Beyond the planet, a vitally important ingredient for producing a potentially living world is, of course, the parent star itself, and all stars are most definitely not alike. There are over two hundred billion stars in the Milky Way galaxy. The largest known supergiant stars are over 1500 times the diameter of our Sun. If such a star were located at the centre of our solar system, it would engulf Jupiter. At the other end of the spectrum are tiny red dwarfs, with diameters from around half that of our sun to as small as a tenth of it. The smallest known star at the time of writing goes by the name of 2MASS JO5233822-1403022, which shines eight thousand times less brightly than our sun and is smaller (but denser) than Jupiter.
As with virtually everything in physics, a good way to make sense of this stellar menagerie is to draw a graph. The most famous graph in all of astronomy is known as the Hertzsprung-Russell diagram, after astronomers Ejnar Hertzsprung and Henry Norris Russell, who drew it independently in 1911. They plotted the surface temperature of the stars (which is directly related to their colour – hot stars are blue or white hot, cool stars are red) against their brightness. It is immediately obvious that the stars are not distributed randomly on the diagram. Most lie on a sweeping line ascending from the bottom right to the top left. This line is known as the Main Sequence. Our yellow sun lies around the middle of the main sequence, and all the stars on this line are generating their energy in the same way – by fusing hydrogen into helium in their cores. These are the ‘standard stars’, if you like, although their masses, lifetimes and suitability for the support of living solar systems are very different.
The basic physics underlying the Main Sequence line is simple. Stars are clouds of hydrogen and helium, which is pretty much all there is in the universe to a good approximation, collapsing under their own gravity. As the cloud collapses, it heats up. This is not surprising – all gases get hot when they are compressed – try pumping up a bicycle tyre. Eventually, the collapsing ball of gas gets so hot that the positively charged hydrogen atoms overcome their mutual electromagnetic repulsion and fuse together in a nuclear reaction to make helium. This releases a tremendous amount of energy, which further heats up the gas, increasing the rate of nuclear reactions and continuing to heat the gas. Hot gases want to expand, and so ultimately a balance will be reached between the crushing force of gravity and the outward pressure exerted by the nuclear-heated gas. This is the current state of our Sun, happily converting 600 million tonnes of hydrogen every second into helium to counteract the inward pull of gravity. For less massive stars, the equilibrium will be reached at a lower temperature because the inward pull of gravity is weaker. Having a lower surface temperature, these stars will be redder than our sun, and also less luminous. These are the dim, red stars at the bottom right of the diagram, known as red dwarfs. We’ve already met an example of a red dwarf – our nearest stellar neighbour, Proxima Centauri. Red dwarfs also have the longest lifetimes of the stars on the Main Sequence, simply because they have to burn their fuel at a lower rate in order to reach a stable equilibrium with gravity.
At the other end of the Main Sequence are the massive blue stars. Ten times the mass of our Sun or more, the inward pull of gravity is strong, and they have to burn their hydrogen fuel at a profligate rate to resist collapse. This makes them hot, and therefore blue, but also short-lived. The largest Main Sequence stars will use up their nuclear fuel in ten million years or less, at which point they will move off the Main Sequence to become red giant stars. The red giants, like the famous Betelgeuse in the constellation of Orion, are stars nearing the end of their lives. Starved of hydrogen in their cores, they begin to fuse helium into heavier elements like carbon and oxygen. These stars are the origin of most of the heavy elements in your body. Their cores become superheated in their ultimately futile battle against gravity, causing their outer layers to expand and cool. This is why the red giants sit at the top right of the Hertzsprung-Russell diagram. They are vast, and therefore bright, but their cool surfaces cause them to glow a deep red. Red giants will last for only a few million years before they run out of nuclear fuel, at which point they shed their outer layers, forming one of the most beautiful sights in nature – a planetary nebula. It is these clouds, rich in carbon and oxygen, which ultimately distribute the building blocks of life into the galaxy. Your building blocks are likely to have been part of a planetary nebula at some point over five billion years ago. Cooling at the heart of the nebula is the fading core of the star, exposed as a white dwarf. These stars populate the bottom left of the Hertzsprung-Russell diagram.
