08 January 2010

Origin Questions and First Special Post: Rare Living Worlds

Most people don’t seem to have any real interest in what I think of as origin questions. I’m not talking metaphysics here… that’s a whole different area of human inquiry and understanding, and I’m definitely not dismissing it, but I’m talking about physical origins here. Questions about why there is something and not nothing, whether the universe exists amid myriad other universes, and what is the ultimate nature of reality, I will set aside. I’m also not talking about the ultimate physical origin of the universe as a whole, which is also a fascinating topic, about which much more is known than even twenty or thirty years ago. 

If you accept as a given that the physical universe had an origin, the Big Bang or whatever it was, and that a milieu of (all but innumerable) galaxies and clusters of galaxies, having certain composition and reasonably well understood dynamics, came into being some twelve or thirteen billion years ago and has been undergoing a sort of evolutionary process (including the accelerating expansion of the space containing them), ever since, that leaves a great deal to talk about in terms of how that milieu led to the here and now on planet Earth, and similar circumstances elsewhere. (The fact that there are an almost unimaginably vast number of elsewheres  is of course relevant, but this, too is a given).

Anyway, this is what I’m focusing on: the physical origin of the environment we live in, given the existence of the universe. This planet; its origin and history, in general terms, leading to the existence of a complex and relatively stable biosphere. Its star, the Sun, on which life on this planet unquestionably depends. The milieu from which the star and its system of planets arose: how, when, where, and what particular processes were involved. How probable it was for this process to have happened the way it did, which has obvious implications for how often something similar has happened elsewhere. The origin and potential alternative developmental histories of life on this planet, or on similarly situated planets. The fact that intelligent animals evolved, capable of manipulating their environment to the extent of possibly even escaping from the surface of their world of origin and going to live elsewhere, potentially replicating the essentials of their original biosphere in new locales (not happened yet, beyond a baby step, on this world). Necessarily related to these questions are questions of whether and how often the universe has produced conditions that have resulted in the evolution of beings comparable to ourselves: the Are we alone? question, and its corollary, first posed by Enrico Fermi more than 60 years ago: [If not], Where are they?

(The “Where are they?” question will be a whole post coming up).

So anyway, if you are one of those folks, the majority of humanity, clearly, whose eyes glaze over when these topics are brought up, and who prefer to simply take the world more or less as it is for granted, (since, quite sensibly, you conclude that there’s nothing we can do to change any of this anyway, so we might as well ignore it), then these rambling thoughts are probably of no interest to you, and you may as well break off right here. But I have been casually interested, as a lay person,  in these subjects for my whole adult life, and have read widely in the popular scientific literature on this subject. While I am hardly an expert, I do have a pretty good understanding, descriptively (as opposed to quantitatively), of the scientific consensus on the most important factors giving rise to the current state of knowledge and understanding in this area.

Of course, this is a huge topic, and serious students of its various aspects have written massive amounts touching on every aspect of it. But what I’d like to talk about is what it seems to me every intelligent person should know about it, particularly since a lot of what was and remains conventional wisdom on these subjects, as embodied in the mythos of the Space Age and Star Trek, turns out to be pretty much entirely wrong.

So bear with me, if you find this at all interesting. Watch this spot, as they say. When I get a chance, haphazardly, over some period of time coming up, I plan to post a series of what I hope will be thought-provoking ruminations on topics related to these questions, which I hope will be interesting and informative, starting with this one (below): Rare Living Worlds and the Origin and Special Characteristics of the Solar System and our World, and Implications for the Formation of Similar Worlds.

Some others I’m thinking about:
Why this is not (yet) the Space Age; what a Space Age might actually be like (and why the 'Star Trek' Universe does not and cannot exist).
Where are they? Are we the first (around here)?  
How and why not all galaxies are likely abodes of living beings (but don’t worry, there are lots of the kind that are).


Rare Living Worlds and the origin and Special Characteristics of the Solar System and our World, and Implications for the Formation of Similar Worlds.

