be possible for a star to have giant planets but show no significant depletion of refractory elements. So while Nibauer’s analysis is interesting, and even encouraging, it does not really eliminate more than a minority of the stars. If you are on a voyage of discovery, it remains something of a guess which stars are of particular interest.

Planets for alien life

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.