Are we alone in the Universe? How may life emerge elsewhere in the Cosmos? What is our future in the grand scheme of things? In this article, we dive into the field of astrobiology and planetary sciences and explore these questions in turn.
by Carmen Chan and Vanessa Lunardi
The Search for Extraterrestrial Intelligence (SETI) Institute may be perceived as a big fancy I-believe-in-aliens fan club, but what started as a search for alien intelligence has culminated into a deeper study on the origin of life, to understand the necessary conditions to sustain life, investigate the possibility of life outside Earth, how we can observe and identify them, and what our future will look like going forward.
At SETI, much of their research is centred around the factors of the Drake Equation, a probabilistic argument used to figure the odds of finding intelligent life in the Milky Way. The equation comprises several variables: the rate of formation of stars suitable for the development of intelligent life, the fraction of those stars with planetary systems, the number of planets per solar system with an environment suitable for life, the fraction of suitable planets on which life actually appears, the fraction of life-bearing planets on which life emerges, the fraction of civilisations that develop a technology that releases detectable signs of their existence into space, and the length of time such civilisations release detectable signals into space.
Given the thousands of exoplanets confirmed so far, the billions of stars in our one galaxy, and the billions of galaxies in the observable universe, what are the odds that we may find intelligent life? While we only have rough estimates for now, it is safe to say we have not been contacted yet. This then begs the question, where are all the aliens?
Understanding Life in the Universe
In the emerging field of astrobiology, scientists are interested in answering the long-standing question of how life began and evolved in the Universe. By combining knowledge and techniques from various scientific fields like astronomy, chemistry, and geology, the multidisciplinary field of astrobiology seeks to test the limits and “extremes” under which life can survive, investigate if life can emerge elsewhere, and contemplate the future of life in the Universe.
In recent years, space scientists have sent multiple probes, rovers, and space shuttles to neighbouring planets and beyond in search of clues that may aid us in understanding a little more about life’s history. Their missions have led to the discovery of viral footprints in our DNA that seem to suggest cosmic origins,1 the discovery of microorganisms from cosmic dust samples on the surface of the International Space Station,2 and more recently, a new class of habitable planets composed of water-rich interiors with massive oceans underlying hydrogen-rich atmospheres.3 Momentum is also building in the scientific community that our Milky Way galaxy may be teeming with ocean worlds – terrestrial planets with significant amounts of water either on their surface or in a subsurface sea. Jupiter’s moon Europa, for instance, is hypothesised to host vast seas under its icy shell and Saturn’s moon Enceladus is known to have watery geysers spewing from its exterior.4
All these findings point to the possibility that life may have cosmic origins and that other worlds could potentially host life, but whether there is life on these celestial bodies is another question altogether. Up until now, the existence of extra-terrestrial life remains open to debate for the simple reason that we have yet to find definite evidence to support their existence. This obvious gap in the hunt for alien life has left many wondering whether we should continue our pursuit at all.
As said by astronomer Michael Hart in 1975 on his thoughts regarding interstellar travel and intelligent extra-terrestrial life:
“[If] they are not here; therefore they do not exist.”
But should absence be taken as proof of non-existence? As the saying goes, the absence of evidence is not the evidence of absence, and some scientists would understandably disagree for several reasons: considering the massive size of the Universe, the rarity of intelligent extra-terrestrial beings, and the short lifetime of civilisations, life could easily exist in some unexplored pockets of the Universe, undiscovered by humanity. Moreover, given that our understanding of “life” is severely limited to what we know on Earth, would we even recognise “life” when we see it?
Defining Life on Earth’s Terms
Here on Earth, life is easily defined by a set of characteristics and properties shared by all living creatures. At its most basic level, all forms of life have the same elemental makeup, use a common set of biochemical building blocks, and share a common ancestor and genetic code. On a molecular level, 99 per cent of life is composed of carbon, hydrogen, and oxygen, with hydrogen and oxygen being the most abundant in living systems, and hence the use of water as a key biosignature for developing life.
