Are We Alone in the Universe? The question is almost as old as humanity itself. For centuries, scientists, philosophers and regular people have gazed at the star-pricked night sky and wondered whether there could possibly be others like us in that great expanse. The question has evoked an entire genre of books, movies and entertainment and is one of the most fundamental questions upon which powerful institutions like NASA are founded. Each year, millions of dollars are spent researching and seeking answers to this question, and hundreds of scientists worldwide collaborate to find evidence of extra-terrestrial life. The search spans all branches of science, unifying Physics, Chemistry, Biology, Earth Sciences, Maths and Engineering in the mammoth effort to find out if there is anybody out there. As a species, we research and wonder and guess and hope to find an answer, to find someone else with whom we can share that great emptiness we call space. But so far, it seems our labours have gone unrewarded. The earth is but one small, rocky planet, orbiting a medium, insignificant star in a non-descript arm of a regular galaxy in a normal galaxy cluster. It seems absurd to suggest that this is the home of all the intelligent life in the entire universe. The universe is so much larger than we can even imagine, and to think we are the only representation of life in that vast domain makes for a very lonely existence. Our best estimates suggest that the current observable universe is 93 billion light years in diameter (or about 880,000,000,000,000,000,000,000,000 meters (8.8Ă—10 26 m)) and contains about 100 billion galaxies, each containing on average 250 billion stars. Data from our own galaxy enables us to assume that each star has an average of at least two planets orbiting it, giving a conservative approximation of 5Ă—10 22 planets in the observable universe. To put that into context, there are approximately
7.5×10 18 grains of sand on Earth, about 7000 times less than the number of planets in the universe. This demonstrates just how many environments there are in the universe for life to flourish, and to think we are the only example of life seems both improbable and impossible. However, the existence of life on Earth is the result of billions of years of everything being just right. Although the circumstances which give rise to life on Earth are largely unclear, what is known is that there are many factors which had to align correctly to give the perfect conditions for life to occur. Factors which influence the development of life on Earth range from the distance from the sun, the existence of plate tectonics and the segregation of ‘right-handed’ and ‘lefthanded’ amino acids. Every aspect of science has factors that contribute to the development of life on Earth and we have to assume that each one has to be perfect in order for life to form. Furthermore, the conditions on a planet have to remain perfect for millions or billions of years in order for intelligent life to develop. On Earth, the earliest evidence of life existed 3.5 billion years ago. Life on Earth had to survive 3.5 billion years on the earth and in the solar system, facing thousands of threats and challenges, such as meteors from outside the solar system or a runaway build-up of greenhouse gases turning the earth into an oven, killing any potential life forms. It is incredible that life has managed to survive so long when there were so many opportunities for it to all go wrong. It is reasonable to ask, “Is it likely that other planets which potentially contain life will also be able to avoid all these truly lifeending catastrophes?”. Additionally, how many other planets can maintain the conditions necessary for life for billions of years? In 1961, American astronomer Frank Drake developed an equation to estimate the number of detectable extra-terrestrial civilisations in our galaxy. The Drake equation combines many factors, all crucial for the discovery of alien life. The equation incorporates the number of stars born per year, the number of stars with planets, the number of habitable planets per star system, the chance that the planet develops life, the chance that the life is intelligent, the chance that the life can communicate across space, the length of time a civilisation sends signals to space and the number of times that civilisations can redevelop. Using Drake’s original values, the equation yields ten communicating civilisations in our galaxy, and using more current (albeit optimistic) data, the equation tells of 756 civilisations. According to the Drake equation, other life in the galaxy seems inevitable! A more modern equation has also been developed Dr Sara Seager, which modifies the Drake equation in line with newer methods used to search for extra-terrestrial life. The Seager equation makes more use of Astrobiology, a new and quickly growing field, and considers how we can detect other life forms. It consists of the number of observable red dwarf stars, the percentage of these stars with stellar flares and disruptions, the percentage of planets which can be observed, the
