Extraterrestrial Worlds: Life Not As We Know It
The Milky Way contains over 100 billion stars, and the whole Universe is made up of more than 100 billion galaxies. Surely there is at least one planet out there, just teeming with life, orbiting around one of those stars? We know that other planets exist in other solar systems. On 6th January 2015, NASA announced the Kepler Space Telescope had discovered its 1,000th exoplanet – a planet orbiting another star in place of our Sun. In fact, as of November 2015, 1,977 planets in 1,257 planetary systems have been found. Nonetheless, of all the worlds discovered to date, only a handful closely resembles the Earth. Instead, they exhibit a truly spectacular diversity, varying immensely in their orbits, sizes and compositions, and have been seen circling a wide variety of stars, including ones significantly smaller and fainter than our Sun. We are starting to see that any kind of world imaginable (within the realms of physics, of course) might be possible somewhere out there, and if that were true then theoretically a huge variety of alien life is possible too. The search for ET often focuses on planets that resemble Earth, the only world known by humans to host life – but does this always need to be the case?
Alien Worlds
Only in the past two decades have astronomers been able to confirm the existence of thousands of worlds orbiting distant stars, and could finally then legitimately question the possibility that some of these exoplanets might be home to extraterrestrial life. However, the conviction and belief that planets outside our Solar System exist goes back long before this. The Catholic monk Giordano Bruno proposed in 1584 the notion of ‘countless suns and countless earths all rotating around their suns’ – he was accused of heresy and burned at the stake. And yet, even in Bruno’s time, the concept of a plurality of worlds was not entirely novel. The scientists and philosophers of ancient Greece also pondered whether other solar systems might exist and whether some would harbour other forms of life. The astronomer Edwin Hubble (1889–1953) used the world’s most powerful telescopes of the day at the observatory on Mount Wilson in California and by 1923 had established that tiny nebulae visible in the heavens were in fact fields of hundreds of billions of stars, located way beyond the Milky Way. Hubble’s observations proved the existence of countless potential worlds out in the darkness of space, and any number of these could be a habitable planet bustling with life.
Too far off from our own planet for direct observation, exoplanets can only be detected through their effects on their host star. Since our Solar System provides an excellent example of a planetary system with life, not surprisingly astronomers began the search for new worlds by examining stars similar to our Sun. Ironically, however, the first genuine discovery of a planet beyond our system came in 1994, when two or three planet-sized objects were found orbiting a pulsar – a dense, rapidly spinning corpse of a supernova explosion – rather than the expected Sun-like star. Although the existence of this small group of planets remains controversial, there is a consensus that these worlds could not support life as we know it, being permanently doused in high-energy radiation. The first discovery of a planet orbiting a star similar to our Sun came in 1995. A Swiss team proclaimed their finding of a new planet at least half the mass of Jupiter, set in a speedy orbit near the star 51 Pegasi. Thus began a surge of exoplanetary discoveries and by the arrival of the twenty-first century, several dozen more worlds had been detected.
Planet Hunters
In the beginning, there was Hubble. This first-generation space telescope, launched in 1990, has provided some of the most breathtaking images ever taken of our cosmos, and has been celebrated as the first to take an image of an exoplanet, Fomalhaut b. Launched in 2003, the Spitzer Space Telescope observes objects in the infrared spectrum and was the first instrument directly to detect light coming from an exoplanet. The data it has collected has revealed the composition, temperature and possible wind patterns on many distant extrasolar worlds. From its launch in 2006, the French CoRoT (Convection, Rotation and planetary Transits) mission, was the first exoplanet-hunting space operation, looking specifically for signs of planets transiting in front of their local star. It was a major contributor to the list of confirmed exoplanets, including some of the best-studied planets beyond our Solar System. CoRoT ceased to function in 2012, and was retired in 2013.
The $600m Kepler mission was launched in March 2009 and its primary mission came to a premature demise in May 2013. Its mission objective to establish how frequently Earth-like planets, in or near the habitable zone of their host star, occur across the Milky Way galaxy was a hunt for Goldilocks planets. It used a specially designed telescope called a photometer (light meter), that continuously records the brightness of stars. To make its discoveries, Kepler targeted a dense field of stars, allowing it to monitor simultaneously and unceasingly 150,000 balls of burning light. As a planet drifts across the face of its star in transit, it blocks a percentage of the light as viewed by the observer, and it is these dips in brightness that enable the planet to be detected. The drops can be miniscule, often around 0.01 per cent for an Earth-sized world, and last between 1–16 hours. The change must occur in a regular sequence to be attributed to a planet orbiting the star. Errors are quite possible at this subtle level, so once a candidate has been identified, Earth-based observatories take over, searching for telltale fingerprints of the host star’s wobble as it responds to the pull of the orbiting planet’s gravity. This is an invaluable double-check and shows that some one in ten of Kepler’s candidates are false alarms. Even though its original mission has ended, it continues to observe the heavens, and scientists and the public alike will be combing through the massive treasure trove of publicly available data for years to come. This is how new exoplanets continue to tumble out of the sky and the number of potentially habitable exoplanets found is still climbing, long after the mission to detect them has ended. Kepler has discovered more than half of all known exoplanets to date, with over 2,000 confirmed and at least another 3,000 unconfirmed candidates.
