The question of what is life has puzzled us for centuries. A new movie, Chappie, addresses this issue in the context of what is a spirit or a soul. Life is simpler but still can be awkward. Because we’re seeking to find civilizations that send out radio waves, we can limit our ideas of life somewhat. Life could be defined as something that reproduces itself using available energy and material resources. To be useful, this life should also be capable of making reproductive mistakes that lead the way to evolution. Without evolution, that civilization could not appear.
In order to figure out if SETI makes sense, we must gather some sort of estimates of the probability of life starting and of it evolving into something like us. We must also determine how many stars harbor planets capable of supporting such life.
Before beginning this peregrination of thought, consider that our version of life here on Earth consists of organisms spawned in water and built of carbon, oxygen, hydrogen, and nitrogen plus some other elements in smaller proportions. Any life must be capable of a complex chemistry and of building rather extensive molecules. Finally, the basic construction materials should be close at hand and in reasonable abundance.
Hydrogen is the most abundant element in the universe and constitutes nearly all of its normal matter. It is found in important simple compounds: water, ammonia, and methane. These also are the simple compounds in which oxygen, nitrogen, and carbon reside. While some have suggested that alien life chemistry might use silicon in place of carbon, the much greater abundance of carbon argues against that route. Similarly, water is not only abundant on the Earth but also throughout space. It has the advantage of being an excellent solvent and the odd characteristic of expanding upon solidifying so that lakes freeze from the top down. All of these features make water the best medium for harboring life by a large margin.
Warmer planets that could not support liquid water would have a problem finding a good liquid solvent. Colder planets and moons, such as Titan, may have vast liquid seas of methane, but the cold means that chemical processes proceed at a very slow rate. A drop of 100°C in temperature reduces chemical reaction rates one thousand-fold. At 200°C below our normal temperatures, life would have to form and progress one million times more slowly than it might here. Given that it took around three billion years to reach multicellular life on Earth, the necessary time for a cold planet or moon of three quadrillion years exceeds the life of the universe.
I put the likelihood of life developing on appropriate planets as very high, nearly certain.
Our solar system has three planets that may have had life begin on them. It survived on only one. Given that recent scientific evidence points to life beginning very early on Earth, it must be either very fortuitous here or very common everywhere. The constant rain of organic molecules from space and the predilection of rocky planets to volcanism early in their lives provides ample opportunity for life to form using that falling space debris, local stuff, and plenty of energy-rich compounds from the crustal upwelling. In short, I put the likelihood of life developing on appropriate planets as very high, nearly certain.
The next steps are perilous. One nearby example, Mars, solidified its core early in its life and so lost its protective magnetic shield. With only a modest gravitational field (38% of that on Earth) and a powerful solar wind (100 times what it is now), the atmosphere was probably stripped from it in only a few million years. The magnetic field of Earth forces those solar wind particles to pass around the Earth because they are mostly charged particles capable of being deflected by such a field.
The other planet, Venus, has a different story to tell. Its gravity is much closer to Earth, about 90%. Its atmosphere is nearly 100 times as heavy as ours (about 90 times) and consists mostly of carbon dioxide (CO2). This CO2 probably accumulated from outgassing of a great many volcanoes in the early life of the planet, the heavy concentration of CO2 resulting in an extreme greenhouse effect that makes the surface hot enough to melt lead. Our planet should have suffered the same fate, but the spectacular intervention of a planetoid collision changed all of that. (See Heldmann’s explanation in the video below.)
That collision had two important results. In the first place, it blew off the entire atmosphere that had accumulated to that point. Earth had to start over and build an atmosphere at a time when its volcanism had subsided substantially from its early days. It just didn’t have enough outgassing to rebuild an atmosphere to more than about 1% of that of Venus. The air probably was mostly methane and CO2, nevertheless.
The second effect of that collision, a rather unlikely one, was to create our moon. Unlike every other planet in our solar system, our moon is very large compared to our planet. This unusual size has created a situation in which our planetary winds are much weaker than they would otherwise be. On Mars and Venus, they run up to around 300 km/hour, faster than a category 5 hurricane. Simulations have shown that our much lower wind speeds are due to our gigantic moon. The importance of wind speeds will become apparent later on.
…around 2% of stars have life on them. That’s a great many when multiplied by ten sextillion. It means that we may find life, if we could travel and look, on some 200 quintillion planets. And that’s a lot of life.
Venus, Earth, and Mars may all have had life on them early on. A billion years later, life probably persisted on only one of these. As we search the galaxy for life, we can expect that plenty of planets at an appropriate distance from their stars had life but that many have lost that initial spark. Maybe 10% of stars out there have planets that might have life. Maybe 20% retained that life. These are very rough guesses. You can make your own adjustments. If those numbers are at all near accurate, then around 2% of stars have life on them. That’s a great many when multiplied by ten sextillion. It means that we may find life, if we could travel and look, on some 200 quintillion planets. And that’s a lot of life.
When we get to the first of those planets, we will likely be disappointed because we’ll see slimes and oozes but nothing like grass or crickets. Even in the seas, there probably will be no fishes or plants. Our planet had nothing but unicellular life for around three billion years. Evolution is funny like that. With no reason to change, it doesn’t. The dominant life forms continue to dominate year after year, century after century, millennium after millennium, and eon after eon for millions of years. The first dominant life forms were simple single-celled organisms.
The next installment covers how we move from this rather dull, to us, seascape to what we see today.
Does SETI Make Sense? Part I: Numbers
Does SETI Make Sense? Part III: Evolution
Does SETI Make Sense? Part IV: Communicating