Is Anybody Out There?
Some 450 years ago, anyone who knew something about anything was convinced, and you better not believe otherwise, that our Earth was the only inhabited planet in the Universe, that it resided at the center of that Universe, with every other celestial body orbiting around it at various distances, and that all of creation, including humankind, had been brought into existence in a miraculous event orchestrated by a Divine being about 6000 years ago.
Well, who’s to say otherwise, at least at the time? While what we know, or what we think we know, has become vastly greater than way back when, at least in some areas, in others we are just as ignorant today as anyone has ever been. This is especially true of some of the most important questions we can ask ourselves about the nature of the Universe and our place in it. How did the Universe come to be? Did it have a beginning or has it always existed? Either way, is it the same today as it was in the past, or has it evolved over time? What will it be like in the future? And what about us, humankind? Why and how did we come to be as we are? What is the meaning or purpose of our current existence, if any? What is our destiny? Do we have any choice in the matter, or have our past, present, and future been pre–determined?
So many questions, not enough time for any of them, even if time, whatever that is, is without end, Amen. Let us focus on just one of these – Is Anybody Out There? Does life exist elsewhere in the Universe? We don’t currently have any convincing evidence of that anywhere but on Earth. If it does exist, is extraterrestrial life common? How common? What about intelligent life, however that might be defined? Some will plausibly argue that the existence of intelligent life on Earth itself has not yet been convincingly demonstrated. “Beam me up, Scotty! Nothing of interest down here.”
The fossil remains of Neanderthals (Homo sapiens neanderthalensis) first appear about 400,000 BP (before the present) and disappear about 40,000 BP. Anatomically modern humans (Homo sapiens sapiens) appear on Earth somewhat later than Neanderthals, as early as 230,000 BP or even earlier. Recent advances in genetic sequencing and population analysis make it clear that the two sub–species coexisted and even interbred for much of that time. And by any commonsense definition of intelligence, previous cynicism notwithstanding, both are highly intelligent compared to any other animal species extant or extinct.
We must begin with asking how likely it is that life can arise on ordinary planets around ordinary stars anywhere in the Universe. If our terrestrial experiences are any guide, then we might well conclude that the simplest forms of life, like single–celled algae and bacteria, may be quite common in the Universe. From what we understand today about astrophysics, biology, and genetics, a minimum of three things are required for life as we know it to exist: 1) A stable environment with average temperatures at which liquid water can exist, 2) The presence of the fundamental building blocks of organic compounds, and 3) A stable source of external energy.
Planets around stars are the most obvious objects that might harbor liquid water in sufficient quantities to sustain life. Why water? Because it is the “universal solvent”. Chemical and biochemical reactions cannot take place unless the reagents are brought into close contact with each other. This does not happen very often, if at all, if the reagents are locked up in solid compounds such as rocks. No, in almost all circumstances they must be dissolved in water for enough of them to come in contact for long enough to react at a significant rate.
But not all stars, nor all planets, have properties suitable for liquid water to exist. Stars come in a broad range of masses, that is, with nuclear fuel in the form of hydrogen. Stars derive their energy by nuclear fusion, where four protons (hydrogen nuclei) come together to make a helium nucleus composed of two protons and two neutrons. The transformation of two of the four protons into neutrons is accompanied by a release of energy in the form of particle motion (heat) and radiation (light). The reaction rate is strongly dependent on the density, pressure, and temperature of the ionized plasma at the center of the star.
High–mass stars have very large reaction rates and, consequently, very hot and luminous surfaces. Low–mass stars have very low reaction rates and very much cooler and less luminous surfaces. Planets around stars have equilibrium temperatures that in turn depend on the luminosity of their parent stars, as well as the distance from those stars. If the star is too hot or the planet too close, temperatures are too high, and the water boils away. If the star is too cool or the planet too far away, temperatures are too low, and the water freezes. Neither condition is conducive to biochemical processes.