The Sun is what’s referred to as a ‘Main Sequence’ star. All this means is that it’s still in the phase of its existence where it is stable, and cooking along doing what stars generally do for most of their lifetimes: fusing hydrogen nuclei (also called protons) into helium nuclei (also called alpha particles) in their core regions (a process that produces a lot of energy, in the form of radiation). As it happens, the Sun is about halfway, maybe a little more, of the way through this phase, which commenced shortly after its formation from interstellar gas (and some “dust,” which just refers to other stuff, i.e, not hydrogen and helium)— that was present in the Milky Way galaxy in the region where the Sun formed. That was about 5 billion years ago, or something like 35–40% of the age of the universe. So the Sun is relatively old.

So what are stars? This is important to understanding what our world is and where it came from. Astronomy textbooks often just skip over this, but it’s actually not all that obvious. Why are there such things as stars anyway? The simple answer is that stars are great globes of incandescent plasma, consisting of the same thing as the universe is mostly made of, about 90% hydrogen, and 10% helium (with some contamination). Plasma is just is ionized gas. Meaning the electrons are stripped off the atoms because they’re so hot, and the electrons and nuclei are floating around in giant hot, very brightly shining, dense sphere, held together by gravity, not attached to one another. Of course it’s more complicated than that, but that serves as a basic picture. These globes form in the first place from gravitational collapse. Clouds of gas (contaminated with a tiny bit of other stuff— we’ll get to that)… in the vastness of space coalesce; they begin to collapse inward, as a result of various processes, not all that well understood. Eventually most of the gas in a particular region of space ends up in a particular place, and forms a roughly spherical ball of hot plasma. Turns out the question is, why doesn’t it collapse all the way to a point and just disappear? And the answer is radiation pressure (although, in most cases there wouldn’t be enough mass there for that to happen anyway, because it takes enormous pressure to actually dissociate protons into neutrons, or, even further, to dissociate them entirely, but it does happen when there’s enough mass collapsing: they’re called black holes).

Yes, light exerting pressure (it does, it’s just not noticeable in the environmental conditions we live in); that’s what holds stars together, preventing them from collapsing into even smaller bodies, which when they use up their fuel, they do (white dwarfs, neutron stars, black holes; but that’s a whole different topic). When enough gas, now plasma, collapses to increase the temperature and pressure to about million degrees Celsius (~Kelvin), nuclear fusion occurs. Protons fuse to become helium nuclei, releasing huge amounts of energy as radiation, which is where the star’s light comes from, after filtering up from the core to the surface. This was all worked out by Fred Hoyle and Margaret and Geoffrey Burbidge (among other people) back in the 1940s. Anyway, without focusing on the details, if there’s enough mass of hydrogen and helium for this nuclear fusion to occur at the core of the collapsing ball of plasma, a metastable state results. The pressure from the intense radiation of the fusion going on at the core holds up the outer layers of the ball, and before long a relatively stable, brightly shining star comes into existence. Later, the star leaves the main sequence because it’s ‘burned’ a lot of its hydrogen and it starts burning other nuclei, but that, too is a different topic. Stellar evolution is fascinating, but only its preliminary phase, the origin of normal, mid-life, hydrogen burning stars, is relevant to our topic.

Main-sequence stars vary, based mainly on mass, and to some degree on composition. The collapsing regions of interstellar gas that form stars vary from quantities not big enough to form shining stars at all (not hot enough for nuclear reactions), which form dully glowing bodies called brown dwarfs, glowing mainly only from the energy of their collapse itself (more like big Jupiters than stars at all, really)— all the way up to very massive, intensely bright stars that shine like beacons over whole vast regions of the Galaxy. More or less, though, the time that the star spends ‘burning’ hydrogen normally is inversely related to how massive it is. In other words, these bright beacons, these massive blue stars we see in our sky (like Rigel, for example), don’t live very long, and end up in spectacular explosions, sometimes after only a million or a few million years, in contrast to the Sun, which is already 5 billion years old.

