NASA’s announcement that there is currently water on the surface of Mars is potentially the greatest discovery in the history of humankind. On Earth, where there is water, there is life. The ultimate question for humankind is whether we are alone, or whether there is life elsewhere.
The deposits, most markedly in fact those slightly to the left on these photographs, are (in the opinion of NASA) almost conclusive proof of current, running water on Mars.
Of course, the really ultimate question is whether there is complex, intelligent, communicating life elsewhere. That is certainly not currently the case on Mars. Yet, unbeknownst to many, we have closed in remarkably quickly in the last two decades on finding it (or, at least, correctly assessing the odds of it). For what it is worth, I remain doubtful that we will find any (at least, any relevant to us) but, given the scale of the question, it is worth assessing what is happening and what our chances may be!
Mars is not particularly relevant to the quest for complex, intelligent life, but the discovery of any form of life would give us a significant clue as to the likelihood of finding that complex, intelligent life elsewhere.
The big question wpuld then be whether life on Mars exhibits the same DNA as life on Earth. If it does, then either all life in the Universe has the same basis, or life on both Mars and Earth originated from the same place. If it does not, life may develop in radically different ways in different parts even of the same stellar neighbourhood.
We should note, of course, that water does not necessarily equal life on Mars, just because it does on Earth.
The best known conversation starter to determine the likelihood of current, complex, intelligent, communicated life elsewhere in our galaxy is known as the “Drake Equation” (after American Dr Frank Drake).
There is no need to complicate the matter with the full mathematics here (not least because I am no mathematician), but essentially this sets out to calculate the number of stars created in our galaxy, and then determine which proportion of those have planets (or moons) capable of harbouring life, then those which actually do harbour life, then those on which that life has become complex or intelligent, and then those on which that intelligent life has chosen to communicate its existence into space – then putting all that into an equation alongside the probability of that communicating life actually coinciding with us in time.
We can begin, better than Drake himself could when he set out the equation in the early 1960s, to answer some of these:
- there may be as many as 250 billion star systems in our galaxy (perhaps 400 billion stars – but around half of stars are part of multi-star systems, a point to which we shall return), with seven stars created annually on average;
- it is now reasonably inferred that very few star systems do not have planets (and moons);
- it is now reasonably inferred that a significant proportion, perhaps over a quarter, of star systems have a planet in what is known as the “Habitable Zone” (sometimes also known as the “Goldilocks Zone” – not too hot, not too cold, just right for life);
- it is now reasonably inferred on the basis of the discovery of nearly 2000 confirmed planets in the last two decades and nearly 5000 candidate planets (most of which, historically, have subsequently been confirmed) that somewhere between a fifth and a half of all sun-like stars have at least one planet of around Earth’s size in the “Habitable Zone”.
The Mars findings are notable because, on Earth, where there is water there is life; therefore we may reasonably now increase the proportion of potentially habitable planets which actually are inhabited by living organisms (albeit not intelligent or even complex ones).
Put those numbers into the equation and most experts had already come up with a number in the thousands – i.e. that there must be thousands of civilisations (complex, intelligent life forms) in our own galaxy alone.
This leads inevitably to the “Fermi Paradox” (after Italian Dr Enrico Fermi), which in fact just pre-dates the “Drake Equation” but runs neatly from it. This infers that there should be thousands of civilisations in our own galaxy alone, but then asks the simple question: “Where is everybody?”
The relatively simple argument is this: humankind has advanced as far as spaceflight in just 12,000 years from the end of the last glaciation (“Ice Age”) and subsequent beginnings of irrigation; or even “just” 2.8 million years from the first bipedal hominid (recently discovered in Gauteng). This is the blink of an eye in cosmic terms – the Universe is 13.8 billion years old; the solar system 4.6 billion; life on Earth 3.8 billion; and it is even 33 million years since the most recent “extinction event” (and even that was comparatively minor). Given those time scales, and what we have recently accomplished technologically, surely we will accomplish inter-stellar travel within the next 12,000 years (or even 2.8 million)? So, then, why has nobody else?
