Are We Alone in the Universe?

The recent trend among scientists in certain fields, so far as I can tell from perusing their writings, is to be sanguine about the chances of there being intelligent life somewhere else in the universe. This trend is really quite remarkable given the typical attitude of scientists in other areas and the paucity of evidence for this life. The same scientific community which stubbornly refuses to acknowledge some very clear indications of life on Mars turned up by the Voyager experiments of the 1970’s is nevertheless cautiously optimistic about finding it elsewhere. Many well respected scientists like Seth Shostak have spent many hours scanning the heavens for some sign of radio contact from another civilization, and a couple have even recently announced that, given the very real possibility of intelligent life evolving elsewhere, perhaps we shouldn’t so quickly scoff at UFO reports.

Notwithstanding this hopeful drift of opinion, we still find ourselves confronted with Fermi’s Paradox, to wit, “If there are intelligent beings out there, where are they?” In other words, given all the time during which intelligent species could have evolved before we did, before our solar system even came into existence, the presence of intelligent species, if they are at all similar to us in temperament, should be pretty obvious in our galaxy. It has been estimated that a single community, by spreading out in a chain reaction of extra-solar colonization, could populate the entire galaxy in as little as 5 million years, and certainly in under 50 million. Given that our galaxy is billions of years old, it is strange that this has not yet occurred.

Many explanations to this paradox have been proffered over the years, some of them seemingly ridiculous, some of them moderately sensible, but only one, in my opinion, is a complete and simple solution to the paradox which does not leave one uncomfortably doubtful about at least one or two of its implications. That elegant and reasonable solution is simply that there is no one else out there. Or if not no one, then at least not many.

The famous astronomer Frank Drake came up with an equation named after him which is an attempt to estimate how many other civilizations exist. It is a series of fractions which eventually whittle down the total number of stars to an estimate of the total number of planets that are home to intelligence. Generally applied only to the Milky Way Galaxy, it starts with an estimate of the number of stars, then asks how many of those have earth like planets, how many of those planets are near enough, but not too near, the sun, and so forth. But all current estimates are fraught with guess work, and Drake himself said the equation is really just a way to organize our ignorance.

Carl Sagan, using the Drake Equation, once estimated a million other intelligent species in our galaxy, and though he later reduced that number there is still much optimism out there that, given the sheer quantity of stars, there simply has to be intelligent life, if not in our galaxy, then at least somewhere in the universe. Since there are, approximately, one million stars in the known universe for every grain of sand on planet earth, this could well be a universe teeming with intelligent life. But I myself am much influenced by The Rare Earth Hypothesis, which states that microbial life is probably extremely common, but that complex and intelligent life is extremely rare or even unique. It is, after all, the most elegant solution to Fermi’s Paradox.

The Rare Earth Hypothesis summarizes what we know about life’s evolution here on earth, tries to fill in the gaps with some speculation, and then applies these tentative conclusions to the idea of life elsewhere. There is much criticism that many of the points raised in TREH are not limitations to life elsewhere, but rather simply descriptions of how we developed here. There certainly could be much validity to these arguments, but for the most part I think the authors have identified some key elements that any system with intelligent life will have. With some of these fractions beginning to come into sharper focus, and in light of these Rare Earth insights, we can recalculate the Drake Equation and perhaps get at an idea of the likelihood of intelligent life existing in our own galaxy.

Let us begin with the number of stars in our own galaxy. Estimates range from 100,000,000,000 – that’s one hundred billion – to 500,000,000,000. The estimate I have seen most often is 200 billion, so I will begin with that and pare away the systems thought to be unsuitable for life. What we will wind up with is a range, with an optimistic figure at one end and a pessimistic figure at the other, of likely suitable worlds where carbon-based, earth-like intelligence could develop. This is as far as we can go with estimates based at least partly on what has been observed. Discussions on non earth-like life are mere speculation and, in my opinion, probably pure fiction.

With our figure of 200 billion stars in the Milky Way, we must now, according to TREH, determine how many of those stars reside in the galaxy’s habitable zone. This is a concept that many find surprising, that there could be parts of the galaxy in which complex life will not develop, no matter how pleasant the sun and ideal the planet. The fact is, the central part of the Milky Way is an area where stars are packed tightly together. There are 40 stars within 16 light years of our sun, the nearest being 4.22 light years away. If our sun were in the center of the galaxy, there could easily be hundreds in that same 17,000 cubic light year sphere of space. This greatly increases the chances of a star being in the vicinity of a super nova, whose gamma ray burst could fry all life in nearby systems. It also means the super nova explosion could be right next door, as opposed to several light years away. The center of the galaxy is also a zone of high radiation which would certainly be inimical to our kind of life.