Owing to its resounding success, new space telescopes capable of finding Earth-sized worlds around nearby stars are being designed to succeed Kepler. At the time of writing, the European Space Agency is scheduled to launch CHEOPS (CHaracterising ExOPlanet Satellite) in 2017, followed by NASA’s launch of TESS (Transiting Exoplanet Survey Satellite) in 2018. By 2024, ESA hopes to have followed CHEOPS with a larger planet-finder dubbed PLATO (PLAnetary Transits and Oscillations of stars). This mission’s objective is to identify and study a large number of extrasolar planetary systems, with the emphasis placed on finding Earth’s twins. If all goes to plan, the European Extremely Large Telescope (E-ELT), currently being built in Chile, will then be able to analyse the atmospheric composition of these newly found planets. By analysing the mix of gases in an atmosphere, E-ELT will be able to determine whether the planet in question is potentially habitable – or even inhabited. In addition to all these missions, the shiny new James Webb Space Telescope (JWST) is a sophisticated new observatory currently under design that is tasked with unlocking some of the greatest mysteries of the Universe, and which could also play a key role in the hunt for alien planets. Sold to the public as a replacement for the Hubble Space Telescope, this $8.8-billion infrared telescope is planned for launch in 2018 and will orbit 1,496,690km (930,000 miles) from Earth, in a region called the Lagrange Point 2. Here, the gravitational forces from the Earth and the Sun essentially cancel one another out, so JWST will be able to maintain a stable orbit while using minimum energy. From this orbital perch, it will be able to stare uninterrupted at stars through its sensitive infrared eyes and allow astronomers to sniff the atmospheres of alien planets to break down their molecular composition.
Habitable Worlds
Everyone wants to find a planet that might have life on it, either now or in the past, whether this is a second Earth or a world with sufficiently similar features and conditions that could make it habitable for life as we know it. An Earth-twin would have an Earth Similarity Index (ESI) of 1.0, and all exoplanets get assigned their own ESI. Astronomers remain committed to the idea that planets and moons with liquid surface water are the best bet for finding life, so the goal is to find a Goldilocks planet or moon. With this in mind, they are searching for one that is neither too hot nor too cold nor too large nor too small, but just right for liquid water. As with the Goldilocks zone of the Solar System, an exoplanet too close to its star would be superheated and any liquid oceans would boil away, while one that is further from its star would be too cold and any oceans would freeze over. But there is also a Goldilocks size to the perfect exoworld. Too small a planet or moon would not be able to hold on to a protective atmosphere, while if it were too huge, it would have an immense atmosphere of hydrogen and helium, making the surface too hot to support life. This holy grail of a perfect world does not need to be a complete replica of the Earth, but should simply enjoy some of its finest features. In fact, the more exoplanetary worlds we see, the more we are starting to consider that the Earth may not even be the gold standard for life. There may conceivably be superhabitable worlds out there that are even better suited than the Earth to support life.
A Periodic Table of Exoplanets divides most of the known candidates into groupings based on mass or dimensions and temperature. Exoplanets in the Hot Zone are too close to their parent star to have liquid water, whereas those in the Warm Habitable Zone are at the right distance for liquid water to be stable, given that they are the right size (from half that of the Earth up to 10 Earth masses). Water will only be in existence as ice for those in the Cold Zone. The Mercurians are low-mass bodies, most likely spherical and lacking an atmosphere, like Mercury and the Moon. The Subterrans are comparable to Mars, Terrans to Earth and Venus, while Superterrans, or Super-Earths, are up to 10 times as massive as Earth, a category with no comparable examples in the Solar System. Neptunians are similar in mass to Neptune and Uranus (are you starting to see a naming pattern yet?), and Jovians are compared to Jupiter and Saturn-sized worlds, or greater. The two largest types of exoplanet are, unsurprisingly, the most commonly detected as they are the easiest to spot. These Hot Jupiters have similar characteristics to Jupiter itself but generally orbit closer to their parent star and so have far hotter temperatures. Giant Neptunes are large gaseous planets, considerably more massive than the Earth but smaller than Jupiter. Both of these types of world are believed to be inhospitable to life.
Searching for Super-Earths
Instead of hunting solely for Earth’s twin (in terms of both the planet and its star), we are fascinated by planets similar to, yet larger than Earth – the so-called Super-Earths. This name denotes the size of the worlds, not their capabilities. These are worlds that feel both familiar and yet completely alien. They may be made of rock and metal, or ice and gas. These planets may have oceans and atmospheres, or contain nothing but hydrogen and helium. The goal, of course, in studying these is to find a rocky Super-Earth located in the Goldilocks zone of its parent star. More than 30 Super-Earths had been discovered up to 2016, with the first found around a pulsar in 1992. The first one discovered around a main-sequence star, the red dwarf Gliese 876, was not observed until 2005. The first discovery of a potentially habitable super-Earth was in 2007, this time around Gliese 581 and on the edge of that star’s Goldilocks zone. In fact, not one but two worlds were discovered, with Gliese 581c getting the most attention. This planet has a mass of at least five Earths and sits on the overly warm side of the Goldilocks zone (conversely, Gliese 581d sits on the cold side). Scientists believe that 581c suffered a runaway greenhouse effect, as is thought to have occurred on Venus.
At the time of writing, the most Earth-like planet yet has been discovered and named the ever-imaginative Kepler-452b. It is the first almost Earth-sized planet to be found in the Goldilocks zone of a star very similar to our Sun. It orbits a star known as Kepler-452, located in the Milky Way and some 1,400 light years away in the direction of the Cygnus constellation. Its size – 1.6 times that of Earth – hints at it being a rocky world that is likely to have an atmosphere, good cloud cover and possibly active volcanoes. This is our best candidate yet for Earth 2.0 and could also be the ideal place to look for evidence of extraterrestrial life. What is most fascinating about it, however, is its age: it is a staggering 1.5 billion years older than our own Earth. This world may give us a glimpse of what awaits our planet in the future. We have also found Kepler-438b orbiting not a red but an orange dwarf star in the constellation of Lyra, which is 470 light years away. It is slightly larger than the Earth and is bathed in 40 per cent more heat from its star than the Earth is from the Sun. Its small dimensions imply that it is a rocky world, and it sits within the Goldilocks zone. Despite being exciting candidates, these worlds will surely not hold the podium finishes for long.