There are a number of answers to this, of course. Firstly, perhaps they have, and we have not yet noticed. Secondly, perhaps they reached a stage of development where they realised it was best to keep themselves to themselves (after all, coming into contact with Europeans did not do slightly less advanced American Indians much good). Thirdly, perhaps there is a certain level of technological advancement beyond which it is impossible to go (simply because it becomes practically impossible, or because any civilisation attempting it destroys itself in so doing – this is often known as the “filter theory”).
Fourthly, though, it is just possible that the development of complex, intelligent, space-age life on Earth is just an absolute fluke – this is normally known as the “Rare Earth Hypothesis“.
(I personally counter the “Rare Earth Hypothesis” on admittedly more philosophical grounds: what is the point in all these spectacular supernovae, nebulae, galaxy mergers, pulsars, dust clouds, rogue planets and all the rest of it, if there are no sentient beings around to experience them? Call it the “Parsley Paradox” – sounds pretty good!)
It is worth, firstly, assessing the cosmic scales of such things. This is usually done by measuring space and time based on the speed of light – around 300,000km per second.
On this scale, the moon is just over one second away (the exact distance varies slightly as the moon’s orbit is not precisely circular – it is at its closest right now, hence the “Red Moon” event last weekend) – meaning we see the moon as it was just over a second ago.
The sun is just over eight minutes away.
The brightest planet in our night sky and nearest neighbour apart from the moon, Venus, can be as close as around two and a half minutes, although it can drift to over ten minutes if it is at the opposite side of the sun during its orbit – Venus is, for reference, almost exactly the same size as Earth. Mars, topically, can come as close as just over four minutes and go as far as over twelve. The furthest easily visible planet, Saturn, is over an hour; the New Horizons space probe has made it out to Pluto at around four hours. At least one comet orbits the sun from over a light year away, and there are probably many more – but they are of course only visible from Earth (or any inner planet) when much closer (within a few minutes of light travel) on their occasional approaches.
The very nearest star system – actually a three-star system with two stars similar in size to the sun and one much smaller – is four years away on the same basis. To compute that back to Earth, if each of the main two stars were the size of a grain of sand, the sun would also be the size of a grain of sand nearly seven kilometres (four miles) away!
Often quoted as being of most immediate interest is the area within around 80 light years – that is the area from which the sun itself would be visible with the naked eye from a planet orbiting another star. In that area alone, current detections indicate there are on average two or three Earth-like planets in the Habitable Zone of Sun-like stars (including one candidate “just” 12 light years away).
Our galaxy is over 100,000 light years across, and four light years is a fairly typical inter-stellar distance. There are also some small satellite galaxies a few tens of thousands of light years further out. Not all of this is visible – about a sixth is hidden behind the busy Galactic Centre.
The nearest major galaxy, as noted above, does not appear in the current equations but, for the record, is over 2.5 million light years away. (The distance to the end of the Observable Universe is over 45 billion light years, but that’s just incomprehensible so let us not go there!)
However, in addition to scales in space, we also have to consider scales in time. If a very advanced civilisation existed in our galaxy five billion years ago; or exists in our galaxy five billion years from now; there is every chance we will know nothing about it.
The age of the Universe is current projected to be 13.8 billion years, give or take a few hundred thousand. As noted above, the solar system is around 4.6 billion years old, so has existed for around a third of that time. However, humankind’s presence would only have been easily detectable in space from 1895, and possibly not definitively until 1937. This is a tiny period, obviously.
Obviously different planets can have come into existence at any time in the past 13 billion years or so. Therefore, even assuming (and it is a big assumption) that other civilisations develop at a similar rate from formation of the planet and the beginnings of basic life (i.e. over billions of years), the chances of finding a civilisation at almost exactly our stage of development (i.e. which is obviously detectable but not significantly advanced) are millions to one – even assuming one can exist elsewhere at all. Realistically, this leaves two options: that we detect life by detecting biology in the atmosphere of other planets (or stars); or we will find a civilisation vastly more advanced than we are. The latter is less likely, particularly anywhere “nearby”, because it would surely already have found us…
It is worth noting that around 85% of stars in our galaxy are red (or orange) dwarfs, invisible to the naked eye (particularly in urban areas) here on Earth. These are not a focus for seeking life because they are much older and cooler, meaning the “Habitable Zone” is much closer to the star than it is in the Solar System – so close, in fact, that any potentially life-harbouring planet would be “tidally locked”, showing one face to the star all the time (as the Moon does to Earth) and thus not experiencing day and night (assumed by many to be necessary for complex life, notably through photosynthesis, to develop). This accounts therefore for maybe 340 million or so of the 400 million in our galaxy (and then even a sixth of the remainder are practically invisible to us as noted above). Although the contention that planets or moons could not possibly develop complex life is contested, they are not seen as prime candidates – after all, if the only planet known to have complex life is Earth, it makes sense to look for star systems and planets like Earth.