There are a handful of other exotic stars out in the Milky Way. The vast blue supergiant stars like Deneb are extremely hot and extremely luminous. Deneb, the brightest star in Kepler’s field of view in the constellation of Cygnus, is almost 200,000 times more luminous than our Sun, and 20 times more massive. It burns its nuclear fuel at a ferocious rate, and will probably explode in a supernova explosion within a few million years, leaving a black hole behind.
The Hertzsprung-Russell diagram, then, is the key to understanding stellar evolution, and also contains vital information for planet hunters. Stars that do not lie on the Main Sequence are highly unlikely to support planetary systems with the right conditions for life. They are either short-lived and ferociously bright, or have had a life history fraught with violence and change. The Main Sequence, containing the stable, hydrogen-burning stars, is where we should look for stability. But even there, the more massive, brighter stars are likely to be too short-lived for complex life to emerge. On Earth, life existed for over three billion years before complex organisms emerged in the Cambrian explosion just 550,000 years ago. We will discuss the history of life on Earth in more detail a little later, but for now we might venture an educated guess that stars with lifetimes significantly shorter than a billion years or so are unlikely to preside over planets with intelligent civilisations. This rules out the blue stars at the top left of the Main Sequence. Even familiar stars like Sirius, the brightest star in the night sky and only twice the mass of the Sun, can probably be ruled out as its lifetime on the Main Sequence is expected to be a billion years at most. We are therefore left with stars on the Main Sequence with masses within a factor of two or less of our Sun as candidates for solar systems that could support complex life.
There may also be a lower limit on the masses of life-supporting stars, although this is very much an active area of research. Around 80 per cent of the stars in the Milky Way are red dwarfs, and many are known to have solar systems. Red dwarfs have potential lifetimes measured in the trillions of years, so there is no issue with their longevity. Despite their frugal use of fuel, however, red dwarfs tend to be volatile and variable in their light output. Sunspots can reduce their brightness by a factor of two for long periods of time, and violent flares can increase their brightness by a similar factor over time periods of days or even minutes. Planets in orbit around red dwarfs are therefore subject to significant and rapid changes in the amount of light and radiation they receive. Furthermore, because of their low light output, planets must be extremely close to the star if they are to be warm enough for liquid water to exist on the surface, irrespective of the details of their atmospheres. When planets orbit close to stars, they become tidally locked, with one hemisphere permanently facing the star and the other always facing into the darkness of space. We only see one face of our Moon for the same reason – tidal locking is inevitable for moons orbiting close to planets or planets orbiting close to stars. This results in a strange kind of climate for potentially habitable planets around red dwarf stars; there will be regions of permanent day, and regions of permanent night.
Despite all these problems, however, recent computer modelling suggests that red dwarf planets may be able to maintain stable surface conditions if they have thick, insulating atmospheres and deep oceans, and life has plenty of time to evolve in these unfamiliar (to us) conditions. The jury is still out as to whether the red dwarfs that populate the low-mass region
of the Hertzsprung-Russell diagram could be candidates for living solar systems.
Where does all this leave us? If we take the conservative path, and focus our attentions on the Sun-like orange and yellow stars on the main sequence, we can look at the Kepler data to estimate how many of these so-called F, G and K-type stars in the Milky Way have rocky planets in the right orbits to allow liquid water to be present on the surface, at least in principle. These planets orbit within what is known as the habitable zone, and this is the number we want to measure and insert into the Drake Equation. This has been done, and the results are surprising. In a recent study, ten planets were identified as Earth-like in the Kepler data set, in the sense that they have the right mass and composition, and are in the right orbits around their parent Main Sequence F, G or K stars, to support liquid water on their surfaces for long periods of time. Applying all the statistical corrections to account for the alignment of the solar systems relative to Earth, the lack of ability to see planets with longer orbital periods, and so on, we can estimate with a reasonable degree of certainty that there are around 10,000 Earth-like planets capable of supporting life in Kepler’s field of view. This in turn suggests that around a quarter of F, G and K stars in the Milky Way have potentially life-supporting planets in orbit around them, corresponding to ten billion habitable planets. If we allow the possibility that planets around red dwarfs may also be habitable, then we can more than double that number.