Gliese 832c is not one of the top three potentially habitable exoplanets but this super-Earth is located only 16 light years from the Earth. It is a rocky planet orbiting in the interior of a planetary system thought of as being similar to a miniature Solar System, with a gas giant in the outer reaches. Gliese 832c has a mass 5.4 times that of the Earth and takes 35.68 days to orbit its sun. Given its large mass, it seems likely it would possess a massive atmosphere similar to that of Venus, making it inhospitable for life, although this is not known for sure. Its Goldilocks orbit should allow for liquid water to persist on its surface; the planet’s atmosphere, however, would determine whether any such water were usable by life. The number one question is, ‘Could a super-Earth such as this support life?’ So far, all super-Earths have been found orbiting smaller dwarf stars (yes, this may have something to do with the fact that they are easier to detect), yet to be capable of supporting life, these worlds will need to orbit dangerously closely to their star to maintain the best temperature for liquid water to be stable on their surface. Super-Earths will also have a larger gravity than the Earth. Life inhabiting any oceans would have no problem with this, as the buoyancy of water would balance out the greater gravity of a super-Earth. On land and in the air, however, it would be a different story. We will describe later what kind of life might evolve, should it live on a world with higher gravity and a thicker atmosphere, but the key message is that it is possible!
Revealing Red Dwarfs
The vast majority of exoplanets detected so far lie within planetary systems orbiting red dwarf stars, which are smaller, colder and dimmer balls of light than the Sun. For a world to be habitable in this type of star system, it must hug a tight orbit around its sun to keep it warm enough for liquid water to be stable. Such a close orbit means there is a high probability that the planet would become tidally locked. This means that one face of the planet would always be looking towards the star, bathed in eternal sunshine, and one would always be turned away, in perpetual darkness, similar to the Moon’s experience as it orbits the Earth. This creates a huge temperature dichotomy between the two sides and could produce global gale-force winds. Neither face of these worlds is attractive for life, although there is a sliver of hope. A thin zone could exist, encircling the planet on the boundary between day and night, sitting in the aura of a never-ending twilight that could potentially have a temperature range suitable for life. Sadly, however, intense heating caused by the proximity of planets to their host red dwarfs would nonetheless be a major impediment to life developing in these systems. There would be a very small circumstellar Goldilocks zone owing to low light output from the star, and this light would be shifted into the infrared spectrum, unlike light from our Sun. Finally, if that were not tough enough for life, many red dwarfs earlier in their life are also far more violent and unpredictable than their more stable, larger cousins such as the Sun, in a matter of minutes erupting with flares that double their brightness and send torrents of atmosphere-stripping charged particles towards any unsuspecting nearby planets. You may therefore wonder why we are bothering to look here? First, red dwarf stars are the most common type of star, making up 73 per cent of all those in the Milky Way. Second, they are very long-lived. Red dwarfs could live for trillions of years (in fact, it is thought that no red dwarf has actually died yet) since their nuclear reactions are far slower than those of larger stars, meaning that life would have both longer in which to evolve and to survive.
Exploring Exomoons
As the name implies, an exomoon is a satellite in orbit around an exoplanet or extrasolar body. The number of natural satellites found in our Solar System orbiting terrestrial worlds and gas giants alike suggests that exomoons should be equally common in other planetary systems. Sizeable moons in a stellar Goldilocks zone could quite possibly outnumber planets. When we focus on moons in our Solar System, there are quite a few with many potentially habitable environments through the presence of liquids, heat and nutrients on and within them, such as Europa, Enceladus and Titan – so why could the same not be true in other solar systems? Furthermore, only four worlds in the Solar System other than the Earth show evidence of current tectonic or volcanic activity, and these objects are not planets but moons. Exomoons are, however, extremely difficult to detect and confirm, owing to their size. Observations from missions such as Kepler have hinted at a number of candidates, including some that may one day turn out to be habitats for extraterrestrial life. As yet, no exomoons have been confirmed, but they are sure to lurk within the vast Kepler data sets, awaiting discovery.
Exomoons may prove to be better candidates for life than exoplanets, as moons can have multiple energy sources. While the habitability of terrestrial planets is generally influenced by the levels of sunlight reaching their surfaces, moons also receive reflected starlight from their parent planet as well as thermal emissions from the planet itself. If a moon were to orbit a planet similar to Jupiter, which is quite possible given how many exoplanets have been classified as hot Jupiters and since Jupiter itself has 67 moons, even more energy sources would be available. A planet such as this would still be shrinking and thereby converting gravitational energy into heat, so it would actually emit more heat than it received from the Sun, providing yet more illumination on to its nearby moons. Besides this, moons orbiting close to a gas giant are flexed (affected) by the planet’s gravity, providing potential tidal heating as an internal, geological heat source. The distance of the moon to the planet would also play a role in determining habitability, because the closer they lie together the stronger this tidal heating would be. Too close and it is likely that the moon would suffer from a catastrophic runaway greenhouse effect, boiling away surface water and leaving it uninhabitable. These complex interactions between a planet and its satellite could affect the latter’s climate enough to make it suitable for life, even if the host planet were completely inhospitable. It seems there is even a Goldilocks zone for exomoons around their exoplanets, as well as exoplanets around their star.