You often hear the Sun referred to as a “garden variety” or “average” star. This isn’t actually true. Most stars (about 75% or more) are small, red dwarf stars, ranging from about a tenth to about 40% of the mass of the Sun. As noted, the lifetimes of stars are also more or less inversely related to their mass. So these small, very dim stars (much, much dimmer than the Sun), live a very, very long time. Far longer than the universe has even existed. They range from ten or eleven billion years old to recently formed, but all of them are young in terms of their own lifetimes, which can range up to over 100 billion years… again, far, far longer than the universe has existed. More below on whether life-forming worlds can exist in Red Dwarf systems, but for now, let’s talk about stars more or less like the Sun.

There is a class of stars (Class K) between Red Dwarfs (Class M) and yellow dwarfs like the Sun (Class G). These are what you might call Orange Dwarfs, and they live longer but are significantly dimmer than the Sun. It’s pretty much a continuum, although age of the star (stars grow brighter continuously as they age in the main sequence), and its composition (how much and what other stuff, other than hydrogen and helium, which astronomers confusingly refer to as “metals”) also affect its brightness and other characteristics. The Sun is most of the way to the brighter (and rarer) end of the “G” category, after which (brighter, shorter lived, and rarer as you go along), come “F” and “A”, and finally “B” and “O.” The latter two classes are extremely rare blue giants, so we can pretty much forget about them. They probably never have planets, and they live such a short time that life-bearing worlds could never form in their orbits, and together they constitute less than 1/4 of 1% of stars.  

So the Sun is somewhere around the 90th percentile of brightness, short-livedness, and rarity, of all stars. Hmm. So much for the mediocrity hypothesis of the Copernican worldview. Turns out our Sun is minimally exceptional. (We knew we were special all along, didn’t we? Just not too special).

Why is this important to understanding our world? Obviously, a star, to be the home star of a living planet like Earth, has to exist in a stable form long enough for that planet to evolve life, and there has to be a reasonable probability that a planet capable of supporting life will have formed at a spot where it receives, like Goldilocks, just the right amount of sunlight. Not too hot, not too cold. The sun fits this bill… it’s medium-bright, and it has a planet just where liquid water can exist, Earth.

Very bright stars don’t live long enough. Very dim stars have such small zones where the temperature is right that planets are actually quite unlikely to form there, and if they did, they are likely to be locked with one hemisphere facing the star all the time, like the Moon in relation to the Earth. That would be not too good for life. Also, red light is low-energy. It’s not inconceivable that life could evolve in a red-star system, but Earth life wouldn’t do too well there. Photosynthesis doesn’t work well in red light. Also, red dwarf stars typically flare a lot. Meaning their atmospheres throw out prominences that increase their brightness by a factor of two or even more. The sun does this too, but the prominences are trivial compared to the overall brightness of the sun, and at 150 million km. away, they are no danger to us. A planet a few million km. from a red dwarf could be cooked by a stellar flare. On the other hand, little red stars are really really common, and they live a long, long time, so life may very well exist on some worlds orbiting red dwarf stars, where conditions have turned out to be just right. Certainly, if life gets started, it should have plenty of time to evolve solutions to its environmental challenges. We shouldn’t expect, if we ever mature enough to explore such things, to find such life to be all that similar to life on Earth.

So, the sun is a medium bright, middle aged star. All these things appear to be pretty much necessary for the evolution of a living world like earth. Such stars are still quite common. What else about the Sun is favorable to life, and if such a factor were absent, life might be less likely, or, perhaps more specifically, complex multicellular life like Earth’s might be less likely?

First, it turns out that the Sun, compared to other stars of similar age (enough time for life to have evolved to a high degree of complexity), is very rich in ‘metals’ (stuff beyond helium on the periodic table). This is important, because it’s precisely this stuff, in which the cloud that formed the Sun was much richer than average, that formed the Earth, with all its trace elements necessary for life. Before I get to what is believed to have caused this unusual richness, we have to consider at least the possibility that it is a necessary prerequisite for complex evolved life. We don’t really know this, but it seems at least plausible. The formation of a rocky world with oceans and atmosphere just right for life, like Earth, would seem likely to be critically dependent on particular composition of the cloud of stuff from which it formed. We now know, from research into planetary detection, that other stars have systems of planets that do not at all resemble the Solar System. Many have massive hot planets very close to the stars. We can’t yet detect planets the size of Earth in orbit around other stars, so there are a lot of questions, but right now it looks like past assumptions that the Earth is quite typical may not be correct. It seems likely that planets similar enough to Earth (oceans, stable climate over long periods of time, nontoxic atmospheres, etc.) for complex life to form, may be rather rare. I’ll get to some additional reasons to believe this to be true later.