Stars like our own Sun are obviously ideal for this purpose, since we by direct observation know we live on a planet with an average temperature at which liquid water can exist. But even our Sun cannot produce the right temperature range on planets too close or too far from it. We don’t have to wander too far from the Earth to find examples. Venus is the next planet in, and its surface temperature is hot enough to melt lead. (There is a concurrent circumstance, that Venus’s atmosphere is almost entirely carbon dioxide, but that is a topic for another blog post.) Mars is the next planet out, and the temperature there is below freezing year–round, actually two years, as that is the orbital period.
There is a so–called “Goldilocks zone” of habitability around any given star, closer to cool ones and farther away from hot ones. Planets must live in this zone for liquid water to exist, and this will depend on things like how common planets are in the first place, as well as how often a planet that is not too small or too large to sustain a gaseous atmosphere exists within the zone. Without getting lost in the details, it seems that virtually ALL stars that we have examined closely enough have planetary systems of some kind, and we expect that at least some of them will have suitably Earth–sized planets. Some fraction of those planets will live in the Goldilocks zone and possess liquid water. What about the stars themselves?
Stars come in a broad range of masses, from midgets having less than one–tenth the mass of our Sun, to behemoths with more than one hundred times its mass. The low mass stars outnumber the high mass stars by a very large margin. It’s like a riverbed. There is a lot of fine mud, a fair amount of sand, fewer pebbles, even fewer stones, and hardly any boulders. Likewise, there is only a small fraction of all stars that have suitable masses, perhaps between 0.5 and 1.5 solar masses, and that is being generous. The rest are too cool (the vast majority) or too hot.
There is an additional requirement that stars must live long enough for life to arise and evolve into more complex forms. On the Earth, it seems that it might have taken as much as one billion years for primitive lifeforms like algae and bacteria to arise, and it is clear from the fossil record that more complex life did not exist at all until a little more than 600 million years ago. Or if it did, there were no hard body parts that could be fossilized in the first place. Regardless, it seems that it takes quite a while to produce even simple lifeforms, and even longer for them evolve into more complex forms, at least here on Earth.
Small stars live a long time, but most are too cool and turn out to be too unstable to be suitable. Large stars are not only too hot, but they have much shorter lifetimes as well. Stars with two times the mass of the Sun live for only about one billion years before they exhaust their hydrogen fuel supply and die. This seems far too short, if a billion years can be considered short, for life to arise and evolve into complex forms, never mind becoming intelligent. The Sun and Earth are thought to be about five billion years old, while our species has existed only for 200,000 years. This is just four one–thousandths of one percent of the age of the Earth. If the age of the Earth were scaled to 24 hours, humankind arose (literally, to walk upright) at about four seconds before midnight. This will be seen to be relevant below.
What about the water itself? It turns out that water (H₂O, two hydrogen atoms bonded to an oxygen atom) is one of the most common molecules in the Universe. Everywhere we look, we find evidence for it. This is because hydrogen is by far the most common element in the Universe (9 of every 10 atoms is hydrogen), and oxygen is relatively common compared to almost all the remaining elements in the Periodic Table. Carbon and nitrogen are also relatively common, and this will be a crucial point when we discuss the organic building blocks of life.
Oh well, why wait? What about those organic compounds? If elements like hydrogen, carbon, nitrogen, and oxygen commonly exist on planets in significant quantities, and it seems certain that they do, then it doesn’t take long to conclude that simple molecules made of those elements, like water (H₂O), methane (CH₄), carbon dioxide (CO₂), and ammonia (NH₃), will also be present in significant quantities.
From these, provided there is a sufficient source of external energy to drive chemical reactions, it is straightforward, if somewhat tedious, to show that all of the more complex constituents of life made from these building blocks, namely lipids (fats), carbohydrates, nucleic acids, and proteins, will also exist in quantity. And where will the necessary external energy come from? Why, from the very star around which the planet revolves. The interrelationships remind one of the ouroboros, that mythical snake that swallows its own tail, round and round, without beginning or end, signifying unity, wholeness, and timelessness.
If you ask me, and you don’t have to because I rarely need an excuse to dispense an unsolicited opinion, planets capable of supporting at least the more primitive forms of life should not be uncommon at all. And if those planets live around stable stars that have sufficiently long lifetimes, I expect that more complex forms of life would also not be uncommon. I say “not uncommon” rather than “common”, because the percentage of habitable planets is still very small compared to the total. But there are lots and lots of stars.