A Pale Blue Dot
If there were an alien civilisation on a planet currently orbiting one of the 100 or so nearest Sun-like stars, what would they see, were they to look towards the Earth? Could they tell it is teeming with carbon-based life forms ready to shake their hand/tentacle? And how is the Earth helping us to identify signs of life in nearby planetary systems? If we look at the Earth from over 4 billion kilometres away, it fits within a single pixel as a tiny pale blue dot, but intelligent alien life could learn a great deal from this infinitesimal speck of light. The Earth varies in brightness over time as clouds and continents move across its surface, so extraterrestrials could infer that this little planetary object has weather, water and even rocky continental bodies. This blue spot on the horizon could also reveal to our inquisitive neighbours what kinds of gases are in its atmosphere. Each gas present will either remove or absorb a separate wavelength of starlight, leaving gaps in the complete rainbow spectrum of light. By looking at the different missing pieces from Earth’s spectrum, the aliens could make an educated guess about what comprises Earth’s atmosphere. There is plenty of water vapour in our atmosphere, which would suggest liquid oceans are lapping across the planet and therefore that a key ingredient for life, as hopefully they know it, is present. The atmosphere would also be found to contain an unusually high amount of oxygen. Since oxygen is a highly reactive gas it normally combines with other substances and does not exist on its own for very long – it should not really be in Earth’s atmosphere at all. But plants and photosynthetic bacteria continually produce oxygen so on Earth, at least, there is always a large amount in the atmosphere. We say that oxygen is a biosignature gas: a gas produced by life itself. This would tell the aliens that there is oxygen-producing life, as well as oxygen-consuming life, on the planet.
ET would also be able to see carbon dioxide, methane and other important trace gases in Earth’s air. Methane is composed of one carbon and four hydrogen atoms stuck together, and can be a tiny, but powerful, biosignature gas as the product of life as well as one of life’s most basic energy sources. Most of our methane comes from countless tiny microbes feasting in the depths of Earth’s oceans and swampland as a by-product of their metabolism. It can also, however, be created without any input from life whatsoever. Abiogenic methane arises when volcanically heated water reacts with rocks containing high levels of iron and magnesium. Because of the heating, hydrogen in the water is liberated. This free hydrogen then meets with carbon that has come from carbon dioxide dissolved in the water. The result is methane that has nothing to do with life forms. And yet while not every process that produces methane is generated by life, the overwhelming majority of known sources are alive. Knowing that methane exists in the atmosphere of a planet serves another life-related function as well: it can inform one about the surface temperature of an exoplanet. Methane is one of the most notable greenhouse gases. Like carbon dioxide, atmospheric methane acts as a sort of planetary thermal blanket; it wraps around the Earth and absorbs surface radiation that would otherwise make its way into space. In fact, of the two, methane is a far more efficient warming agent and has at least 20–25 times the global warming potential of carbon dioxide. Fortunately for us, however, methane only persists for a few years after it is produced; otherwise things on Earth would be pretty toasty.
Interestingly, and sadly, alien astronomers would also be able to see evidence of humanity itself and the negative effect it has had on the world. The Earth is surrounded by at least 500,000 pieces of greater-than-marble-sized debris or ‘space junk’ – natural fragments of meteoroids and leftover remnants of now-defunct spacecraft, satellites and launch vehicle stages. Who is to say whether distant observers could spot this orbiting belt of debris or perhaps even glimpse sunlight reflecting off from it? In the atmosphere, they would see evidence of an industrialised civilisation and its pollution. Chlorofluorocarbons (CFCs), in particular tetrafluoromethane (CF4) and trichlorofluoromethane (CCl3F), are the key ingredients in many mass-produced products on Earth, for everything from holding hair in place to eating holes in our ozone layer. These are compounds that can only be produced by advanced industry. Alien searchers might be tempted to shake whatever passes for their heads in sadness, and move on to contemplate other options.
Designer Life
What might life look like on some of these exoplanets, assuming they had sufficient conditions and the time available for complex multicellular plants and animals to arise? If life developed on a world with a lower or higher gravity than that of the Earth, or with no landmass, or it orbited a red dwarf star instead of a Sun such as ours, what adaptations would have developed to enable it to survive? What common traits might we expect to evolve on any planet regardless of the starting conditions? Would we even recognise this life as ‘living’?
The Degrees of ‘Alienness’ …
There may be a number of ways to arrange and assemble organic molecules to create a living being, but it is incredibly hard for us to imagine one better suited to run and support the functions of life than the system of DNA/RNA storage, the operating manual of a cell and proteins. If any life form arising in the Universe were to be built with a similar sugar-phosphate backbone, then these alien beings would essentially be terrestrial and biochemically similar to us. Many scientists also believe the symbiotic evolution of the eukaryotic cell and the building of multicellular organisms to be all but inevitable.
Owing to the distribution of organic material in the Universe and carbon being the third most abundant element in the cosmos, the first degree of alienness is a population based on the same organic building blocks as Earth-based life, such as amino acids and sugars, but assembled in different ways or using different sets. To recap from Chapter 2, all proteins within life on Earth are built from a combination of just 20 amino acids, yet a catalogue of more than 70 different types of amino acid has been found within rocks from space. Most are alien to terrestrial life. Also, most molecules within a cell have a particular chirality, described earlier. As such there are two possible mirror-image versions of a molecule, which are called enantiomers. All terrestrial life uses one enantiomer – biologically-produced amino acids are left-handed and sugars are right-handed. Perhaps on another world life will use amino acids, but right-handed forms instead of left. And as mentioned in detail back in Chapter 2, life on Earth depends on water and carbon. The most alien life we could imagine would not be based on water and maybe not even on carbon.
