The size and apparent nature of the universe are gut punches to the human ego. There could be over a trillion galaxies, of which our Milky Way is a fairly ordinary one. Our galaxy, in turn, contains something like a hundred billion stars, of which our Sun is a fairly ordinary one. And most stars have one or more planets, of which our Earth is a fairly ordinary one, with a single glaring exception: it’s the only place we know of where life has arisen. Is it the only such place in reality? As we continue exploring the answer, we are now at the midway point on our tour of the Drake equation:
N = R* × fp × ne × fl × fi × fc × L
In this equation,
- N is the number of active, communicative extraterrestrial civilizations in the Milky Way galaxy
- R* is the rate at which new stars are created in our galaxy
- fp is the fraction of stars that have planets
- ne is the average number of planets per star that might support life
- fl is the fraction of life-supporting planets that actually develop life
- fi is the fraction of developed life that becomes intelligent
- fc is the the fraction of intelligent life that sends signals into space
- L is how long a signal-sending civilization survives and sends those signals
So far, we have estimated R* = 2 per year, fp = 0.99, and ne = 0.35. Now comes the really squishy part: trying to draw conclusions about life on other planets from what we know about life on this one.
The origin of life is one of those subjects where science and religion collide rather violently. In the Western world, we seem to have a choice, for example, on the age of our Earth. The prevailing religious view suggests it is 6000 years, while the prevailing scientific view puts it at more like 4.5 billion years. I was raised Catholic, but I have also loved math and science since my earliest memories of school. The Catholic school where I was educated from 2nd through 8th grade was extremely adept at combining religion and science without ever suggesting one invalidated the other. And their library had lots of great books on science, including one entitled “Four Billion Years Ago”. One thing I have always believed since those formative years is that science and religion need not be mutually exclusive, even when we are exploring mysteries like the beginnings of Earth and life. I’m going to talk in terms of the scientific view here, but the reasoning could be applied to other views as well.
Science calls the beginning of life on Earth abiogenesis – the process by which life arises from non-living matter. We talked a bit in the last post about life forming in water. When Earth first formed 4.5 billion years ago, it was too hot to have liquid water. But it may have only taken a couple hundred million (0.2 billion) years after that to cool down to a temperature where the oceans could form. We have found evidence of microbes in Northern Quebec, in rocks that may be nearly 4.3 billion years old. There is additional evidence in other locations on Earth that life arose over 4 billion years ago. So, speaking on these vast time scales, it would appear that life began quite soon after the necessary conditions were established. We just don’t know exactly how. Scientists have conducted experiments where they place all the right material together in conditions similar to what we think prevailed back then, but thus far they have not been able to generate life from the non-living ingredients. At first that might seem disheartening, but even as “quickly” as life appears to have emerged on Earth, it still likely took tens to hundreds of millions of years after the formation of the oceans. We just don’t have that kind of time, pandemic or not.
Even though we don’t know exactly how life began on Earth, there’s a logical chain of reasoning. The fundamental molecule of life – deoxyribonucleic acid, more affectionately known as DNA – has the distinct characteristic that it can make copies of itself by unwinding and with the assistance of certain enzymes. This is how life persists and proliferates, and DNA that encodes more useful features in its parent organism will make that organism more likely to survive. Occasionally, errors will occur in the copying process, and these lead to mutations. Some become the scourge of cancer. Others have little to no effect. And still others may lead to significant new features – some of which give an advantage to the organism and its offspring. Extrapolating backwards, you can imagine that DNA has been evolving this way all the way back to the beginning over 4 billion years ago, to the very first molecule that was able to make a crude copy of itself. Once that first copy was made, you can also imagine a cascading effect – molecules that can make copies of themselves will eventually be quite plentiful compared to other types of molecules. The earliest microbes appear to be similar to those we have found in hydrothermal vents at the bottom of the ocean, so perhaps that is where this process played out as well.
If Earth is “typical” of a planet in the Goldilocks zone, then it is only a matter of time between planetary formation and the beginning of life. We also know organic (carbon-based) matter is fairly common in objects outside of Earth, and we’ve even found amino acids – the building blocks of proteins – in meteorites and comets, so it is also possible that the ingredients for life on a given planet have a head start in the rocks that come together to form that planet. On top of that, there is every indication that planets like our Earth are quite common – rocky worlds located in the habitable zone around their respective suns – and in fact we’ve already estimated such a world orbits every third star in the galaxy. Put it all together, and without any evidence that Earth is special beyond our knowledge that life exists on it, there is a compelling argument to assign a pretty high percentage to fl in the Drake equation.
Let’s drill down by the process of elimination to a good number. Assigning a value of 0 seems to make little sense given the argument above, and of course we also know 0 is impossible given that we are here to debate the matter. Assigning a value of 1 seems like overkill – surely there are habitable worlds where things just didn’t go right for one reason or another. Assigning a value of 0.5 would be a nice compromise between the extremes – but I think the 0 extreme is significantly more absurd than the 1 extreme. What if we split the difference again? That would make fl = 0.75, and that’s a number I think I could live with. No, it’s not exactly the scientific method, but let’s face it, we’re spitballing no matter what we do. And just as life evolved on Earth, thus evolves our Drake equation:
N = 2 × 0.99 × 0.35 × 0.75 × fi × fc × L
Four down, three to go. But so far, we have determined that every other year, a star that forms in our galaxy will eventually preside over life on one of its planets. In the next post, we will try to figure out how much of that life reaches a point where it decides it might as well start a blog.