The search for extraterrestrial life, long the stuff of science fiction, has gained momentum in recent decades as advances in technology have allow scientists to look into the cosmos like never before. As of 2023, 5,026 exoplanets have been directly observed and confirmed by the Kepler spacecraft, with almost a tenth of those being Earth-sized and in their star’s habitable zone [1]. However, the implications of such discoveries run deeper.
1. Introduction to the Search for Extraterrestrial Life
This article presents a historical overview of how humanity has come to believe Earth is not unique in having intelligent life. First is a discussion of the earliest thoughts about extraterrestrial life forms since civilizations’ conception. These thoughts gradually turned into a scientific possibility following the development of the scintific method. The next two sections focus on the search for extraterrestrial intelligence during the 20th century. The first section examines scientific efforts to detect radio signals from other civilizations. The next activity describes how searches for extraterrestrial intelligence have expanded to include places where life may exist locally and beyond Earth. The final section discusses the sometimes contentious cultural and philosophical implications of the beliefs that other life forms exist [2].
Over the years, there have been many directions considered to test whether or not Earth is unique in housing intelligent life. However, all theories and models currently in use overwhelmingly suggest that there are billions of planets in the universe in a position to support life as it is known on Earth. While the likelihood of such planets hosting intelligent life capable of creating radio technology or driven past Earth would be small, they still would number in the millions. Given the age of the universe, even if one civilizations were to evolve billions of years before humanity, there would still be a window of time, in which they could have travelled throughout the galaxy and colonized planets alike. Thus, panpsychism, the belief that complex thought exists in pure form, would become popular in academia.
1.1. Historical Background
The search for extraterrestrial life has fascinated humanity for thousands of years. The belief in the existence of life on another planet dates back to ancient times. Many philosophers speculated about the nature of planets and whether they were inhabited. However, these ideas remained speculative by nature [1]. Then, advances in technology in the 19th century allowed systematic attempts to collect evidence of extraterrestrial life. An observer with an account of blasting gases from volcanoes and cities on the Chain Islands wrote about them to the New York Times in 1870. Its publication inspired many readers, and soon a debate about life on Mars began between scientists and the public. Mars was of particular interest because with a similar length of days, two polar caps, and changing dark markings, it could support water and vegetation like on Earth. Lewis Wells proposed that by taking the Hawarden telescope, a gentleman from England could decipher the surface of Mars and see cities and railroads like those on Earth. The publication of the findings aroused interest, and soon subsequent finds were published.
In 1909, observations from the largest Earth-built telescope were published, which stated that Mars was barren. Observations on Earth about the complete absence of signs of civilization on Mars followed this discovery. In 1938, the fictitious report of Mars invasion read on the radio led to public panic that there was a real invasion from Mars. Similar interests in astrobiology were present in other cultures. The Ancient Arabic scholars and philosophers believed in living stars, while Indian Buddhism witnessed a debate about other worlds. Feasible proposals for finding extraterrestrial life began in the 19th century when it became known that galaxies were separate star systems. The focus was on Mars. A letter of Harvard Professor Adams in the Evening Post in 1877 assured those who believed in the existence of life on Mars that their hopes would soon be realized [2]. A search for Mars could be initiated from Earth by determining a major diameter of canals on Mars, transmissions by illuminating them with a powerful beam of light from Earth when Mars is closest to Earth, or by Mars telescopic photographs published in the morning papers.
1.2. Signs of Life in the Universe
The Search for Extraterrestrial Life has intrigued mankind for centuries, and discussions on “life in the universe” rank among the oldest and furthest-reaching human inquiries [1]. The assumption that the Universe is void of life is not only improbable, but alternatively, if life exists elsewhere, Earth is not unique [2]. The stars possess planets and satellites, and some should possess the ability to support life. With the advancement of technology, humanity’s capabilities to explore its planetary environment have grown. The upcoming few decades will represent an eventful period, and boldly going beyond the Moon and the solar system will be the frontier of human exploration. Searching for extraterrestrial life represents the next and ultimate goal of future space exploration and is probable to be the most expensive scientific endeavor ever undertaken.