percentage of stars with rocky planets in the habitable zone, the percentage of planets with life and the likelihood of a detectable bio-signature gas on the planet. Seager’s original values anticipate 0.45 planets with detectable life in our galactic neighbourhood, while optimistic present-day numbers suggest 750 planets. For any optimist, it seems certain that there are other planets with life in our galaxy, and even Seager herself believes we may make a discovery in the next ten years. So, what makes a planet habitable? Firstly, and perhaps obviously, a habitable planet must form around a star. This may seem like a prerequisite for any planet, but there are thought to be countless exoplanets wandering in deep space with no attachment to any star. A star provides a planet with an energy source, which is the first and primary necessity for life. However, the planet must orbit at the correct distance from the star, in the so-called ‘Goldilocks zone’ where the surface of the planet receives just the right amount of energy from its parent star. This is because the temperature on the surface of the planet must be between 0-100°C in order for there to be liquid water at the surface. Furthermore, the planet must be a rocky planet, accreted from solid matter in the early solar system. The alternative is a gas giant such as Jupiter or Saturn which has no solid surface on which life could develop. The planet must also be ‘just right’ in terms of orbit and spin. For example, both our neighbours, Mercury and Venus, have years shorter than their days, which leads to very high temperatures on one side of the planet and very low temperatures on the other; conditions which cannot readily support life. The parent star must also be stable, with minimal solar flares or disturbances which could threaten the planet. So far then, a habitable planet is a rocky planet orbiting a stable star in the ‘Goldilocks zone’ with a sensible orbital period. Once we have looked at the position of the planet in space, we must now consider the structure of the planet. Our own planet is segregated into three distinct layers; the core, the mantle and the crust, and each layer is equally important for the development of life. The core of the earth is made up of dense metallic elements, for example iron (Fe), tungsten (W) and nickel (Ni). The core gives the earth most of its mass, and thus creates a strong gravitational field for a planet of Earth’s size. This means the earth can hold onto more of its atmosphere, and so creates conditions more suitable for life. Venus is of a similar size to Earth, but it has a gravitational attraction only 90% of Earth’s and thus the depth of the atmosphere is less than Earth’s. Additionally, the outer core in the earth is less compressed, so it exists in a liquid state. This is vital for life on Earth as it allows a magnetic field to form, which provides protection from fatal solar winds, streams of charged particles emitted from the sun. Without a magnetic field for protection, any life forming on planets around sun-like stars would be unable to survive. The mantle is also important for the development of life on Earth as it is the driving force of plate tectonics. Contrary to popular belief, the mantle is not in fact liquid, but instead a highly viscous but very hot solid. Furthermore, heating in the mantle does not form regular convection cells but forms more erratic regions of
heating and cooling at the surface crust. The differential heating of the surface crust has led to the crust breaking up into different plates and given rise to plate tectonics and volcanism. This has been crucial for life on earth for so many reasons. Early volcanoes led to the development of the atmosphere as they released gases such as carbon dioxide and nitrogen from the mantle. The input of gases from volcanism allowed the atmosphere to thicken and mature to provide a greenhouse effect and provide an input into chemical cycles essential for life, such as the carbon cycle or the sulphur cycle. Many of the volcanoes outgassing to the surface are located in the ocean, at constructive ocean ridges. This has helped to develop seawater composition, and, as we shall see, may have been vital to the origins of life. Furthermore, plate tectonics created continental crust through volcanism, which is ultimately where life thrives. The crust is important for obvious reasons. It is where all known life in the universe survives, and has been home to that life for 3.5 billion years. Therefore, the crust has had to maintain its conditions fairly well in that time to allow for steady evolution. This means the crust must be stable, made from molecules which are largely not radioactive and do not break down quickly. The crust must also have all the building blocks of life readily available, either from the land, sea or atmosphere. Furthermore, all the correct ingredients to create and sustain life must also be available in the right places and at the right times for life to be successful. So, what we have deduced so far is that a habitable planet is a rocky planet orbiting a stable star in the ‘Goldilocks zone’ with a sensible orbital period. The planet has a solid metal core creating a relatively strong gravitational field, a liquid metallic outer core creating a magnetic field, convection in the mantle leading to plate tectonics and a crust with readily abundant material for life. For a planet to be habitable, it must also