There is one final point worth making about habitable zones around stars. In our solar system, Venus, Mars and Earth are within the habitable zone as commonly defined, but there are other places where life may exist. Several of the moons of Jupiter and Saturn are planet-sized worlds, and it is known that the Jovian satellites Europa and Ganymede, and quite possibly Saturn’s giant moon Titan and the small but active Enceladus, have sub-surface oceans or lakes of liquid water. Europa in particular is considered to be one of the most likely places beyond Earth that may support life, even though it is outside the more commonly defined habitable zone around the Sun. If we admit the possibility that planet-sized moons may extend the habitable zone around stars, then the number of potentially life-sustaining worlds in the Milky Way increases significantly.
Over 50 years after the Green Bank meeting, the first three astronomical terms in the Drake Equation are now known from experimental data, and they are encouraging for SETI. There are, of course, large uncertainties, and one can find differing interpretations of the data in the academic literature. What is absolutely clear, however, is that the number of potential homes for life in the Milky Way is measured in hundreds of millions at the very least – most likely billions. From an astronomical perspective, the Milky Way could be teeming with life. The next three terms in the Drake Equation are biological; they concern the probability that life will emerge spontaneously on a planet that could support it, and the probability that the necessarily simple life that first appears evolves into complex, intelligent beings capable of constructing a technological civilisation. It is to these difficult questions that we now turn.
ORIGINS
Earth formed 4.54 +/-0.07 billion years ago out of the flattened disc of dust orbiting our young Sun. The planet was far from hospitable for the first few hundred million years of its life; it was an intensely hot and volcanic world, bombarded by asteroids and comets and, at least once, it collided with another planet, which resulted in the 23.5-degree tilt of our spin axis and the formation of the Moon.
Slowly, the solar system became a more ordered place, and Earth cooled to the point where liquid water could exist on its surface. There is evidence that liquid water existed as far back as 4.4 billion years, but it is certain that our planet was blue by the end of the late heavy bombardment 3.8 billion years ago, and around this time we find the first evidence of life. Structures known as microbially induced sedimentary structures were discovered in 2013 at a remote site in the Pilbara region of Western Australia. They were found in a sedimentary rock layer laid down in the early Archean period, 3.48 billion years ago. Similar structures are found today along ocean shorelines and in rivers and lakes, formed by the interaction of microbial mats with sediments carried through them by water currents. They indicate the presence of a complex microbial ecosystem, most likely a purple layer of slime that thrived in the warm, wet, oxygen-free environment of the early Earth, filling the atmosphere with the sulphurous stench of anaerobic breath. Early Earth would not appear welcoming to our eyes or noses.
Beyond 3.5 billion years, there is indirect evidence for the existence of life as far back as 3.7 billion years. Geologists studying some of the oldest sedimentary rocks on Earth in the Isua Supracrustal Belt in Western Greenland analysed the ratio of carbon isotopes in sedimentary rocks. The ratio of the heavier carbon 13 isotope to the more common carbon 12 can be used as a biomarker, because organisms preferentially use the lighter carbon 12 isotope in metabolic processes. Around 98.9 per cent of naturally occurring carbon is carbon 12, and if the concentration is significantly higher in a particular rock deposit then this is taken as evidence that the carbon was laid down by biological processes.
What can this evidence tell us about the probability of life emerging spontaneously on other worlds? The problem is that Earth is a sample size of one, so it would be erroneous to draw firm conclusions. It is interesting to observe that life emerged very early in the Earth’s history – probably as soon as the conditions were right. The first half a billion years after Earth’s formation is known as the Hadean Eon, named after the Greek god of the underworld. It is likely that the carbon dioxide atmosphere, volcanism and frequent bombardment from space made life impossible on the surface during the Hadean. From the start of the Archean Eon 4 billion years ago, and certainly after the violent period of the solar system’s history known as the Late Heavy Bombardment – which is known from analysis of lunar rocks to have ended 3.8 billion years ago – Earth became a more stable planet, and this date coincides with the earliest evidence for life. It is tempting, therefore, to suggest that life began on Earth pretty much as soon as it could have done after the violence of its formation. If this is taken as a working hypothesis, then we might venture that the probability of life arising on a planet that could support it – the term fl in the Drake Equation – is close to 100 per cent. This is, of course, speculative to say the least, and we would know this number with much greater certainty if we found that life arose independently on Mars, Europa, or one of the many bodies in the solar system that had or still have large bodies of liquid water on or below the surface. This is one of the most important motivations for the exploration of Mars and the moons of the outer solar system.