Assuming that life beyond Earth exists, the unaccountable question is: “What should be the signs of this?” In other words, assuming that life is an evolving, organic structure, what would be the phenomena emitted by this structure – light and dark, noise and quiet, movement and stagnation – that would be intriguing for the observers? Is there a set of phenomena, which, regardless of the complexity and diversity of life forms, could be the potential indicators of the existence and investigation of this life? The signs in question are presented in the literature. On a global macroscopic scale, dying planets, disequilibrium, inhomogeneity, ordered structures, asymmetries, surface colors, and temperature anomalies are the phenomena that have been attributed to population by provided mechanisms. Earlier works studied the consequence of population by unequal active-dead phases, disease, and massive extinction events. Recently, far from equilibrium properties of the dynamic systems have been studied, due to the fact that in living systems entropy is locally decreasing. On a macroscopic global scale, living matter would represent a system in an open far-from equilibrium steady state with respect to a source of energy and material.
2. Methods and Technologies in the Search for Extraterrestrial Life
Astronomers have been striving to decode the mysteries of the universe, abundantly littered with stars, planets, satellites, and various celestial phenomena. Space exploration has been undertaken with suitably equipped unmanned spacecraft that have traversed millions of kilometers in their journey to learn about nearby planets, moons, and asteroids. So far, space exploration has helped unravel a few mysteries, but the majority remain unresolved. The quest for extraterrestrial life is one of the foremost problems and can be understood in two parts. The first part is scientific and addresses the question of whether life exists or has existed beyond Earth. With billions of stars and planets in the universe, many of which are Earth-like, the statistical argument overwhelmingly favors the chances of life. Nothing has been found yet, nor has any evidence been uncovered to clarify this issue. The second part of the quest is philosophic, whereby the anthropogenic question needs to be answered: If an advanced intelligent form of life exists, why it has it not contacted us? Possibly, many civilizations have risen and perished in as-quickly-as-thought lifetimes, leaving behind little or no trace [3] .
Preliminary explorations of the Moon, Venus, and Mars revealed barren wastelands where no life was found. Further exploration of the Solar System led to discoveries of several icy moons believed to harbor great oceans beneath thick ice crusts. Spacecraft and Earth-based observatories have detected organic molecules around comets, asteroids, moons, and planets. Several of them have also been targeted for exploration by suitably equipped probes. The search for extraterrestrial intelligence (SETI) commenced with the pioneering work of Frank Drake and was focused on radio searches, utilizing the then most recent discovery of radio waves. Evolving technologies and developing radio networks turned SETI into a scientific discipline, thoroughly vilified and starkly exposed to ridicule for a long time, but gradually becoming more respectable. The radio search is based on the premise that advanced technological civilizations must transmit signals into space to amazing civilizations or discoveries. The prime parameter involved in the search is the signal reception sensitivity, which depends on the receiver tuning over interesting spectral windows within the whole radio frequency domain [4] . The biggest and most powerful search instrument is the Arecibo dish located in Puerto Rico. Past breakthroughs in natural sciences led to the successful development of new methods and technologies that allowed scientists to expand their research fields and answer profound fundamental questions.
2.1. Radio Telescopes
Radio telescopes have played a key role in humanity’s search for extraterrestrial life for more than 50 years. They have the unique capability to observe the majority of the sky in microwave wavelengths day and night and to detect faint astronomical signals over a wide range of frequencies. The most troubled radios were the same. Different civilizations would thus likely use technology similar to ours and transmit signals that would be detectable by our current radio telescopes [5]. An idea was proposed that such a radio signal would be continuous and deliberate, aimed at us. An optimal approach was developed for radio observations. These days, with the development of more advanced radio receiving systems and signal processing technology, the quest for extraterrestrial intelligence (SETI) has gained a new significance. Such a signal from an extraterrestrial civilization would have a small bandwidth around the hydrogen line (a 10−10 width). A high Bandwidth is critical in eliminating false alarms. With very wide band observations, a star has to be in the beam for many seconds for a positive detection, and Earth has to be near the plane of the beam [6].