have an atmosphere. The earth’s atmosphere is made up of approximately 78% nitrogen (N), 21% oxygen (O 2 ), 0.95% argon (Ar) and 0.05% other gases, including carbon dioxide, neon, helium, hydrogen, methane and water vapour. However, it has not always been this way, and on the early earth the atmosphere had a much higher concentration of carbon dioxide, methane and water vapour, and much lower levels of nitrogen and oxygen. As the earth aged, these concentrations changed to modern levels, which have been important for the development of life. If carbon dioxide levels had been maintained, or even raised, then Earth may have experienced a runaway build-up of carbon dioxide leading to an enhanced greenhouse gas effect, similar to that experienced by Venus. Venus’s atmosphere consists of approximately 96.5% carbon dioxide, a molecule which is excellent at storing energy in its bonds and allowing little infrared radiation to escape to space. This has led to extremely high temperatures at Venus’s surface and the creation of dangerous molecules such as sulphuric acid which has inhibited the evolution of life. Although it is not completely clear what caused the build-up of carbon dioxide in Venus’s atmosphere, it is thought to be an uncontrolled output of gas from the very active
volcanism, along with the lack of large carbon sinks on Venus’s surface. It is lucky, therefore, that the earth’s volcanism is more controlled and the ocean acts as a large carbon sink. The decrease in both carbon dioxide and methane through Earth’s history can be accounted for due to the rise in life forms, which are both built from the same elements as methane and carbon dioxide but also take in carbon dioxide in order to synthesise their own food. The decrease in water vapour concentration may be accounted for by the decrease in greenhouse gases reducing the temperature at the earth’s surface and leading to the condensation of more water vapour. The increase in oxygen concentration is a more interesting problem which scientists do not quite know the answer to yet. It is theorised that the development of life meant that there were large abundances of oxygen released, which led to the increase of the atmospheric oxygen concentration. However, calculations suggest that the rate of oxygen production from organic matter would take six billion year to produce the volume of oxygen required to bring concentrations up to today’s values. As Earth is only 4.6 billion years old this cannot be the sole reason for the increase in oxygen in the atmosphere. Therefore, it cannot be organic matter which is entirely responsible for the rise in oxygen in the atmosphere to current levels, which means there is currently an unknown source of oxygen which fed into the atmosphere, but no longer does today. There are still mysteries surrounding the development of the atmosphere, but in general, there must be an atmosphere rich in oxygen and low in greenhouse gases in order for a habitable planet to be formed. One of the biggest gaps in human knowledge is the process by which life actually began. Many theories of the origin of life have been proposed, but it is difficult to either prove or disprove them, which means no theory is fully accepted. In order for life to form, there must be all the correct building blocks, which means there must be conditions to form complex organic molecules. Central to life is the element carbon, which is unique in the periodic table as it can form four bonds, is stable and in great abundance in the solar system. Carbon is the element on which all life on Earth is based, so any potential habitable planet must have a ready supply of carbon. Water is another molecule essential to life, as it can react with hydrogen to form organic molecules, which make up all of life. Other elements, such as nitrogen, sulphur or iron are also important, but none so much as carbon, hydrogen and oxygen. In fact, the general formula for all organic molecules is CH 2 O. With carbon, hydrogen and oxygen, a massive range of molecules can be created, including molecules in the four main classes of organic molecules from which life is formed. These are carbohydrates, lipids, amino acids and nucleic acids. These four molecules all have different jobs in the building of cells and are all vital to life. Carbohydrates are the source of energy for a cell. Burning carbohydrates such as glucose releases energy for the cell to use to power operations. Lipids are the storage of energy for cells, for use in emergencies, as well as the materials which build some important structures such as the cell membranes. Amino acids are essential for life, but there are only twenty naturally