Of the remaining 15%, it is currently estimated that around half are multi-star systems (although there is a fair measure of doubt about this, as multi-star systems tend to be easier to detect but there may be some dispute about whether the stars really are part of the same system). As noted above, the closest to us is a three-star system. Again, these are not seen as prime candidates – it is unclear whether complex life could develop with the complexities of two or more stars. For the record, there are some planets which orbit the entirety of such a system (one even orbits four stars at once); there are others which orbit only one of the multiple stars in the system. The latter would probably be more stable and may be candidates for complex life, but they are currently not generally prioritised.
Of the remaining 7-8% or so, some (albeit a minority) are far too big realistically to support life within their system. These may be hundreds or even thousands of light years away, and tend to be short-life stars anyway, around which complex life may simply not have had time to develop (their lives can be a short as tens of millions of years, whereas life has existed on Earth for several billions). They would also be extraordinarily hot, forcing their “Habitable Zone” well out, potentially into areas vulnerable to objects such as comets.
That leaves around 5% – although a third of those easily visible with the naked eye – which are sole stars (or at least wide orbiting binary stars) of average age like our Sun. These are themselves split into three categories – “generally Solar-type” stars, of which there are a few dozen within 20 light years, are roughly the same age but may have little else in common; “Solar Analogs”, three of which are sole star systems 10-20 light years away, are quite similar in many ways but may have a significant discrepancy in heat or make-up; and “Solar Twins”, the nearest of which is just under 50 light years away, are very similar indeed in all regards (age, heat, size, metallicity etc), and thus have a very similar “Habitable Zone” to the Solar System’s. The focus of our search understandably focuses on these, as they provide conditions nearest to those which we already know support intelligent life.
The first planets found outside the Solar System (known as “exoplanets”) were confirmed in 1992, but these were around a pulsar, not a star. They began being found around stars from 1995, with particular advances being made by the Hubble Space Telescope (a wide detail telescope placed in orbit around the Earth, although its focus was more on the origin of the universe rather than the search for life) and the Kepler Mission (which chose a particular area of sky in our neighbourhood in the galactic “suburbs” known to host a high proportion of Sun-like stars to look for exoplanets, with a particular aim of establishing roughly how common they are).
(The Solar System is in the galactic “suburbs” about two thirds of the way from the centre to the edge, excluding the much less populated outer “halo”. It is therefore well away from the more populated and potentially more dangerous centre, and even from other denser “arms” of stars coming out from that centre. Many astronomers argue such a galactic location is essential for life to develop over the long term to the levels of complexity and intelligence now found on Earth.)
As the only complex life we know exists on a planet, those seeking life outside our solar system focus on planets (and occasionally their moons).
Despite vast advances, it is too early to be definitive about all of the trends we are finding, but we can say some things:
- very few stars have no planetary system at all;
- so-called “Hot Jupiters”, large gas giants orbiting closer to their star than the inner Solar System planet Mercury does to the Sun and thought to inhibit the development of complex life, are not as common as first thought (they were found in great numbers early on because they are the easiest planets to detect) – probably fewer than 3% of Sun-like stars possess one;
- the most common size of planet may in fact be one which does not exist at all in the Solar System, between the size of Earth (the largest inner planet) and Uranus (the smallest gas giant); and
- nevertheless, small (potentially Earth-like rocky) planets may be very common, existing in the “Habitable Zone” of perhaps over 30% of stars.
For those seeking life, the balance of these outcomes is surely positive. Furthermore, we have found confirmed planets over 20,000 light years away (and we even have candidates in other galaxies).