2.2. Space Probes and Rovers
The lithosphere is believed to cover a habitable subterranean cavern that contains liquid water and is geochemically active. Microbes are thought to have existed there for billions of years. Life detection methods are needed for future landers or penetrators capable of directly sampling and analyzing subterranean rocks. The latest analysis methods include: Mass spectroscopy of organic molecules and inorganic materials in deep martian meteorites can distinguish between biotic and abiotic organics using carbon isotopology; Composite CIs characterize mineral assemblages via electrochemical/environmental conditions and temperature and can distinguish biotic and abiotic assemblages; and Carbonate veins in drilled comets can be characterized via ion microprobe analysis [7]. These methods are inexpensive and require no new development for future penetrators or landers.
The exploration of Mars and the icy ocean moons of Jupiter and Saturn are two high priority goals in the search for extraterrestrial life. Currently, six spacecraft are in transit or have arrived at Mars since 2003 with several more planned for launch in the next decade, while the recent discovery of plumes and interior oceans on Europa and Enceladus has revived interest long dormant since the Galileo mission [4]. The vision for future exploration of these planets includes orbiters carrying high resolution visible/near infrared imaging and spaceborne synthetic aperture radar, landers with aqueous sample collection and C-hawk methods, and penetrators to access sub-surface habitats. However, at present, there is a lack of technology capable of detecting extant extraterrestrial life from any of these platforms.
3. Exoplanets: Potential Habitable Worlds
The search for extraterrestrial life beyond Earth naturally leads scientists to search for planets similar to Earth with conditions conducive to life. These exoplanets should lie in a circumstellar habitable zone (HZ) with a surface temperature allowing the presence of liquid water, have a rocky composition, and possess an atmosphere with suitable pressures and acceptable gases to allow life as we know it [8]. Scientific instruments increasingly detect planets orbiting other stars, referred to as exoplanets or extrasolar planets. For a long time, planets in our own solar system were believed to be solar system objects just like planetary moons, asteroids, and comets. The first confirmed detection of a planet outside our solar system came in 1992 and in the last decade, the search for exoplanets gained momentum. Several dozen have been confirmed with their detections reported, along with dozens of candidates awaiting confirmation. Exoplanetary discoveries have had dramatic effects on ground-based, airborne, and space-borne observatories. They have also led to the hope of eventually finding life-like worlds elsewhere in the universe. Others are expected to lead to results in the near future with the same hope. Consequently, the study of exoplanets has emerged as an exciting new branch of astronomy. There are currently over two thousand exoplanets confirmed, and they come in a surprising diversity of sizes, compositions, and orbits. Some are only a few times larger than Earth, while others are more massive than Jupiter. Many orbit much closer to their host stars than do the planets of our solar system, while others live in regions between the gas giants of the solar system where no planets were predicted. Some are likely made of rock or ice, while others may be entirely gaseous [9].
The Kepler mission has already found over 2000 candidate exoplanets that occupy diverse regions of parameter space, including many Earth-sized and potentially habitable candidates that orbit Sun-like stars in their HZs. It is becoming increasingly clear that another example of habitable worlds could exist with climates very different from Earth’s. These exoplanets with atmospheric pressures less than the triple point pressure of water might be HZ planets of M dwarf stars. Considering their low gravity and very small sizes, the present study aims to determine their possibility of sustained surface liquid water or habitability. Also, it examines the effects of determining the vapors and clouds of water on the mantling strength of these vapors on surface liquid water and investigates the habitable ranges of stellar effective temperature and radiation in the region of visible quality.