occurring variations in nature. These twenty amino acids can combine in hundreds of different ways in order to create many forms of proteins. They can combine to make extremely complex molecules, for example haemoglobin, which has the chemical formula C 2952 H 4664 N 812 O 832 S 8 Fe 4 . Finally, there are nucleic acids, which are the information carriers of a cell. They form two types of molecules deoxyribose nucleic acid (DNA) or ribose nucleic acid (RNA). These molecules contain all the information about the cell and allow the cell to selfreplicate. These four classes of molecules are essential to life; however, it is unclear how they were initially formed and combined to create a cell. There are additional complications which make it even more difficult to find a good explanation; for example amino acids can form in either a ‘right- ‘or ‘left-handed’ format, but every living being only uses ‘left-handed’ amino acids in all their cells. The question of how life began is one of the biggest and most complex unsolved problems facing humankind in the twenty-first century. To solve it would be to shed light on our own origins, and by extension the origin of all life in the universe. Many places have been suggested as the appropriate environment were life began. Since we do not quite know how life began, it makes it difficult to hypothesise where it began. However, what we do know is that there must be a source of energy, a source of carbon, hydrogen and oxygen and a safe, stable area for these ingredients to combine and form life. All kinds of environments have been suggested, from the underwater hydrothermal vents to pools of acid to salt pans. Some even suggest that life did not originate on Earth, but it was brought here on an asteroid which collided with Earth in its early formation. There is very minimal evidence of where life came from on early Earth. The best evidence are fossilised stromatolites, which are the excretions of a special type of ancient bacteria. Stromatolites are found in shallow waters, were there are supplies of the essential ingredients of life from the atmosphere, the sea water and the oceanic crust. However, few believe that stromatolites are the first examples of life on Earth, so life’s origin remains a mystery. Once life on Earth began, it quickly spread to cover most of the Earth. The vast majority of life forms are bacteria, and even today bacteria outnumber macroscopic life by many orders of magnitude. However, through the mutation of DNA sequences and the principle of evolution summarised as the ‘survival of the fittest’, larger life forms with substantial structures began to emerge during the Cambrian explosion 543 million years ago. As life quickly evolved, it had to deal with several catastrophes which had the potential to wipe out all life on Earth. A wellknown example is the meteor which killed the dinosaurs. This meteor impacted Earth 66 million years ago in modern-day Mexico and was anywhere between 11 km and 81 km in diameter. The shockwaves from the meteor were one million times more forceful than the largest modern atomic bomb and caused a mass extinction of all macroscopic life. Other mass extinction events have been caused by large volcanic outpouring or glaciation events, which rapidly altered
atmospheric composition and therefore conditions on the earth’s surface. Through all these changes in Earth’s history, life has managed to survive, adapt and evolve, which has ultimately led to the rise of homo sapiens. It is therefore essential for any potentially habitable planet to be unhindered by any cataclysmic extinction events. While Earth has survived mass extinctions, it would not have survived larger asteroid impacts or other more drastic changes to its structure or composition. To summarise, a habitable planet is a rocky planet orbiting a stable star in the ‘Goldilocks zone’ with a sensible orbital period. The planet has a solid metal core creating a relatively strong gravitational field; a liquid metallic outer core creating a magnetic field; convection in the mantle leading to plate tectonics and a crust with readily abundant material for life. A habitable planet has an atmosphere rich in oxygen and low in greenhouse gases, and an abundance of organic material at the surface to form the four classes of molecules essential for life. While it is unclear where, how and why the organic molecules combine to create life, once life has formed it must evolve and survive extinction events in order for a habitable planet to be formed. All these requirements make the likelihood of finding another habitable planet rather slim, but remind yourself of the 5×10 22 planets which exist in the observable universe. Additionally, these are requirements to find life similar to that found on Earth. Life on Earth is not necessarily the only type of life in the universe. While some factors, such as an atmosphere on the planet, seem almost essential to life, there are other factors, such as the requirement of liquid water, which some scientists argue are not so paramount. Turning our attention closer to home, Astronomers and Astrobiologists have considered planets in our immediate vicinity to be hosts to some forms of life. Bodies like Mercury or the moon are totally inappropriate for the formation of life, due to extreme temperatures and lack of atmosphere or both. Up until the 1960s, scientists believed life on Venus was very probable, and despite the proof of crushing pressures, boiling temperatures and a complete lack of liquid water, some scientists are still working on the possibility of extremophile bacteria which can survive in such conditions. The planet with the most promise for extra-terrestrial life in our close neighbourhood is our neighbour Mars, the red planet. Even though Mars has a thin atmosphere and extremely low temperatures during Martian nighttime it is possible that Mars is host to microbial life, either currently or in the past. Scars on the surface of the planet are easily explained by the presence of large oceans of liquid water, and the planet does sit inside the ‘Goldilocks zone’. Contrary to popular belief, water does still exist on the red planet, except it is all locked up in giant ice sheets under the surface at the planet’s poles. Furthermore, there is evidence of organic molecules originating from Mars, both on the planet itself and in meteors which have crashed to Earth from the planet. While it is unclear whether these molecules are the result of life on Mars, it is promising
