Human Universe (14 page)

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.

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 f
_l
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.

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 f
_i
, the fraction of planets with life that go on to develop intelligent life, and f
_c
, 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, with 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 f
_c
. 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.