We are looking for life in the middle range of alienness, put together using the same building blocks as Earth life but arranged slightly differently. Furthermore, its organic chemistry would have been designed (evolved) to help the organism survive the environment within which it existed. We are not looking nor do we expect to find life identical to that on Earth. For a start, it is a very narrow-minded point of view, but also, if it were found, there would be no way of proving it were alien and not simply terrestrial contamination. Even focusing on the possible variants of life as we know it, there turn out to be a number of familiar features we might not be too surprised to see staring back at us …
The Predictability of Evolution
The most difficult question about evolution is ‘Why?’, since this is not posed by evolution itself – it is not working to a game plan or plotted timeline. Instead, the current prevailing conditions represent the only place where evolution occurs, and organisms change in response to fluctuating opportunities or crises imposed by their environment. Evolution is driven by random mutations and natural selection. If an adaptation (or mutation) is useful to a lineage, chances are that it will be preserved and passed on to future generations. Individuals best suited to their particular environment will prevail while those that are not will perish. It is rational to assume, therefore, that an alien organism developing on another world in the distant reaches of our or another galaxy would be subject to the same evolutionary demands as terrestrial biology.
Useful adaptations for life seem to emerge throughout evolution again and again. Evolution is not particularly original but it is innovative, upcycling existing proteins to play new roles and turning previously negative traits into a positive adaptation for survival. This has its limits, however – everything does – and Mother Nature seems to hit upon the same designs over and over again. If it worked well the first time, then why not a second time – Earth’s own ‘If it ain’t broke, why fix it?’ philosophy. We call this convergent evolution – a process whereby species not closely related live in similar ways and/or in similar environments, and by having to face the same environmental challenges are likely to evolve similar traits. An example of convergent evolution is the similar nature of the wings of insects, birds, pterosaurs and bats. All four designs of the wing serve the same function, to enable the organisms to fly, and are similar in structure, yet each evolved completely independently of the other. Convergent evolution is extremely common on Earth. In their own exploration of what life might lie waiting in the Universe, popular British science writers Jack Cohen and Ian Stewart, a reproductive biologist and mathematician respectively, listed the four F’s of universal evolution in terrestrial biology: fur, flight, photosynthesis and … sexual reproduction.
The late evolutionary biologist Stephen Jay Gould, in his 1989 book Wonderful Life: The Burgess Shale and the Nature of History, discussed the Burgess Shale fossils described back in Chapter 4. You may recall that they are a collection of strange alien-style life forms that inhabited the Earth’s oceans about 520 million years ago. Many species from this time in the Cambrian have since died out because they were not fit enough to compete for survival or were in the wrong place at the wrong time during volcanic eruptions, asteroid impacts or other extinction events. Gould theorised that life today would have been very different had history unfurled in another way, that life is a result of the outcomes of past accidents – historical contingencies. Random mutations and chance extinctions would build on each other, driving the evolution of life down one path or another. In Gould’s view, the existence of every animal, including humans, was a rare event that would have been unlikely to recur if the tape of life were rewound to the Cambrian Period and replayed. For example, it is widely believed that the chance asteroid impact 65 million years ago that killed off the dinosaurs allowed mammals to arise and humans to become the dominant species on the Earth. Without this impact, would we even be here?
Life is a lottery of convergence and contingency, a potluck of inevitable adaptations and some lucky chances. The same interactions between convergence and contingency may play out on other planets, with many features of an alien being partially predictable, while others can only be based upon quirks in their evolutionary and environmental history. If extraterrestrial life has faced similar evolutionary pressures as life on Earth, future humans may discover aliens that have convergently evolved to resemble us, and have intelligence similar to ours. On the other hand, if contingent events build on one another and are responsible for driving the development of life down unique paths, as Stephen Jay Gould suggested, then extraterrestrial life may be remarkably strange. This means, however, that in order for us to speculate on what alien life might look like we should pay attention to the convergent adaptations life has created, as the number of lucky breaks is probably infinite. Furthermore, if an alien were evolving in an environment very different to the Earth, then it might have developed a number of unique design solutions to allow for its survival. One of the most important questions in evolutionary biology is what features of organisms are universal and might be expected to re-appear every time life arises or is restarted, regardless of the planet or moon it finds itself on.
A Body and Brain
In popular culture, we predominantly portray ET as humanoid with four limbs, standing upright and with a forward-facing head. Although this probably is not alien enough, we know it is as a result of the budgets placed upon costume departments and that it is more engaging and believable to witness Captain Kirk talking to a human-like alien than a gelatinous blob of goo. However, there is some truth behind the idea. On Earth, birds, reptiles, fish and insects are all constructed with bilateral symmetry. This means the left and right halves of their bodies are reflections of each other. Alien life may quite possibly have chosen the same layout. Yet it could also have a different fundamental plan, such as the radial symmetry of jellyfish, whereby it has no left or right side, only a top and a bottom. Most vertebrates have an entrance and an exit – mostly separate, sometimes shared – and some form of skin to contain all their organs. There is also a common need for lungs or gills in creatures, once they reach a certain size, in order to be able to access oxygen. And before you ask, there is good reason to suppose that alien life would enjoy a deep breath of oxygen as much as we do. Burning organic carbon-based fuels, such as glucose in oxygen, provides greater levels of energy – enough to satisfy the enormous power demands of animal life.
Larger animals colonising the land would also require some kind of support or scaffolding to hold their body together against the force of gravity, as well as a frame against which muscles can push. They will therefore most likely have a skeleton that can either be an ‘innie’ like ours or an ‘outtie’ like that of crustaceans. But what about limbs? These are obviously needed in some form for grasping objects and in most animals for movement, but how many would an alien have? Humans and all land vertebrates have a four-limbed body plan (two arms and two legs), as a result of contingent evolution – our fishy ancestors by chance had two pairs of lobed fins, and so we have four limbs. An alien ancestor could just as easily have had three pairs of limbs, creating hexapod descendants along the lines of terrestrial insects, and maybe even have adapted these limbs so that the front pair are no longer used as legs but as arms or claws similar to those of crabs on Earth. Six legs would be a very fortunate adaptation for life on a world with stronger gravity, supporting the movement of much heavier, although not necessarily larger, life forms. At the end of our limbs, humans have 10 fingers and 10 toes, but we could balance or grasp objects just as well with four or six digits, just as long as we kept our opposable thumbs. Who knows how many fingers an alien might have – the point is that the chances are they will have them.