evidence to fuel the research into the red planet. Life on Mars is a very popular branch of research, with scientists from institutes like NASA, ESA and SETI all focused on the search. It is, by current predictions, the most likely place, other than Earth, to be home to life in the inner solar system. The search for extra-terrestrial life is not restricted to our closest neighbours. There are many other possibilities for life elsewhere in the solar system. The most promising location is on Europa, one of the Galilean moons of Jupiter. Europa has a solid ice crust, but there is evidence of a liquid water ocean below it, driving the ice on top in convection patterns similar to plate tectonics on Earth. This ocean is heated by tidal flexing from the massive influence of Jupiter nearby, which makes it very possible that the ocean is at an appropriate temperature for life to form. NASA have even recently confirmed a mission to explore Europa in search of extra-terrestrial life. Europa is not the only moon in the outer solar system worth considering when looking for life other than on Earth. There are several other possibilities, for example Enceladus and Ganymede, which are also both Jovian moons, or Titan, the largest moon of Saturn. All these bodies have shown the promise of water at the surface as well as carbon-bearing molecules. These moons are all being investigated for the potential of life, either primordial or developed, and many missions have been proposed by various organisation in order to visit them and gain a greater understanding of the conditions on each moon. Of course, our solar system is just one small corner of the great expanse of the cosmos. It is entirely plausible to suggest that the conditions necessary for life do exist on other planets elsewhere in the universe. As of June 2019, 4003 exoplanets had been discovered by the Kepler Space Telescope, with a potential 2955 more still to be confirmed. This large number of exoplanets is very promising in the search for extra-terrestrial life, and several have been confirmed as objects of interest, meaning they may play host to extra-terrestrial life. One example is Proxima Centauri b, which is the closest known exoplanet to Earth, only 4.2 lightyears away. It has a similar mass and similar radius to Earth and is heavily researched in the search for extra-terrestrial life. As of August 2019, there are between seventeen and forty-seven potentially habitable exoplanets, and the list is growing all the time. “Are we alone in the universe?� immediately suggests the search for intelligent communicating forms of life but, in reality, modern research concentrates on detecting simple amino acids or complex organic molecules. This essay has considered all forms of life as a generalised group, and to consider the likelihood of finding basic bacteria structures versus evolved complex civilisations would require a whole other essay altogether.
With the rapid advancements in science through the twentieth century into the modern era, the search for extra-terrestrial life has gone from fanciful pondering to a coherent and ambitious exploration of our universe. The subject interlaces all the branches of science and asks some of the most fundamental questions of human existence. To me, projects like the search for extra-terrestrial life in any form – evolved or simple – are among the greatest endeavours of human beings, uniting the globe and looking for answers to the most ancient of questions. One day, perhaps, we may be rewarded for our labour, and we may find someone else with whom we can share that great emptiness we call space.
References http://www.bbc.co.uk/earth/story/20160610-it-took-centuries-but-we-now-knowthe-size-of-the- universe https://www.space.com/14200-160-billion-alien-planets-milky-galaxy.html https://informationisbeautiful.net/visualizations/the-drake-equation/ https://www.livescience.com/1804-greatest-mysteries-life-arise-earth.html https://www.scientificamerican.com/article/how-did-life-begin1/ https://www.sciencedaily.com/releases/2018/05/180524141736.htm https://www.livescience.com/reasons-to-believe-life-on-mars.html https://www.theguardian.com/science/2018/jun/17/the-most-likely-cradles-for-lifeour-solar-system-mars https://en.m.wikipedia.org/wiki/Lists_of_exoplanets https://en.wikipedia.org/wiki/List_of_potentially_habitable_exoplanets ‘How to Build a Habitable Planet: The Story of Earth form the Big Bang to Humankind’ Charles H. Langmuir and Wally Broecker, Princeton University Press 2012, ISBN
978-0-691-14006-3
Last updated: August 2019 Image from: https://pixabay.com/photos/milky-way-starry-sky-night-sky-star-2881461/