Apart from water, there are also primordial biomolecules, which include alcohols, amino acids, purines, pyrimidines, and sugars, that are present and shared by virtually all living organisms known to date.5 These are the standards by which we understand life and the measures we have been using to detect life outside of Earth.
In 1975, the National Aeronautics and Space Administration (NASA) launched two Viking spacecraft to Mars to look for microorganisms in the Martian soil. The landers conducted three biology experiments that involved searching for photosynthesis and organisms capable of utilising a nutrient solution. The results registered gaseous release, which suggested some sort of chemical activity. However, its gas chromatograph mass spectrometer (GCMS) indicated no Martian organics.6 It was only much later in 2008 with the Phoenix mission, where scientists detected high levels of perchlorate in Martian soil,7 were we able to understand that when a sample was heated with perchlorate in the GCMS, it would decompose any organics present in the sample.8
In other studies, upon analysing geomorphological evidence, it seems that liquid water may have existed on the Martian surface at roughly the same time that life first appeared on Earth between 3.8 and 3.5 billion years ago.9,10 During this time, we can expect to see hot springs, lakes, salt pools, and so forth on Mars as we would have seen on Earth. Such microenvironments would have been viable for the formation of life, but we have yet to know for how long Mars had these Earth-like environments,11 and so it may be possible that Mars harboured life at some point and died away. While life remains to be found on Mars, the red planet still draws considerable interest from the scientific community as it may hold, beneath its icy surface, the best record of events leading up to the development of life.
Yet, these considerations are made based on what we know from life on Earth. If we were to consider a second genesis vastly different from our own, we can expect the environment and conditions that have enabled “life” on Earth to be completely different. Instead of water, amino acids, or the fundamental elements we recognise today as the building blocks of life, there may be other requirements for life to arise and survive.
Consider the possibility of an alternative structure for genetic codes in alien planets apart from our typical DNA and RNA. If we search for extra-terrestrial life based on Earth’s planetary atmospheres, bioindicators, surface, and industrial biosignatures, it could be misleading as we may very well come across a present or past signature of life and simply not know it.
Considering Life Beyond Water
Beyond Mars, a billion kilometres away, lies Titan, Saturn’s largest moon and the only known moon with an atmosphere. Its complex atmosphere is 98 per cent nitrogen and 2 per cent methane, where it rains liquid methane, forming seas and lakes of ethane and methane. On Titan, there are clouds and there are seasons; there are valleys and there are mountain ridges. A celestial body that seems to have followed a vastly different evolutionary path from Earth, the suggestion that Titan may harbour life in its methane lakes would challenge our concept of liquid water-based life.
What can we expect from an alternative genetic code? The idea that an alien life form could be based on a non-water solvent has been discussed in recent years by biochemist Steven Benner12 and Chris McKay.13
In the journal Icarus, McKay argued that if methane-based life were to exist on Titan, complex hydrocarbons (like ethane and acetylene) produced by sunlight would likely be an energy source when reacted with hydrogen in the atmosphere. This would be similar to chemical energy in hydrocarbons on Earth when reacted with atmospheric oxygen. In consuming these complex hydrocarbons and producing methane as a result, it would explain the presence of methane on Titan.13
In 2010, a study published in Icarus reported hydrogen data that are consistent with conditions that could potentially produce and sustain methanogenic life.14 Furthermore, a separate study published in the Journal of Geophysical Research the same year mapped hydrocarbons on Titan’s surface and reported a lack of acetylene,15 a complex hydrocarbon posited by McKay to likely be a source of energy of methanogenic life.
However, despite the possible evidence, it is important to consider all non-biological explanations before biological ones. Mark Allen, principal investigator with the NASA Astrobiology Institute Titan team, suggested one possibility for a lack of acetylene could be that sunlight or cosmic rays transform the acetylene in the atmosphere into more complex molecules that would settle to the ground, leaving no acetylene signature.
Testing the Limits of Life With Extremophiles
While we have briefly considered a methane-based life on Titan, we will, for now, have to content ourselves with what we have on Earth.