What caused the Sun to be rich in metals? Current thinking goes something like this. Stars often, maybe even always, form in clusters. The cluster that included the Sun was probably relatively small, because it has spread out to the point that we can’t really tell which stars in the star stream of the Galaxy formed with it, indicating that the cluster was small enough that in 5 billion years it's drifted into a diffuse population embedded in the general star stream of the Galactic disk. But the cluster in which the Sun formed was not too small, which we know because there is strong evidence (radiation effects on meteorites) to suggest that the cluster that formed the Sun experienced a number of supernova explosions prior to the formation of the Sun. These explosions probably actually triggered the collapse of matter from which the Solar System formed, and the explosion of stars enriched the medium with metals. (That’s where metals come from: other than water and organic materials that contain hydrogen, almost everything we’re made of and that exists around us came from the interiors of stars that blew up and disgorged this stuff back into the interstellar medium… before the Sun and Earth even formed). Supernovas result from very bright, very massive stars that end their lives in spectacular explosions. These still occur, about once a century or so in a Galaxy like the Milky Way, but it’s thought they were more common billions of years ago, when star formation was a bit more ramped up than at present (a lot of the matter has been used up). 

An interesting aside here. While the Sun is unusually rich in metals for a star of its age, stars more recently formed, due to the continual enrichment of the interstellar medium over time, are somewhat more likely to be similarly rich in metals. What this implies is that to the extent that metal richness of the formative medium is a necessary prerequisite for complex life, and intelligent life in particular, we may be pioneers. The universe may be very young from this point of view... complex worlds may just be getting going, and ours is a forerunner. Most of the stars that will ever form in the universe have already formed (another topic!), but it may well be that most of the complex forms of life that may one day evolve have yet to do so. (Good, we'll get on the ground floor of the greatest real estate grab ever!) 

Anyway, the particulars of the cluster that formed the Sun may also account for the presence of comets in the Solar System. There’s a current school of thought that says that comets are not typical of stars in general, but form when there is interaction between stars in close proximity during their formation stage in a relatively (but not extremely) dense cluster. This is important because it’s also now generally thought that the origin of the Earth’s oceans may well have been bombardment by millions and millions of comets, especially early in its history. (Comets are mostly water). If a star more or less like the sun had a rocky world more or less like Earth, but no comets, that world might remain dry and uninhabitable forever.

These characteristics of the formation of the Sun were not unique, but neither were they universal. Most stars of comparable age and brightness are different. They have lower levels of metals, and may well have very different histories and planetary systems. So you can’t blithely assume that earthlike worlds will be common orbiting such stars. They may well not be. On the other hand, there’s no reason to believe that conditions have to be exactly like Earth for complex life to form. A star may be quite a bit dimmer or brighter, and conditions on the planet of life-origin could be rather different. We don’t have a clear picture what the parameters are. Some think that Mars may have had an ocean early in its history, and may have developed some kind of life, although that remains speculative. So it’s at least plausible that conditions could vary quite a bit from what we have here and still result in living worlds.

What are some of the other factors? Robert Brownlee and Peter Ward published a book in 2000 called Rare Earth which discusses a number of factors that they contend make the Earth virtually unique as an abode of complex, multicellular life. (See here). They talk about a ‘galactic habitable zone’ (you need to be the right distance from the center of the Galaxy to have the right kind of stars and no disruptive cosmic events in the necessary time). They talk about the need for a Jupiter like planet to shepherd the colliding asteroids of a forming solar system, and keep a planetary surface relatively impact free for long periods of time (look at the Moon… big impacts were common in the early Solar System). They talk about orbital stability, and reasons to believe it may be more or less fortuitous in our case. They talk about mass and composition of an earthlike planet, which may be more or less coincidental. The Earth, it turns out, formed from the collision of two planets, which then formed the Earth/moon. The presence of the moon may have affected mantle convection, plate tectonics, axial tilt stability, and other factors that may or may not be crucial for complex life. It’s also just not known how likely the origin of life itself is; and how likely some of the other steps in the evolution of a complex biosphere may be. Optimists tend to assume that these things are inevitable, given the right conditions, but we just don’t know. All this may add up to the conclusion, as Brownlee and Ward argue, that while simple unicellular (bacterialike) life may be rather common where temperate liquid-water environments form, complex multicellular life, and intelligent life in particular, may be spectacularly rare in the universe.

