With more sophisticated telescopes, we would like to determine an Earth-like exoplanet’s surface temperature and whether it has liquid water. A spectrum might provide that, but you might only be able to see light from cloud tops or the upper atmosphere. In fact, Venus is veiled in this way. For exoplanets that are similarly obscured, we’ll need to infer the amount of greenhouse gases from absorption lines in spectra and calculate a surface temperature. Together with an exoplanet’s atmospheric composition, you could then infer whether liquid water is present. It might also be possible to look for an ocean through glint, which is the bright reflection spot on a smooth body of water at glancing angles. Finally, we should keep an open mind about anti-biosignatures. Microbial life will readily eat hydrogen or carbon monoxide gas, so an abundance of either of these might be considered an anti-biosignature for a planet in the habitable zone. Basically, anti-biosignatures say ‘no one home’.
The search for extraterrestrial intelligence (SETI)
A different approach to finding life on exoplanets is to hypothesize that technological civilizations exist. If they do, we can look for their communications. This endeavour is the search for extraterrestrial intelligence (SETI) or more generally the search for technosignatures.
In 1959, Guiseppe Cocconi and Philip Morrison advocated looking for broadcasts of extraterrestrial civilizations using large radio telescopes. Others have since suggested watching for transmissions in visible light. There are practical reasons to look only for these signals. For example, X-rays are absorbed in the upper atmosphere, whereas radio and visible light are easy to generate and detectable across space. The main question for SETI is whether it’s worth looking. Are there enough civilizations out there?
In 1961, Frank Drake (then at Cornell University) described a way to evaluate the potential number of transmitting civilizations in our galaxy. If we estimate the average number of civilizations that come ‘on air’ each year and their average lifetime, we can multiply those two quantities together to give the current number of transmitting civilizations. To give an analogy, if the number of new freshmen starting at a university is around 6,000 per year and they spend an average of four years on campus, the total undergraduate population at any one time is 6,000 × 4 = 24,000. Drake went further by devising six factors that determine the number of ‘freshman’ transmitting civilizations appearing per year. In full, we calculate N, the number of communicating civilizations, by multiplying the six numbers and the average lifetime, as follows:
N civilizations =
This is the famous Drake Equation. The first number R, is the birth rate of stars suitable for hosting life. Astronomers observe about ten new stars per year of types G, K, and M. All the other factors are as follows:
fplanet is the fraction of such stars having planets;
nhabitable is the average number of planets per planetary system that are habitable;
flife is the fraction of those planets on which life originated and evolved;
fintelligence is the fraction of inhabited worlds that developed intelligent life;
fcivilizations is the fraction of those worlds that developed civilizations capable of interstellar communication;
and L is the lifetime of those communicating civilizations.
Several factors in the equation are unknown. But, what the heck? Let’s use the Drake Equation anyway. Current exoplanet searches suggest that at least two-thirds of all stars have planets, so let’s take fPlanet as 2/3. Data from the Kepler mission are still being analysed, but suggest at least that 1 in 100 of planets are habitable, so let’s set nhabitable = 1/100. We will simply have to guess all other parameters. Life developed rapidly on Earth, so let’s presume that life originates on half of the habitable planets, i.e. flife = 1/2. Let’s suppose that the fraction of biospheres that develop intelligence is fintelligence = 1/8. Let’s also guess that one in ten intelligent biospheres develop civilizations capable of interstellar transmissions, so fcivilizations is 1/10. Finally, the lifetime, L, of communicating civilizations is sociological speculation, but let’s say 10,000 years. So how many communicating civilizations in the Milky Way do we get? The answer is four, from 10 × (2/3) × (1/100) × (1/2) × (1/8) × (1/10) × 10,000. Fine. But can we constrain the probabilities better?
Exoplanet studies will eventually nail down the second and third terms in the Drake Equation and might have something to say about the probability of life arising, flife, if biosignatures are detected. Optimists think that a planet with liquid water and the right materials develops life easily. At present, we don’t really know.