In redefining our present understanding of what constitutes “life”, astrobiologists have tried to look for answers from within Earth, though not in a way that has been done before. Instead of comparing Earth’s criteria and characteristics of life with that of other planets, researchers have begun to reconsider the boundaries under which life can survive by studying the endurance and limits of extremophiles in space-like conditions and that of other worlds.16,17
Extremophiles, as the name suggests, are organisms that inhabit extreme environments. From ice and boiling waters, to acids and the water core of nuclear reactors, extremophiles can not only tolerate but also thrive under conditions initially thought inhospitable for life. In fact, extremophiles have been found at 6.7 kilometres deep inside the Earth’s crust and more than 10 kilometres deep within the ocean, showing their capacity to withstand extreme pressure. For example, the archaea Thermococcus piezophilus CDGS, isolated from deep-sea hydrothermal vents, can withstand up to 125 MPa. Some species have also demonstrated remarkable tolerance to extremely acidic and basic conditions. The most extreme acidophile and alkaliphile microbes known to date, Picrophilus oshimae and Serpentinomonas sp. B1, have been found to survive at pH 0 and pH 12.5 respectively. Other types of extremophiles like 116 and Planococcus halocryophilus Or1 can survive under the extreme temperatures of deep-sea hydrothermal vents at 122 degrees Celsius and sea ice cores at –15 degrees Celsius respectively.16
To explore the boundaries of life in a planetary context, scientists have tested the resilience of extremophiles under space-like conditions. In 2012, to support the exploration of Mars, a study found that several microorganisms were able to withstand extreme levels of radiation and even survive for hundreds of days when exposed to space-like conditions.17 The spores from the Black Mold fungus, for one, survived a trip to the Earth’s stratosphere, which represent the key conditions on Mars, even when they were exposed to intense UV radiation.18 In a more recent study, scientists conducted an experiment outside the International Space Station and discovered that some bacterial species can survive in space for years as well. The bacteria Deinococcus radiodurans, in particular, can survive at least three years in space.19
With accumulating evidence to support the durability of extremophiles, these microorganisms have become a valuable resource to uncover the physicochemical parameters of life on Earth as well as its origin. But more importantly, they highlight the surprising resilience of Earth life to survive, adapt, and possibly thrive in harsh, foreign environments. This brings to the table a new possibility – sending Life out of Earth.
Living on the Edge, Inhabiting Other Worlds
“The earth is a cradle of humanity, but humankind cannot stay in the cradle forever.” — Konstantin Tsiolkovsky, Rocket Scientist
As the Sun ages and burns more hydrogen, it gradually grows more luminous and in about 1.1 billion years, will cause Earth to be inhospitable to life. However, it is unlikely that humanity will be a part of that future. Perhaps we have finally perished in the worsening climate, obliterated by an asteroid, or evolved into new beings who have conquered interstellar space travel. Nonetheless, compared to microbes, complex organisms such as we would not stand a chance of surviving such an apocalypse.
Even if microbial life were to prove resilient against all odds, in about 5 billion years, our Sun will run out of hydrogen. With no hydrogen to fuel its core, gravitational forces will take over, compressing the core, and allowing expansion to take place. Ballooning into a red giant, the Sun will devour the inner planets of our solar system, including Earth.20 Life on Earth as we know it will truly cease to exist.
In such a case, how can we ensure Life goes on? Today, synthetic biology has allowed us to manipulate the genetic code, producing specialised organisms for use as tools in pharmaceutical production, environmental remediation, and crop fertilisation, among others. Following this line of thought, if we were to take the hardiest microbial life, modify them in some way, and send them to the stars, who is to say that Life may not evolve out there? If Darwin’s theory of natural selection could occur in the same way as on Earth, Life could very well continue—even in the absence of humanity—taking on different paths of genesis and evolution, bringing forth endless forms most beautiful.
- Wickramasinghe, N.C., 2012. DNA sequencing and predictions of the cosmic theory of life, Astrophysics and Space Science, 7 September 2012.