But the outer zones are poor candidates for life as well, since they are metal poor. In astronomy, a metal is any element heavier than hydrogen and helium, and these metals are made by the super condensing power of stars which, when they die and explode, release their contents into space. When the next generation of stars form, they do so with more metal rich material, but on the thinly populated outskirts of our galactic metropolis, very little of this metal-building has gone on, making terrestrial planets very unlikely. It is therefore argued, and I think the argument is a very strong one, that complex life will exist in an inner ring of a spiral galaxy, away from the lethal center and inside the sparse outer edge. Estimates of the number of stars in the Galactic Habitable Zone range from 5% of the total population to 10%. The pessimistic figure leaves us with 10 billion stars, while the optimistic one leaves us with 20 billion.

Another factor to consider is the type of star. The large, powerful stars are unstable and short lived. While life on earth was probably present 3.8 billion years ago, very soon after the putative collision that is supposed to have resulted in our moon orbiting us, complex life was not present until about 580 million years ago. There is a great amount of uncertainty here, but if intelligent life requires billions of years to develop, then the largest stars, types O, B and A, will explode and destroy any planets they have long before this limit is reached. Small stars are also poor candidates, because in order for a planet in the system of an M type star, the smallest kind, to have liquid water it would have to be so close as to be in constant danger of solar flares and radiation, not to mention being tidally locked with the sun. Being tidally locked means that the same side of the planet is always facing the star, which would not, it is reasoned, be conducive to life.

This means that M type stars are out, in addition to the O, B and A large stars. The dimmer K type stars and the brighter F type stars are also probably poor candidates, which leaves only G type stars, like our sun, along with possibly the bright K’s and dim F’s. The smaller the star type, the more populous it is in the heavens. M type stars account for over three quarters of all stars in the sky. G type stars represent only 7.6%. Combined with a small portion of the K types and a small portion of the F types, it seems reasonable to conclude that only 8 to 12 per cent of the stars in the galaxy are suitable to the long term evolution of life. With our pessimistic 8% figure, we are down to 800,000,000 stars; with the optimistic figure we arrive at 2.4 billion. Even speaking optimistically we are down to little more than 1% of the stars in the Milky Way. And there are many steps ahead of us.

We must now ask what proportion of the stellar population has terrestrial planets. Until recently this was nothing more than speculation, but a recent survey of other stars found that between 20% and 60% had the kind of dust that some astronomers think is the by product of terrestrial planet creation. That is a very large margin of error, but at least it gives us something to work with. Pessimistically, we are down to 160,000,000, and optimistically we come to 1.44 billion.

A terrestrial planet that supports life will be constrained by size in all likelihood. The current thinking which I have read and heard is that, in order to maintain plate tectonics (discussed more below) a planet must be at the very least half the size of earth. A too large planet is also considered a poor candidate. So what is the ratio of acceptably sized planets to stars? There is little to guide us here, but we do have at least a vague notion of things, and I cite two lines of thought to arrive at what might be a plausible range of values.

One, we have now discovered about 200 exoplanets, all but one of them thought to be gas giants like Jupiter and Saturn. What is becoming clearer and clearer is that the larger the gas giant, the fewer of them one finds, despite the fact that the larger ones are easiest to find. In fact, a gas giant of Jupiter’s size – there are far larger planets out there – is many times more prevalent than some of the super giants. It seems reasonable that a similar bias might eventually be discovered amongst terrestrial planets, which would mean that earth sized planets are not uncommon. Furthermore, it is supposed that we have a range from half earth sized to slightly larger to work with. My second line of reasoning is simply to look at our own solar system, excluding the Earth for anthropic reasons. In our system we have very small Mercury, nearly earth-sized Venus, small Mars and a very small planet that never quite formed, leaving us instead with the asteroid belt. This admittedly minuscule study at least coincides with what we are beginning to discover about gas giants: smaller planets are more prevalent.

With these two lines of reasoning, a healthy dose of arbitrary lay-opinion, and very little idea how many terrestrial planets typically form in a star system, I’ll assign a 1 to 4 ratio of correct sized terrestrial planet to sun as pessimistic and, say, 4 to 5 as optimistic, realizing that some systems, like ours, will have two planets of acceptable size and will thus drive up the real ratio. The pessimists now have 40 million and the optimists have 1.152 billion.