The Face of Life
The development of the eye is seen as a universal evolutionary feature. It makes good design sense to have a way of seeing where you are going and, to that end, placing the eyes at the front of a head. It is clever to put the best surveying organ available close to the brain and for it to be protected by a hard shell (the head). The human eye is an exquisitely complicated organ that acts in a similar way to a camera, collecting and focusing light and converting it into an electrical signal that the brain translates into images. Instead of photographic film, it has a highly specialised retina that detects light and processes the signals using dozens of different types of neuron. Humans and most other vertebrates, and cephalopods (which includes octopuses, cuttlefish and squid) have camera-type eyes, which took shape in fewer than 100 million years. Eyes first evolved during the Cambrian Explosion from light-sensitive proteins based in a single illuminated spot, and were used to monitor circadian (daily) and seasonal rhythms. They then evolved into light-sensitive pits, into compound insect eyes and finally our optically and neurologically sophisticated eyeballs by 500 million years ago.
Complex, image-forming eyes have evolved independently some 50 to 100 times, which is not particularly surprising, given that any model or method allowing organisms to see and rapidly focus on an image would confer an enormous evolutionary advantage for survival, regardless of whether they are based on land, air or sea. Many genes are responsible for making the eye. One holds instructions for making light-sensitive pigments, another provides information for making the lens, and other genes orchestrate it all, directing various parts when and to where they need to be assembled. These are called master control genes, and for eyes the most important one is Pax-6. The ancestral Pax-6 gene controlled the formation of the first very simple eye and still controls today’s most complex incarnations. Given that a variety of eye-type structures has arisen independently numerous times, it seems highly likely that an alien life form will have eyes. To be at the pinnacle of evolutionary adaptation, it would quite possibly have an eye similar to that of an octopus – one step better than the eye of a human as it lacks the blind spot created where the optic nerve leaves the eyeball.
Sensory organs give animals an acute awareness of changes in the environment around them and throughout their bodies, so that they can physically respond. A very satisfactory evolutionary adaptation, these touchy-feely organs enable animals to avoid hostile environments, sense the presence of predators and hunt down sources of food. Animals can perceive a wide range of stimuli that includes touch, pressure, pain, temperature, chemicals, light, sound, movement and position of the body. Some animals can also sense electric and magnetic fields. The special senses of smell, taste, sight, hearing and balance are particularly useful, and the organs used for these relatively complex. Life forms need to process this sensory information as fast as possible so most have a centralised nervous system, and to reduce data transfer times nature houses this near to the organs. It seems, therefore, that the development of a head at the front of the body may prove to be universally essential among higher animals, although the positioning not so much.
Skin and Bones
Human skin colour ranges from the darkest brown to the lightest pinkish-white, even yellowish, hues. Human skin pigmentation is the result of natural selection; it evolved primarily to regulate the amount of ultraviolet radiation penetrating the skin, controlling its biochemical effects. There is a direct correlation between the geographic distribution of UV radiation (UVR) and the distribution of indigenous melanin skin pigmentation around the world. Areas that receive higher amounts of UVR, generally located closer to the Equator, tend to have darker-skinned populations. Areas far from the Tropics and closer to the poles have a lower intensity of UVR, which is reflected in lighter-skinned populations.
Alien life would not, however, necessarily follow the same human melanin spectrum. It might more closely resemble that of terrestrial plants than Earth’s vertebrates. Might aliens live up to their fictional reputation and be a greyish green? This colouration of particular life forms on Earth is a consequence of two things: in plants it comes from the photosynthetic pigment chlorophyll; and in animals from the need to be camouflaged and remain hidden within our planet’s vast vegetation. Some biologists believe that chlorophyll may be a universal molecule, used by any number of organisms to soak up the light of their parent star. However, the wavelength of light emitted by a star varies, depending upon its temperature. A star cooler than the Sun, such as a red dwarf, will shine more in the infrared than in the visible portion of the electromagnetic spectrum, so any photosynthetic pigment would need to be tuned to absorb this differing wavelength of light. It might well be perfectly camouflaged to its own vegetation but to our eyes would not look green. If an alien did appear green to us, then it is a pretty fair bet that its sun would be very similar to ours. An alien might also appear green if it used chlorophyll to provide the organism with a source of nutrients. Unfortunately, although perfectly adequate for the energy budgets of plants, photosynthesis cannot provide nearly enough energy to meet an animal’s high demands, especially powering muscles and a brain. Complex life must be carnivorous, devouring the nutrients contained within plants and other animals (which themselves may feed on photosynthetic plants) to draw the energy it urgently needs. There is nothing to say, however, that a carnivorous life form could not have a chlorophyll-rich skin for times of famine or for use as a top-up source of energy and nutrients for survival in harsh or seasonally challenging environments.