The next term concerns intelligence, so how probable is that? A relevant issue might be that only some biological solutions work to solve specific problems. In zoology, we often find the same trait shared by organisms in unrelated lineages, a result of what is called convergent evolution. Convergence occurs for specific functions and ecological niches. For seeing, eyes have evolved in at least forty different animal groups. A dog’s life, of all things, is an example of a convergent niche. The Tasmanian wolf (a marsupial) evolved dog-like traits similar to the Mexican wolf (a placental mammal). Evolutionary convergence is so common that it means that organisms with legs, pairs of eyes, and certain ecologies might be inevitable because only these are physical solutions to the problems of walking, stereo vision, and occupying particular niches.
Whether technological intelligence is unique or convergent is contentious. On Earth, it was slow to appear. It took four billion years to build up sufficient O2 to allow animal life. Then another few hundred million years passed before technological life. Dinosaurs reigned for about 170 million years and yet we find no sign of technology, neither fossil tools nor dinosaur microwave ovens. On the other hand, there are some lineages where brains appear to have had value and grown. Brain mass itself is a poor measure of intelligence because a big body often needs a big brain to run it. Instead, the Encephalization Quotient (EQ) is used, which is the ratio of the brain mass of an animal to the brain mass of an average animal of the same body mass. Thus, an average animal has an EQ of 1. Scores higher than 1 are brainy and scores below 1 more brainless (Fig. 11). Humans and chimps have EQs of about 7.4 and 2.5, respectively, which means that they have larger brains than expected by these factors. A rabbit has an EQ of 0.4, which would please Elmer Fudd. As Aristotle noted, ‘man has the largest brain in proportion to his size’.
11. Brain and body mass for some different mammals. Animals that plot to the left of the diagonal line have more brain mass than typical
Palaeontologists have discovered an increase in EQ in certain lineages over time, most notably the genus Homo. The EQ of Homo habilis, a hominin that lived about 2 Ma, was only 4, for example. In toothed whales (dolphins, sperm whales, and orcas), EQ increased around 35 Ma, when they developed echolocation to find fish and friends. ‘Friends’ may be the key. Generally, intelligence is larger in social animals, such as 5.3 for a dolphin. Intelligence probably aids survival and getting a mate, which, in turn, promotes more offspring. However, there’s much debate about which evolutionary pressures are behind intelligence.
A final question about SETI is called the Fermi Paradox, and was conceived by Enrico Fermi (1901–54), the Nobel Prize-winning physicist who built the world’s first nuclear reactor in 1942. His idea arose from a lunchtime conversation. Stars in the Milky Way disc date from nine billion years ago, whereas the Earth is only 4.5 billion years old. Thus, if intelligent life is common, many technological civilizations should have arisen long before us.
Assuming that they developed space travel (or self-replicating robots to do the space travel for them), they should have spread throughout the galaxy by now. ‘Where are they?’ Fermi asked.
Fermi’s Paradox has three solutions. One is that we’re alone in the Milky Way because one factor in the Drake Equation is vanishingly small. A second is that Fermi’s premise is incorrect. For all manner of reasons, civilizations might not go around colonizing the galaxy. Finally, th
ere are civilizations but they’re hiding their existence from us (or most of us). Scientists particularly dislike the third option because it’s an ad hoc hypothesis that’s untestable, but it’s beloved of science fiction writers, tabloid newspapers, and (according to polls) one-third of American adults who believe that extraterrestrials have visited. The reasonable options are the first two. Obviously, all efforts to find life elsewhere bear upon the question of whether we’re alone and the first solution. If SETI succeeds, we might also gain insight into other civilizations, if we could understand their signals. At the moment, the only known life is here, so, like Fermi, we can only wonder, ‘Where are they?’