The next category is relatively uncontroversial: the solar habitable zone. The planet must be at the “Goldilocks” point, where the water will not all evaporate or freeze. In our own solar system, it is thought that the habitable zone – sometimes called the continuously habitable zone in recognition that the sun varies in brightness over time – extends from 0.95 Astronomical Units – 1.0 AU being the average distance of the earth from the sun – to 1.15 AU, which leaves Mars and Venus just outside the zone. Of course a planet which reflects a lot of sunlight might conceivably move in a little closer and still maintain life, and other considerations might allow to move a little further out, so the limit is not a clear line of demarcation, but it is something to work with. If we consider the asteroid belt, which can go out to about 3.0 AU, as the furthest that terrestrial planets will form in our system, and if we accept that our system is most likely average – meaning that for every system with wider boundaries there is one with narrower limits to balance it out – and if we also assume that a planet is as likely to form in one part of the terrestrial zone as another (possibly a bad assumption), then a planet has about a 7% chance of being in this zone. Some of these assumptions are likely good ones, others are quite uncertain, so let’s make 5% the pessimistic extreme and quadruple it for the optimistic one. This leaves the pessimists with 2 million adequate planets in an adequate zone in an adequate part of the galaxy, while the optimists still have 230,400,000.

Another must for intelligent life, so it seems, is that the system have planets with stable orbits. An eccentric orbit and the wildly varying weather it brings would make it difficult for advanced life to develop. Unfortunately for the SETI crew, of the planets surveyed so far, the vast majority have elliptical or otherwise unstable orbits. There are a number of planets with stable orbits, but almost all of these are gas giants orbiting very close to their suns. Perhaps, upon drawing close to a sun, a planet’s orbit smoothes out a bit, but any such gas giant, which must have formed further out according to what we now understand, would completely upset any terrestrial planets closer in when it started its move. We are left with very few planets with stable orbits, and our list of candidate planets is about to shrink a great deal. Let’s take 1% at the low end and 8%, probably unreasonably high, at the upper end. The most planets we are likely to have left is 18,432,000, and we may already be as low as 20,000 with more elimination steps coming.

The size of the gas giants in the system is a consideration. It is thought that a gas giant of sufficient size will reduce the number of potentially deadly meteorite impacts, but if the gas giant is too big it will disrupt the orbits of the other planets, even if its own orbit is a circle. However, some computer models have suggested that if Jupiter had instead been the size of Saturn, the number of meteorite impacts on earth would increase. Of course, for reasons discussed below, this could be a good thing, so there is a great deal of speculation on this one. There may be very few of the super giants, as discussed above, so it is likely that Jupiter sized planets are fairly common. I am unaware of how distance from the star, beyond simply being outside the terrestrial area, affects these conditions. Let’s set limits at 20% and 50%, leaving us with 4,000 planets and 9,216,000 planets respectively.

Plate tectonics is thought to be, for a number of reasons, important for the evolution of advanced life. One reason is that it recycles carbon back into the ecosystems, an important thing for carbon-based organisms. Another reason is that, by moving the continents over the face of the planet and throwing up mountain ranges, it is constantly creating new climates and micro climates, leading to diversity of species. This diversity, apart from its obvious benefits, is also important for surviving disasters like asteroid impacts. These disasters, in earth’s own history, have killed off as many as 70% of the species of the planet. With less diversity, it is conceivable that all complex life might have been extinguished and, left with only unicellular organism, the planet would have had to start over.

This consideration leaves us with two further paring-points. One, what elements, and in what ratios, make up the interior of the planet. While tectonics are not completely understood, scientists are confident that radioactive decay is indispensable to their existence. Since we are already dealing with metal rich stars, as evidenced by planetary formation itself, it is unlikely that the percentage of these planets without the right elements in its core is extremely low, and could well be very high. There is so much uncertainty that we must leave the range of values very wide open (we must also consider that iron in the core, which provides for a protective magnetic shield, is also important). Let’s go with 20% and 80%, giving us 800 planets on one end, and 7,372,800 on the other.

The second paring-point is the amount of water, which acts as a lubricant for the subduction zones that are a core part of plate tectonics (not to mention being a key component of life). Once again, it is very difficult to estimate with any confidence. Most water is believed to be carried in from further reaches of the solar system by asteroids in the early part of a planet’s life, but too little water and too much water are thought to be bad. Too little and the tectonic process will not run well and the water will most likely lose its liquid form. Too much and no continents will ever rise above the surface of the oceans, and it seems quite clear that the most intelligent species evolve on land (even dolphins, reputedly highly intelligent, evolved on land before returning to the water). Let’s assign the same broad range of 20% and 80% to this category, which has the same profound uncertainties. We now have 160 planets left if we are pessimists and 589,824 if we are optimists.