An internal circulatory system is needed in any life form to move nutrients effectively to where they are needed, as well as to remove waste products. For terrestrial life, rich red blood is the main mechanism, although in actual fact blood can be a number of colours, including colourless, as a result of the specific chemicals it carries. Humans and most other vertebrates use the iron-containing protein haemoglobin, and so have red blood. Haemoglobin is a respiratory pigment and plays a vital role in the body, ferrying oxygen to cells and helping waste carbon dioxide return to the lungs where it can be exhaled. When it is oxygenated, i.e. full of freshly breathed-in oxygen, it is bright red in contrast to when it is deoxygenated, when it is a deep, dark red. It is a commonly held myth that deoxygenated blood is blue – after all, if you look through your skin at any of your veins carrying deoxygenated blood away from your body’s cells, they have a definite blue-grey hue. However, this appearance is in fact caused by the interaction of light with both the blood and the skin and tissue covering the veins. There are some creatures, however, for whom blue blood is the norm. Unlike haemoglobin, which is bound to red blood cells, haemocyanin can float freely in blood and contains copper instead of iron. When deoxygenated it is colourless, but blue when carrying oxygen. This is the shade of blood found in spiders, crustaceans and some molluscs, octopuses and squid. Green blood also exists in some segmented worms, leeches and sea cucumbers, which contain chlorocruorin or a mixture of haemoglobin and chlorocruorin. Finally, violet blood has been found within marine worms and brachiopods as it contains haemerythrin as the oxygen transporter. What is perhaps most interesting about the varying colours of blood is that it showcases evolution coming up with different solutions to the same problem – in this case, how to transport oxygen.
One adaptation I consider particularly useful, and could easily imagine any alien life form adopting, is being streamlined. It turns out that there are not many ways of remaining streamlined while pushing yourself through the water. On Earth, salmon, whales, penguins and water boatmen all come from very different lineages and yet have independently converged on the same body plan of a sleek bullet-like shape helped by fins and flippers. There is no reason why this may not have occurred elsewhere as well. Perhaps on another world, jet propulsion would be a more dominant mode of movement. Terrestrial squid contract an outer cavity to move backwards in pulses and it has been speculated that animals larger than sharks on another world could also propel themselves this way by contracting water or air through a hollow tube running the inside length of their bodies. The need for an aerodynamic body is not just good for swimming, but for the one thing we all wish we could do – fly.
A Flight of Fancy
The evolution of flight might be expected or even inevitable on any terrestrial planet. But before there was to be flight, life needed wings. The way in which these evolved on the Earth is not precisely known although some theories perceive an evolutionary step from arms used by bipedal animals to leap into the air to capture small prey, which evolved into large wings to assist in said leaping. Perhaps wings arose from gliding ancestors who began to flap their gliding structures in order to produce thrust so as to move faster and further. We know that flight evolved millions of years ago in all of the groups that are capable of flight today and the reasons differ depending on the species in question. These range from helping them to escape from predators or catch flying or speedy prey, aiding movement from place to place (leaping or gliding), freeing the hind legs for use as weapons, or gaining access to new food sources or an unoccupied niche.
Gliding – a controlled descent using gravity as the driving force – has evolved many times and in many different groups including frogs, lizards, snakes and several different mammals, and even the seeds of some plants. Gliding is also known among some fossil reptiles. On the other hand, active flight, during which flapping powers an organism through the air, has only evolved four times in nature: once in arthropods and three times in vertebrates. It has only become extinct once … in the pterosaurs. The first animals to evolve flight were insects some 410 million years ago, and the pterosaurs were the first vertebrates to evolve active flapping flight, although the origin of this adaptation is something of a mystery. The transformation from non-flying, perhaps gliding, animal to fully-flying pterosaur probably occurred in the forests of the Middle Triassic. Unfortunately, this environment rarely yields fossils, so the search for the oldest or even original pterosaur may be in vain. Shortly after the Cretaceous, bats appeared – the first active flying mammals – and about 50 million years after that another group of vertebrates achieved the ability to fly. Instead of evolving wings directly, this group used its ingenious brain to skip that step and build machines to fly for them: this was our species.
On Earth, the energetics of staying airborne impose stringent limits on the size of flying animals, but this may not be the case on planets and moons with different gravities and thicknesses of atmosphere. Planets smaller than the Earth have a lower gravity and so tend only to hold on to a thin atmosphere. This would mean that flapping wings would generate less lift and flight would be rather difficult, even though the physical pull of gravity holding the organism to the ground would be reduced. Wings with very large surface areas and spans of up to 75cm (2.5ft), similar to those of prehistoric dragonflies, might help. Flight would actually be easier on a larger world with a stronger gravity, as its pull on the atmosphere would be stronger, and the air would be denser, maintaining a thicker, more flight-friendly environment. Animals as massive as elephants would be able to glide through the air and sky-whales could dominate the clouds, carried on thermals and slowly flapping their immense wingspans.
If I had to design a flying alien species, I would give them wings modelled on those of a bat. Bat-style wings have evolved several times in mammals. They use up less energy during flight as a result of flexible skin membranes and are held together by more than 24 joints. As a matter of fact, bats are operating with the same skeletal structure as humans. Every joint in the human hand is found in a bat’s wing, and indeed a couple more. The wings provide more lift and less drag, thereby increasing manoeuvrability.
Lighting up Life
Bioluminescence is the production and emission of light by a living organism. It is a form of chemiluminescence, whereby light energy (luminescence) is released as the result of a chemical reaction. Bioluminescence occurs widely among animals, particularly in the open sea, including jellyfish, comb jellies, crustaceans and cephalopod molluscs. This characteristic is also present in microorganisms including some bacteria and fungi, and terrestrial invertebrates such as insects. The most famous, perhaps, are the dinoflagellates frosting breaking waves in a blanket of phosphorescence and weaving light ribbons across the sea, while on land fireflies are amazing to watch as they dance around on a warm summer’s evening. Fireflies actually provide so much light that they were once carried underground in jars to provide light for miners working deep in the bowels of the Earth.