Chapter 8
Controversies and prospects
The Rare Earth Hypothesis
The big, unanswered questions of astrobiology generate controversy. One debate surfaced in 2000 when Peter Ward and Don Brownlee, my colleagues at the University of Washington in Seattle, published a bestselling book, Rare Earth. In essence, their Rare Earth Hypothesis is that the fortuitous circumstances that have allowed complex life on the Earth are so uncommon that Earth might harbour the only intelligent life in the Milky Way. Amongst their arguments were the good fortune of Earth being in the right place in the galaxy, having Jupiter in our Solar System to capture comets that might otherwise collide with the Earth, Earth’s unusual recycling of volatiles by plate tectonics to keep the atmosphere going, the contingencies in obtaining an oxygen-rich atmosphere, and the luck of having a large Moon that stabilizes the Earth’s axial tilt and so its climate.
Rare Earth was a polemic that railed against the Copernican Principle. The latter idea (named after Nicolas Copernicus, whose Sun-centred system knocked the Earth from its perceived place as the centre of the universe) holds that there’s nothing special about our location. Astronomers note several factors in its favour. First, the Earth is surely one of many rocky planets in the universe. Furthermore, our Sun, a G-type star, is not special because around one in ten stars are G type. We also live in a humdrum location in the galaxy, along one of many spiral arms. Finally, our galaxy is unremarkable among many in the observable universe. As Stephen Hawking has put it, ‘The human race is just a chemical scum on a moderate-sized planet, orbiting around a very average star in the outer suburb of one among a hundred billion galaxies.’
Advocates of the Copernican Principle also note how the great advances of science highlight the folly of assuming that we’re special. We need think only of Galileo’s The Starry Messenger (1610), which reported his observations of moons of Jupiter and the surface of the Moon, showing that these bodies were not heavenly perfections as supposed by theologian philosophers but explained by the same physics that we have on Earth. Similarly, Darwin’s Origin of Species (1859) overturned the conceit that humans exist outside the rest of biology.
The problem with the Rare Earth Hypothesis is that it assumes too much knowledge about habitability, whereas, in reality, much is uncertain. Recently, for example, it’s been discovered that the wandering of stars means that the galactic habitable zone is a less concrete concept than previously thought. The results of NASA’s Kepler mission also show that planets around other stars are common, including plenty of Jupiter-size bodies. The argument that having a Jupiter-sized planet in just the right place lowers the rate at which comets or asteroids hit the Earth has also been challenged. Certainly, Jupiter mops up some impactors like a cosmic vacuum cleaner. In 1994, the comet Shoemaker–Levy 9 smashed into Jupiter, while in 2009 and 2010, the scars of further impacts were seen on Jupiter. It’s well established that Jupiter helps to deflect and eject comets that come from a halo of such icy objects, the Oort Cloud, which surrounds the Solar System at about 50,000 AU distance. However, near-Earth asteroids and comets from within the plane of the Solar System represent more than 75 per cent of Earth’s impact threat and Jupiter can actually destabilize those objects. So Jupiter is two-faced. Some calculations suggest that Jupiter is actually a net foe rather than friend.
Other arguments for Rare Earth are also ambiguous. In the Solar System, Earth uniquely has plate tectonics. However, for plate tectonics, a planet must be big enough to have adequate internal heat to drive the plate motion and it probably needs seawater to cool oceanic plates and lubricate their movement. In the Solar System, only the Earth qualifies. But that doesn’t mean that Earth-like exoplanets in habitable zones might not also be suitable for similar tectonics.
While the Rare Earth Hypothesis is correct that planets without a large moon will suffer larger axial tilt variations than Earth, climatic variations at low latitudes might be benign. In fact, thicker atmospheres, more extensive oceans, and lower rotation rates of an exoplanet can smooth the climatic differences between pole and tropics caused by a varying tilt. Finally, the question of oxygen not accumulating on other Earth-like planets might go in the other direction to that assumed in the Rare Earth Hypothesis. Some planets might be more favourable for oxygen-rich atmospheres than Earth because their volcanoes pump out a smaller proportion of gases that react with oxygen. Oxygen might build up more easily.