Consider our own Milky Way Galaxy, a typical collection of perhaps one trillion stars. Our best observations of the Universe also suggest that there may be as many as one trillion galaxes in the regions of space that we are able to observe. In scientific notation, one trillion is 10¹² , so a trillion trillion is 10²⁴. Let me write that out, a 1 followed by 24 zeros: 1,000,000,000,000,000,000,000,000. This is approximately the number of stars in the observable Universe. The percentage of all stars with masses between 0.5 and 1.5 Solar masses is approximately 8.8% of the total, so if we restrict our attention just to the Milky Way with “only” its one trillion stars, there ought to be about 88 billion stars in our own galaxy capable of supporting life – in principle. How many have planets that are Goldilocks?
In the last couple of decades, we have discovered some 5,000 exoplanets within about 3,000 light years from Earth. Of these, there are just a handful of possible habitable planets, perhaps one–tenth of one percent of the total, although none of these have been confirmed. The volume of space we have searched is only about 3.5% of the volume of the galaxy, so a very crude estimate of the number of habitable planets in the Milky Way might be about three million. This could easily be wrong by a factor of at least ten in either direction, but at present the best we can do is to say that there might be between 300,000 and thirty million of them.
Of those, I expect that a significant majority of them will have evolved primitive lifeforms like algae and bacteria. I suspect most of those will harbor more complex life in the form of marine flora (plants) and fauna (animals). If any of those were washed up or crawled up on land, they might well be flourishing there too. And of those, how many might we estimate to have had enough time and luck to evolve large brains, or a form of locomotion that frees up grasping appendages? Of those, how many have learned how to use tools, invented language, technology, and civilization, be capable of exploring space near their planet, and use electromagnetic radiation for communication purposes? There is no way to know this at present.
There is one other complication. Consider humankind, a putatively intelligent species which has only existed for some 200,000 years, and which has possessed technology adequate for the exploration of space and long–distance communications for less than 100 years. One might reasonably ask how long our civilization might endure, assuming we do not exterminate ourselves in the very near future or destroy the ability of Earth to sustain life with that same technology.
The Roman Empire as such existed for only a little more than 1,000 years, give or take, and please if you are a history buff can we not get into that? For us to co–exist or even communicate with an extraterrestrial intelligent species would require that our advanced cultures and civilizations exist at the same time, that our levels of technological progress be not too different, that we be close enough to each other to overcome the light speed barrier to communication, and even if we could, that we be able to find a common language in which to converse.
That exoplanet 3,000 light years distant? It would take a question transmitted electronically 3,000 years to get there, and another 3,000 years for the answer to get back. I get antsy waiting more than 10 minutes for a reply to an email or text message. I’d like to be able to phone home, but nobody picks up anymore, not even my baby sister.
I would love to speculate further, but I cannot. I can only describe my thinking and what I believe might be possible, if not probable. I believe that there are millions of planets in our Milky Way galaxy alone capable of supporting some kind of life, and which actually do so right now or have done in the past. I believe that at least some, if not many of these have evolved more complex lifeforms, both in the sea and on land. I believe that some of that life is sufficiently complex for the possibility of intelligence to have evolved at some level.
The uncertainties in all the factors that go into the discussion are so large, that I know of no straightforward way to support my beliefs in a quantitative manner. But we cannot possibly be the only world in the galaxy, much less the entire observable Universe, on which life in some form has arisen. For a much more nuanced discussion, try this excellent American Scientist article, Alone in the Universe, by Howard Smith, a distinguished Harvard astrophysicist.
There are things in heaven and on earth that could well remain mysteries forever. Science frequently runs out of answers. We can always turn to other means of how to know (epistemology) and of what is or is not (ontology), but these involve philosophy and metaphysics, not science. And a good thing too – they are so very interesting in their own right.
So, Is Anybody Out There? If they are, have they come to visit us already?? Don’t ask me. Well, OK you can ask, but don’t expect an informed, sensible answer. And if anyone else claims to know better, I don’t know about you, but as for me I would ask elsewhere or just give it up altogether.
All the best,
From Broomall, PA on Sunday, 02/20 at 8:00 PM,
Rex