- Grebennikova, T. V., Syroeshkin, A. V., Shubralova, E. V., Eliseeva, O. V., Kostina, L. V., Kulikova, N. Y., ... & Tsygankov, O. S. (2018). The DNA of bacteria of the World Ocean and the Earth in cosmic dust at the International Space Station. The Scientific World Journal, 2018.
- Madhusudhan, N., Piette, A. A., & Constantinou, S. (2021). Habitability and Biosignatures of Hycean Worlds. The Astrophysical Journal, 918(1), 1.
- Gohd, C. (2020, June 22). Our milky way galaxy may be teeming with Ocean Worlds. Space.com. Retrieved September 27, 2021, from https://www.space.com/milky-way-teeming-ocean-worlds.html.
- Malkan, M. A., & Zuckerman, B. M. (Eds.). (2020). Origin and Evolution of the Universe: From Big Bang to Exobiology, 189-212. World Scientific.
- Biemann, K., Oro, J. I. I. I. P. T., Toulmin III, P., Orgel, L. E., Nier, A. O., Anderson, D. M., ... & Lafleur, A. L. (1977). The search for organic substances and inorganic volatile compounds in the surface of Mars. Journal of Geophysical Research, 82(28), 4641-4658.
- Hecht, M. H., Kounaves, S. P., Quinn, R. C., West, S. J., Young, S. M., Ming, D. W., ... & Smith, P. H. (2009). Detection of perchlorate and the soluble chemistry of martian soil at the Phoenix lander site. Science, 325(5936), 64-67.
- Navarrosy-González, R., Vargas, E., de La Rosa, J., Raga, A. C., & McKay, C. P. (2010). Reanalysis of the Viking results suggests perchlorate and organics at midlatitudes on Mars. Journal of Geophysical Research: Planets, 115(E12).
- McKay, C. P. (1986). Exobiology and future Mars missions: The search for Mars’ earliest biosphere. Advances in space research, 6(12), 269-285.
- McKay, C. P., & Stoker, C. R. (1989). The early environment and its evolution on Mars: Implication for life. Reviews of Geophysics, 27(2), 189-214.
- McKay, C. P., & Davis, W. L. (1991). Duration of liquid water habitats on early Mars. Icarus, 90(2), 214-221.
- Benner, S. A., Ricardo, A., & Carrigan, M. A. (2004). Is there a common chemical model for life in the universe?. Current opinion in chemical biology, 8(6), 672-689.
- McKay, C. P., & Smith, H. D. (2005). Possibilities for methanogenic life in liquid methane on the surface of Titan. Icarus, 178(1), 274-276.
- Strobel, D. F. (2010). Molecular hydrogen in Titan’s atmosphere: Implications of the measured tropospheric and thermospheric mole fractions. Icarus, 208(2), 878-886.
- Clark, R. N., Curchin, J. M., Barnes, J. W., Jaumann, R., Soderblom, L., Cruikshank, D. P., ... & Nicholson, P. D. (2010). Detection and mapping of hydrocarbon deposits on Titan. Journal of Geophysical Research: Planets, 115(E10).
- Merino, N., Aronson, H. S., Bojanova, D. P., Feyhl-Buska, J., Wong, M. L., Zhang, S., & Giovannelli, D. (2019). Living at the extremes: extremophiles and the limits of life in a planetary context. Frontiers in microbiology, 10, 780.
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- Cortesão, M., Siems, K., Koch, S., Beblo-Vranesevic, K., Rabbow, E., Berger, T., ... & Moeller, R. (2021). MARSBOx: Fungal and bacterial endurance from a balloon-flown analog mission in the stratosphere. Frontiers in microbiology, 12, 177.
- Kawaguchi, Y., Shibuya, M., Kinoshita, I., Yatabe, J., Narumi, I., Shibata, H., ... & Yamagishi, A. (2020). DNA damage and survival time course of deinococcal cell pellets during 3 years of exposure to outer space. Frontiers in microbiology, 11, 2050.
- Wendel, J. A. (2019, August 7). When will the sun die? Space.com. Retrieved September 27, 2021, from https://www.space.com/14732-sun-burns-star-death.html