What does seem to be correct about the Rare Earth Hypothesis is that microbial-like life should be much more common than intelligent life. Microbes have a remarkable range of metabolisms and can live in a far wider variety of environments than complex organisms. But any definitive statements about the prevalence of complex life—one way or the other—simply lack data to support them and we should be sceptical. As Carl Sagan famously remarked, ‘It pays to keep an open mind, but not so open that your brains fall out.’
Prospects for astrobiology and finding life elsewhere
The excitement of astrobiology is that it tries to answer questions such as the origin of life and whether we’re alone in the universe. With advances in technology, it’s increasingly likely that major discoveries will be made in the coming decades.
In the area of understanding early life, it’s likely that realistic self-replicating genomes will be made in the lab. This would provide great insights into the origin of life. Several groups are studying the RNA World or its variants. There are also projects to drill deeply into old sedimentary rocks in South Africa and Australia, which will surely make new discoveries about the earliest life and environment.
In the Solar System, Enceladus ought to be one of the highest priorities for the world’s space agencies. Enceladus has a source of energy (tidal heating), organic material, and liquid water. That’s a textbook-like list of those properties needed for life. Moreover, nature has provided astrobiologists with the ultimate free lunch: jets that spurt Enceladus’s organic material into space. Technology certainly exists to build a spacecraft to swing by Enceladus and sample the organics in the jets. Better still, the material could be returned to Earth for analysis.
In fact, spacecraft to collect extraterrestrial samples and return them to Earth, which are sample return missions, are the future in understanding the history of Mars and Venus and whether either of these planets was once inhabited. A sample return mission for Venus is in the distant future, but one for Mars is a strategic goal of NASA’s and ESA’s current programmes.
Around Jupiter, Europa is probably the best prospect for life. The first step would be a Europa orbiter to study the moon in detail and determine the thickness of the ice above a subsurface ocean or lake lenses. The next steps might involve landers, and possibly robots to melt through the ice using radioactive heat generators. Eventually one imagines submarines diving though a Europan ocean.
Apart from Enceladus, Titan is a target of astrobiology amongst Saturn’s moons. A huge scientific leap could be made if a lake lander—a sort of interplanetary boat—could float on the lakes in the polar regions of Titan and find out which substances make up the organic liquids. Furthermore, a Titan orbiter could do the kind of reconnaissance that would determine the depth of Titan’s subsurface ocean and study Titan’s surface.
One certainty for the future is that exoplanet discoveries will continue
to spur an interest in astrobiology. I anticipate the discovery of many Earth-like planets inside the habitable zone of other stars, dead planets with almost pure carbon dioxide atmospheres, water worlds covered entirely in glinting oceans, and young Venus-like planets sweating off their oceans into space from runaway greenhouse effects.
When astrobiology came to the fore as a discipline in the 1990s, some questioned its future and wondered if it might be a fad that fades, perhaps because of disappointment in not quickly finding extraterrestrial life or a failure to answer questions about life’s origin. However, the discovery of Earth-sized exoplanets in habitable zones will ensure that the possibility of life elsewhere becomes more relevant than ever. Astrobiology is here to stay.
Further reading
Chapter 1: What is astrobiology?
Much lengthier introductions to astrobiology are found in the following textbooks:
J. O. Bennett, G. S. Shostak. Life in the Universe. (San Francisco: Pearson Addison-Wesley, 2012).
D. A. Rothery et al. An Introduction to Astrobiology. (Cambridge: Cambridge University Press, 2011).
K. W. Plaxco, M. Gross. Astrobiology: A Brief Introduction. (Baltimore: Johns Hopkins University Press, 2011).
J. I. Lunine. Astrobiology: A Multidisciplinary Approach. (San Franciso: Pearson Addison Wesley, 2005).
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