Recently my son & I finished listening to a podcast series called Astronomical presented by Professor Alan Duffy. Over 8 episodes we’re taken on a guided tour of the efforts to find and make contact with aliens. Serious minds and money through Breakthrough Listen and SETI are working to find what they all seem to believe is inevitable – first contact with intelligent life in the universe. They even introduced the idea that earth has already had contact with aliens and which the Pentagon investigates from time to time. But I imagine it goes something like this.

But most of the series concentrates on serious scientists doing serious science things all in an effort to find a planet, other than ours, that has life. As Alex Berezow says,

The academic debate now is not whether life exists but in what form. Many scientists assume that the commonest form of life is microbial — a fair assumption, given that on Earth, humans are a relatively modern invention while microbes have been around for 3.5 billion years — so many astrobiologists are spending their days examining the atmospheres of exoplanets for telltale signs of bacteria-like creatures.

We are effectively alone in the universe

Most of them also believe that intelligent life surely exists and with the soon to be launched or finished space telescopes, earth bound arrays they’ll be able to check not just thousands but millions of planets for signs of life and in the next 5-10 or 20-30 years will surely find them. The reason they are sure that life exists is because the galaxy is really, really big. I’m not talking universe, just our galaxy. It’s massive. 100,000 light years or more from one side to the other and in that vast amount of space are somewhere between 200-400 billion stars and somewhere between 300 and 600 billion planets¹. The reason they are sure intelligent life exists is because the universe is really old and so surely someone has been around for a billion years longer than we have and they must be really, really cool. This allows very intelligent people to do massive amounts of wild speculating while being able to maintain the belief that this is science.

Whether you take 300 or 600 billion as your number it’s a number so huge that these cosmologists conclude there simply must be life on other planets. How could there not be? And so armed with that conviction and absolutely nothing else they proceed to proclaim with a great deal of certainty that aliens exist.

Now I’ve been known to say something like ‘just look at the universe, it’s so huge and vast and finely tuned, there must be a God’ and I’m met with the demand for evidence. Belief without evidence is just wishful thinking I’m told. So with that in mind just because something is really, really big this is not by itself evidence. So it’s quite interesting to be the skeptic while listening to the religious convictions of the atheist cosmologist.

But I think we can get from 600 billion to 1 without too much trouble. Although there could be life in ways we can’t imagine, if we are going to do, you know science, we have to start with what we know and the only place we know that has life is right here.

So we assume for life to exist there must be a star relatively similar to ours. Well, that number is estimated to be around 20%. 20% of 400 billion. That leaves us with 80 billion possible stars and maybe 100 billion planets. That’s still a BIG number. But let’s make it smaller.

We’ve already discovered and confirmed the existence of 4472 exoplanets of which 1368 are earth like. Either way we reckon of those 4472 planets there is life on just 1, ours. Every time we discover another planet without life, the odds increase in my favour. But for now let’s divide 100 billion by 4472. Suddenly we’re not talking billions any longer but millions. 22 of them. 22,361,359 to be precise.

Then we need to know how many of those 22.3 million planets are in the Goldilocks zone (not too hot or too cold). Well, let’s use our own solar system as a guide which has a relatively generous eight planets which is (I’m told) above average. But only one of those has life. So let’s divide 22.3 million by 8. That leaves us with 2,795,169 potential planets. Still, I concede, a big number.

But our planet also benefits from a large outer planet that protects us from asteroids by it’s massive gravitational pull so we’re not wiped out by our climate being destroyed. Although an occasional one every 100 million years or so might sneak through. Let’s be generous and divide 2,795,169 by 8 again. Now we have just 349,396 planets to hunt for life.

But life exists here because we have a moon at just the right size distance to keep our planet’s axis stable and for our oceans to have tides, which as far as we know, because we don’t know about any other planet with life or water, is also key to life. No one knows the odds of having a moon that’s just right that so let’s just use 8 again, why not?

Alright, we’re down to 43,674 which is a LOT smaller than 100 billion. Our planet has managed to sustain life for 4.5 billion years which is a long time and that makes it incredibly lucky because there are ALL sorts of things that can wipe it all out. Well, what are the odds of that happening twice? We’re already at a vanishingly small number but life on other planets rests on a pretty big assumption. That life evolves pretty easily.

Alex Berezow again,

Decades of research have yielded little in the way of identifying the mechanism of abiogenesis — the formation of life from non-living matter. There are several different theories on the origin of life, and none of them are any good. In the laboratory, we have had some success in creating biomolecules such as amino acids from gaseous precursors; the Miller-Urey experiment is the most famous of these. But scientists have yet to come even close to reproducing life in the laboratory. This strongly implies that life does not evolve easily.

We are effectively alone in the universe

And I could have added that not enough stars give off enough light for photosynthesis and a fair number of other factors.

Since red dwarfs are by far the most common type of star in our galaxy, this result indicates that Earth-like conditions on other planets may be much less common than we might hope. This study puts strong constraints on the parameter space for complex life, so unfortunately it appears that the ‘sweet spot’ for hosting a rich Earth-like biosphere is not so wide.

New study darkens hope for Earth-like planets

Not only that but even if we could find life somewhere else we have absolutely no means of ever getting there or being able to talk with them. Because the galaxy is really, really big.

That’s my maths and reasoning (which I’m open to much correction and instruction) as to why I think life is so rare that it might only be here. But because I believe in a good creator I’m not filled with existential dread but instead with awe and wonder at being a unique being on a planet that is one in 600 billion.

1. or even 3.2 trillion! [↩]