Of course, the universe is enormous. There’s at least one such world in the Milky Way (Earth), so it seems inevitable that somewhere out there in the vast sea of galaxies, there are others. But maybe Carl Sagan’s (and others’) assumptions that there were billions of civilizations in our Galaxy alone were, well, wildly overoptimistic. None of this rises to the level of certainty, but it’s a safe bet that the truth is not exactly what people like Sagan thought, although it may also not be quite as bleak as the Rare Earth folks think either.

One factor that is quite puzzling is the way the Earth has maintained a habitable (water liquid) temperature for a long, long time, despite the fact that the Sun, like other stars, has grown brighter during that time. (About 20% since the formation of the oceans). Somehow, atmospheric conditions have changed, possibly in a Gaia-like global feedback system, so as to keep global temperatures relatively stable, within a range where life is possible. How likely is that? No one really knows, but it’s another possible pitfall in the evolution of a stable living world like ours.

So what does this all add up to? If you read or watch science fiction based on the tradition dating back to Buck Rogers, there seems to be a conventional assumption that outer space, (realistically, the stars around us in the galaxy), are teeming with living worlds somewhat like Earth, and that alien intelligent beings are common. But from what we actually know, the Sun is not unique but also has a number of uncommon characteristics; and the formation of the Earth has some unusual circumstances as well. There are reasons to believe that some of the characteristics of the Earth that make it habitable may be rather unlikely to develop. So, I think it’s a pretty reasonable assumption to say that complex life, especially intelligent life, has arisen on Earth through a series of at least somewhat unlikely events, and that worlds similar to Earth, with complex biospheres and intelligent inhabitants, are likely to be relatively rare in the universe, and those that do exist, necessarily, are likely to be not too close to us.

Sobering thoughts, contrary to a sort of unspoken conventional wisdom of the past half century or so, at least among those whose minds have tended to even consider these questions. To me, because life like that on Earth may well be rather unusual and rare, is all the more reason why we, as stewards of the biosphere we have evolved the ability to destroy, must be all the more careful to preserve it and protect its richness and complexity. We are unlikely in any foreseeable future to encounter its like. 

To anyone who actually read this whole post, I am glad to assure you that follow-on posts will be substantially shorter. Thanks for your interest.


  1. Let's see:
    1. Sun somewhat unique.
    2. Earth somewhat unique.
    3. Earth unique and a first for our type of life.
    4. Biosphere not likely to be found elsewhere.
    5. Take care of our home or we're, er...cooked.

    I think all of these things can be true along with the fact that there may be quite complex and intelligent life elsewhere in the universe, a la Star Wars if you will, that would have the ability to support our kind of life if we ever got wise enough and brave enough to go look for them.

    But we are a horribly fear-ridden species. Other intelligent life has probably learned to get along with other (alien) beings better than we have managed even to get along with each other (read: cooperate or die). Knowing we'd be likely to assume the worst and attack, I believe we are being deliberately avoided by other intelligent life from other parts of the universe.

    Alas, it still seems likely we are on a path of self-destruction (by destroying our biosphere) that most of the beings living on this planet will not survive when the end comes. But, as I say, maybe a few of us will be brave enough to try for refuge elsewhere.

    Thanks for the info. I did not find it too long. I look forward to whatever else you have in your interesting little grab-bag.


  2. Barbara, Thanks for the encouragement.

    What you're referring to is the so-called "Galactic Zoo" hypothesis, which goes like this: "of course they're out there, but they find us insufferably volatile and primitive, so they've left us alone and put an embargo around our crappy little planet." I'll talk about that in my (eventually upcoming) "Where are they?" post.


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