Finally, we come to the moon. This is a subject I am not entirely sold on. It is argued in TREH that a large companion moon is essential because of the way it stabilizes the earth’s rotation and wobble. Some argue that this is not necessary for complex life but is merely what happened in our case. With some reservations, I am inclined to incline towards Ward and Brownlee, the authors of TREH. Though, as hinted above and discussed a bit more below, climate variation is important for spurring evolution on, it could well be that the erratic wobble of the globe without a moon would be entirely too much for complex life like mammals. It is also worth mentioning, even if it is only one data point, that if this moon formation is as infrequent as studies suggest, then the fact that we wound up with one might carry profound significance, for if moons are unnecessary then we could still be here today and yet would most likely not have a moon. The fact that we do have one, uncommon as they appear to be, combined with the stabilizing effects we know it has, is enough to put me tentatively on the side of the moon inclusionists. At any rate, recent studies indicate that the sorts of collisions that probably form such proportionately large moons amongst terrestrial planets occur infrequently but not rarely, in 5-10% of star systems with planets. However, we must remember that just because it happens does not mean that it happens to the particular terrestrial planet that sits in the habitable zone. Let’s take 5% for the optimists and 1% for the pessimists, leaving us with, after rounding, as few as two planets and as many as 29,491. This upper limit is, in my opinion, very likely to be far too optimistic, but if it turns out that a companion moon is not necessary the prospects get a good deal better.

We have not established that life will develop on these planets, only that these are the ones where complex earth-like life, if it does form, will likely be found. Any other sort of life, if it can possibly exist, is nothing more than speculation. At least with carbon-based earth-like life we have a little bit of data to mix in with our guesses. There is still a great amount of uncertainty, which explains the large disparity between 2 and 29,491, but if the moon is a vital element than I feel a moderate amount of confidence that the real number is in that range. Eliminating the moon requirement for the optimists, we get a range of 2 and 589,824, and I feel quite confident that the real figure will be found in that range, probably far closer to 2.

In crunching these number, this time without ranges but rather with my best guess as to the real percentages, I arrive at 1,853 suitable planets in suitable systems (that’s with the moon requirement). This doesn’t mean that I believe there are 1,853 other civilizations out there in our galaxy, just that there are 1,853 planets where they might have developed or, if the system is still young, where it might develop given more time. And this is where the guess work gets very fuzzy indeed. I would be willing to bet that simple life forms will evolve on at least 1,852 of these planets. Apart from the fact that life arose almost immediately on this planet when conditions were right, biologists and chemists are increasingly noting how abundant the building blocks for life are, and how easy it might be to get life started. I think microbial life is all over the place, but complex life is likely a very different matter.

It took over three billion years to get from simple bacteria to more complex life forms. Furthermore, it does not appear to have been a steady climb to complexity, suggesting that certain conditions must be present, or perhaps certain events set it off. One thing that is nearly certain is that a great deal of oxygen must be present for organisms to gain a great deal of complexity, and this takes a while to build up in an atmosphere. Another factor often cited is that catastrophic events, while killing off a great number of species, actually clear the way for future progress. The great leaps forward in the history of evolution are fairly closely tied with some sort of crisis event on the planet, whether it be Snowball Earth or an asteroid impact. If this is the sort of precarious trail that leads to complex life, this need for disaster and a diverse ecology to survive it, it is reasonable to suppose that not all of these 1,853 suitable planets will develop it. Some of the disasters will prove too much, some to little. As to intelligent life, even less is known. It does appear that animals may have increased in intelligence over time, but the sudden spurt of intelligence in the genus homo is quite anomalous, at least in earth’s history (We must also note that if complex life can develop on the moon of a gas giant in the habitable zone, the odds will get better).

So how many intelligent species are there, at least with characteristics similar to us? The humble blogger is beyond the point where he is willing to speculate. Due to issues of time, we must eliminate the young systems, so it is likely that the number of suitable planets in suitable systems, if the 1,853 figure is remotely correct, is around 1,000. It will be exciting in the coming years to try and narrow down our limits and ranges with new data sure to be forthcoming from the new telescopes that they plan to launch into orbit. Perhaps these ideas will be overturned, perhaps confirmed. But for my money, the best answer to Fermi’s Paradox is still that we are alone or nearly alone in the galaxy, possibly the universe.

For this reason, SETI is probably a waste of time for some talented astronomers as well as a waste of tax payer money, but the humble blogger will expound on that in a future blog post.