A BRIEF HISTORY OF LIFE ON EARTH
At this stage in the analysis of the Drake Equation, it’s looking promising for the alien hunters. There are billions of potentially habitable worlds in the Milky Way galaxy, and it is possible to interpret the early emergence of life on Earth as a hint (evidence would be too strong a word) that simple life may be inevitable, given the right conditions. The next term in the equation turns out to be more problematic for the optimist, however. We need to estimate fi, the fraction of planets with life that go on to develop intelligent life, and fc, the fraction of those worlds on which civilisations develop the technology to be contactable. As for the origin of life, the only evidence we have can be found in the history of life on Earth, so let us briefly summarise what we know.
The first population of living things whose ancestors survived to the present day is commonly known as LUCA – the Last Universal Common Ancestor. These four words mean something very specific; because all living things on the planet today share the same basic biochemistry, including DNA, we may assert that all living things are related and share a common origin. Specifically, if you trace your personal lineage back – to your parents, grandparents, great-grandparents and so on – you will find an unbroken line stretching all the way back to LUCA. It is possible that life emerged more than once on Earth, wi
th different biochemistry, but we have no evidence of it. LUCA may have been unrecognisable when compared to today’s life – they may not even have been cellular in nature, but rather a collection of biochemical reactions involving proteins and self-replicating molecules, possibly contained inside rocky chambers around deep-sea hydrothermal vents. They would certainly have been simpler than the earliest known microbial mats, but somewhere in your genome there will be sequences of DNA that have been faithfully passed down across the great sweep of geological time, and if you have children, you’ll pass these four-billion-year-old messages on to them.
Our task is to try to estimate how likely it is that, given enough time, LUCA will evolve into organisms capable of building a civilisation. This is, of course, not precise; no accurate scientific statements can be made with a sample size of one! All we know for sure is that it happened here. The best we can do is trace our lineage back through time and try to identify potential bottlenecks along the way.
Our species, Homo sapiens, emerged around 250,000 years ago in the Great Rift Valley of East Africa. Given that Homo sapiens is the only species to have built a civilisation, the probability of our evolution from earlier hominin species is what we need to know to estimate fc. To summarise, the emergence of Homo sapiens was undoubtedly fortuitous, dependent on many factors including, it appears, the geology of the Rift Valley itself and the details of cyclical changes in the Earth’s orbit. But given enough time and the existence of large numbers of relatively intelligent animals on Earth, it is at least possible to imagine that some other creature may have made the long journey towards civilisation at some point in the future had we not emerged when we did. This is, of course, simply my opinion, and you should make up your own mind after reading further. Incredibly fortunate as we are to exist, therefore, I don’t think the ascent from primates to humans is the most important evolutionary bottleneck in the road to technological civilisation, given the pre-existing biological diversity on Earth and a few tens or hundreds of millions of years of stability into the future. Rather, I think we should direct our attention back over the much longer time periods between the origin of life on Earth and the emergence of the first intelligent animals. We are mammals, which first appeared 225 million years ago in the Triassic era. Dinosaurs also appeared around this time, a subgroup of archosaurs to which birds and crocodiles are related. The first evidence of large numbers of complex animals can be found around 530 million years ago, during a period of rapid biological diversification known as the Cambrian explosion. The earliest fossils of multicellular organisms, known as Ediacaran biota, have been identified as far back as 655 million years. Many of these organisms appear sponge-like or quilted, and nothing like them survives today. There is evidence of animal-like body plans in some Ediacaran fossils, with a clearly differentiated head, but because of their soft bodies fossils are rare and relatively little is known about them. Beyond 655 million years ago, there is no evidence of multicellular life on Earth.
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