One notable negative is that early indications are that the eccentricity of the orbit of planets is much higher than the Solar System average. Earth’s, itself below the Solar System average, ranges from almost zero to nearly 0.06, averaging just under 0.02 (it is currently slightly below this, and decreasing for the next ten millennia). However, the average for exoplanets appears to be 0.25 – much higher even than Mercury, by far the most eccentric planet in the Solar System. This is a problem because it would create significant seasonal discrepancy between hemispheres, among other things. However, this figure will likely decrease as more exoplanets are found, not least because it may become apparent that some orbits put down as belonging to the same planet in fact belong to two different, albeit nearby, bodies.
One obvious question in all of this is an incredibly simple one: what is life? As a hopeless biologist, I am in no place to answer this directly!
One thing which is apparent is that we make an assumption that Earth is ideal for the development of complex, intelligent life (certainly exponents of the “Rare Earth Hypothesis” base their whole argument on this). I would instinctively dispute this. It strikes me that Earth is hit by asteroids (like the one which destroyed the dinosaurs 65 million years ago) rather more often than the “ideal”; Earth’s eccentricity is fairly low but still not zero; and the variations between glacial and non-glacial periods over hundreds of thousands of years may be relatively extreme. Earth is also, arguably, a little on the small side for supporting intelligent, energy-sapping life.
On the other hand, it is clear that some freak occurrences have enabled intelligent life to develop, some of which may yet prove to be unique. Life itself appears only to have developed once on Earth, as has intelligent life capable of making itself known to outer space. One of the main drivers of our advances has been industrialisation, itself dependent on some remarkable and potentially freak advances right back to someone having the foresight to rub two stones together fearlessly. Another main driver, of particular interest to me, was the development of complex language which seems unmatched elsewhere in the animal world (tied seemingly to humankind’s unique voice boxes, although the fundamental origin of language remains poorly understood), and which may be essential to reach our level of technology. There is also a real question over whether nature in fact rewards intelligence at all, given the length of time dinosaurs dominated the land and sharks have dominated the seas – could the coming to prominence of intelligent homo sapiens be nothing more than a complete fluke? Contrary to these, it does appear that complex organisms have developed on over 40 separate occasions on Earth; and it does appear that some other mammals at least share our broad sense of wonder about the world and even universe around us, hinting that the fundamental pre-requisite to great understanding (and philosophical and industrial advances) may not be unique.
Basic disputes even remain over whether “life” need necessarily be “biochemical”. These are well beyond the scope of my own knowledge and comprehension!
The presence of water on Mars indicates the possible, perhaps even probable, presence of life on Mars – possibly even currently living organisms of some sort. This coincides with the discovery of vast numbers of planets orbiting other stars, many of which are in the “Habitable Zone” also capable of developing water and very possibly therefore also basic life of some sort.
That we are in all probability not the only place where life has developed leads to perhaps the most fundamental question of all: are we alone in this galaxy as complex, intelligent life forms capable of indicating our presence into outer space? In this piece, I have tried to establish some of the parameters of how we may seek to answer that question based on current, rapidly expanding human knowledge.
The parameters are that life needs certain conditions to begin and then thrive, and that these conditions are almost certainly met on planets (and even moons) orbiting other stars, particularly stars like our own Sun in single-star systems like our Solar System. However, this life may develop a long way or even a long time away from where we are.
However, that is no guarantee that the conditions to develop complex, intelligent life to our level of philosophical and technological advancement exist anywhere. Evidence from the history of Earth is patchy, and evidence from elsewhere in the galaxy (not least the apparent absence of any advanced civilisation) suggest it is in practice odds very much against in any given system, even where theoretically ideal conditions exist.
What we do know is that we are in a position to focus in the right areas in the search for life elsewhere, with a real chance of making exciting discoveries very soon.
As a final note, the sum of human knowledge expands twenty-fold every twenty years. So, what we can say for certain is that twenty years from now our understanding of our own planet, of our own Solar System and of our own galaxy will have expanded beyond anything that is even currently imaginable, just as has happened since the first confirmed discovery of a planet around another star in 1995. We may very well know by 2035 how common life is elsewhere in the galaxy – and even specifically where it exists.
It is a most remarkable human endeavour, and we are privileged to be living in such an age of discovery.