Bioluminescence can serve several functions depending upon the needs of the organism using it, which can range from counter-illumination camouflage, in which the animal matches the overhead environmental light as seen from below (such as in the oceans), to attraction of a mate or, conversely, attraction of prey, snaring food in glowing threads and webs. Furthermore, it can be used in defence and in warnings, for communication, mimicry and, of course, simple illumination.
In some animals, the light is not their own, coming instead from symbiotic organisms such as Vibrio bacteria. Other organisms on Earth contain a light-emitting pigment called luciferin and the enzyme luciferase, which reacts with oxygen to create light both within and outside cells. In evolution, luciferins generally vary very little, with one in particular, coelenterazine, present in the light-emitting pigment of nine ancient groups of organisms. Not all manufacture coelenterazine themselves – some obtain it through their diet. Overall, bioluminescence has arisen over 40 times in evolutionary history and is found within at least 70 genera of squid. Most marine light emission is in the blue and green light spectrum. However, some loose-jawed fish emit red and infrared light and the genus Tomopteris emits yellow light.
An Alien Greenhouse
Unsurprisingly, convergent evolution is also common in the plant world and can give a number of clues as to what vegetation might arise on an exoplanet or moon. Plants, especially those on land, need to satisfy four fundamental constraints to survive. They must be able to catch as much light as possible from their Sun to allow for photosynthesis to work and they must be able to disperse pollen or seeds as far as possible to ensure the survival of their species. They also need to ensure mechanical stability so as not to topple over and must have ways of retaining, or not losing too much, water. Depending upon the demands of the local alien environment, plants might meet these requirements in a number of ways. Slow-growing vegetation arising on a world with a low availability of light would be likely to develop wide, flat canopies to maximise the amount of light intercepted (although this could cause a stability problem, increasing the risk of being uprooted in strong winds); while those on water-poor worlds might evolve in a similar way to squat cacti with highly modified leaves, such as spines. As well as defending against hungry herbivores, spines help prevent water loss by reducing airflow close to the cactus and providing some shade. As such, the shapes of trees and plants in an alien forest are likely to resemble those found in similar environments on the Earth and might be recognisable to us. On a planet with high wind speeds, mechanical stability would be of paramount importance and trees may resemble terrestrial firs, or seaweed with its submissively flexible stem. Worlds with a higher gravity might have low-lying trees with stout trunks and fewer branches. Potentially on these worlds, plants might evolve novel ways to reach the sunlight, reproduce and access water. Perhaps instead of desperately trying to grow towards the sky, alien plants would simply float upwards. Photosynthetic plants on Earth use the hydrogen produced by the splitting of water molecules to generate food, and release oxygen as a waste product. If an alien plant released this hydrogen inside an inflatable sac, it might float into the sky like an airship to find what it needed, anchoring itself to the ground with a vine. Going one step further, this vine might have the ability to detach during reproduction to allow the plant to be carried in the wind and disperse its seeds across vast distances. This adaptation is found in the seas on Earth when kelp forests release seeds contained in small flotation bladders, pumped full of oxygen or carbon dioxide.
We would hope to recognise the shapes and even life strategies of a plant or tree on another world, but would we recognise its colour? Would an alien biosphere be as green as the vast majority of healthy vegetated areas on Earth? If alien plants were found on a world orbiting a Sun-like star and use chlorophyll as a pigment, then quite possibly yes. However, even the Earth hosts a variety of organisms other than green plants that photosynthesise other than green plants. On land, plants may also display foliage that is red, yellow, orange, cream, purple or demonstrate a variety of other effects, while underwater algae and photosynthetic bacteria are also found in a wide palette of shades. The dominant colouration of extraterrestrial foliage will depend on how alien photosynthesis evolves in response to the spectrum of light received from its parent star, combined with the filtering effects of the planet’s or moon’s atmosphere (which may be very different to our own) and, for aquatic creatures, of liquid water (or the liquid they are residing in). In general, plants on Earth use the broad spectrum of visible light (red–orange–yellow–green–blue–indigo–violet) from the Sun, profiting most from the blue-green range. The Sun transmits predominantly red photons (particles of energy from light or other electromagnetic radiation), useful for their quantity, though lower in quality than blue photons, which supply more energy. Green photons in between are lacking both in such energy and numbers, so vegetation on Earth has evolved to screen much of these and thus reflect green.
As always, the answer to the puzzle of what vegetation may be found on other worlds lies once more in the stars. Astronomers grade stars according to their colour, which is related to their temperature, size and longevity. It is clear that only certain types of star are long-lived enough to allow the evolution of complex life to occur. From hottest to coldest these are the F-, G-, K- and M-class stars (our sun falls into the G category). F stars are larger, burn more brightly and bluer, and exhaust their fuel over a couple of billion years. K and M stars are smaller, dimmer, more red in colour and survive longer. We know that light of any colour from deep violet through to near-infrared could power photosynthesis. For another Earth-type planet orbiting around a G-class star, we can comfortably predict green, yellow or orange plants. Around stars that are hotter and bluer than our Sun, the abundance of blue photons would be so overwhelming that plants might need to shield themselves against it, using a pigment similar to anthocyanin, so that they would reflect blue. Since, however, they would still try to absorb a percentage of the energetic blue light, they would actually appear green to yellow to red. The span of M-star temperatures would make possible a wide spectrum of colours in alien plant life. A planet around a cooler M star, such as a red dwarf, would receive about half of the energy that Earth gets from the Sun, with less visible light and a spectrum closer to the near-infrared range. With less energy available, plants might try to absorb as much as possible, and with little light left over to reflect, appear black.
Once we have observed and hopefully recognised life on another world, be it animal, plant, microbe or maybe even mineral, what do we do next? Why … we try and say hello, of course.