3.1. Characteristics of Habitable Exoplanets
In August of 2021, a planet outside of the solar system, or exoplanet, potentially in the habitable zone was discovered. The habitable zone is a region around a star where a rocky planet can have surface water at temperatures conducive to life [10]. When looking for other planets orbiting stars, astronomers look for these Earth-like subsurface water worlds known as exoplanets. Exoplanets are classified into two main categories: rocky or terrestrial planets and gaseous giants. These two classes of planets can be identified by their mass, radius, period or distance from the parent star, eccentricity, density, atmospheric composition, surface temperature, and surface features. Other factors that broaden the scope of habitability beyond the Goldilocks scenario include eccentric or tilted orbits, initial runaway conditions, variability of the parent star, or even moon-earth systems [1].
Once an exoplanet candidate is identified, there are many more analyses and extensive calculations that take place so that it can be confirmed and characterized. A mass upper limit can be calculated based on the star’s Kepler data. The conclusion can be drawn that Earth-sized planets are statistically best detected on long-period orbits outside a star’s HZ. A transit observation or photometric period on the order of several years can help determine the planet’s mass. Using transit photometry, there is a change in the depth of the transit light curve, which should yield the planet-to-star radius ratio. With mass-radius models, it is then possible to constrain the density range of the planet more securely. Further investigations, if warranted, would have to involve detailed atmospheric modelling simulations.
3.2. Kepler Mission and Exoplanet Discoveries
The Kepler spacecraft is NASA’s first mission capable of finding Earth-size planets orbiting Sun-like stars [11]. Launched on March 7, 2009, its primary science goal is to determine the frequency and distribution of small planets in or near the habitable zone of solar-like stars. This will be accomplished by monitoring the brightness of approximately 150,000 stars in the constellations Cygnus and Lyra for planetary transits: predictable dips in brightness as a planet passes in front of the star. An astounding 408 planetary candidates have been found in 170 multiple systems. Many systems contain small planets (hot Neptunes and super-Earths) and are compact. Kepler’s results are consistent with a formation model in which solid bodies are implanted in nearly flat orbits in a protoplanetary disk, leading to dynamical interactions that can excite orbits and cause orbital removal, either by ejection or collision. The existence of compact, flat systems containing small planets is unexpected and presents new challenges for formation and migration models.
Images and time-series data from the Kepler mission are publicly available at the Mikulski Archive for Space Telescopes (MAST). Data from the 2008 January Antarctic, engineering, and commissioning campaigns, including Kepler’s first monochrome images of stars, are hosted at MAST. After a year of on-orbit testing of the response to bright stars, nearly 250,000 stars with Kepler estimates of R < 15 were selected as possible long-cadence targets, including the 6,862 stars that were added in response to the first round of guest observer (GO) proposal selections [12]. Bright stars were selected to increase S/N for Kepler photometry and to mitigate systematic effects. Since stars where Kepler eyes were closed would only produce a “Did not observe” alert, fewer Kp < 15 stars were originally included in the target list.
4. Astrobiology: The Science of Life in the Universe
Astrobiology is the scientific discipline of life in the universe. It bridges the divide between the terrestrial and the extraterrestrial. Astrobiology encompasses the search for the origins, evolution, and extinction of life on Earth and elsewhere. It includes the birth of the planets, the birth of life, the evolution of life, the extinctions of life, and the search for life elsewhere.
Astrobiology research is focused in four areas tied together by observations made with ground-based, airborne, and space-based facilities. These areas are habitability, early environment of the Earth, early evolution of life and search for life. Habitability is defined as the characteristics of a planetary environment that permit the existence of life, with an emphasis on terrestrial planets.
4.1. Definition and Scope of Astrobiology
The field of astrobiology seeks to better understand the varied dimensions of life, with the objective of determining whether life exists beyond Earth. This science is concerned with a broad definition of life, and also pursues some concepts that relate specifically to Earth alone. Astrobiology considers life on planetary scales. The science of astrobiology uses life as a starting point to consider the nature of planetary environments, as well as events in the Solar System and the Galaxy that affect the habitability of planets and planetary suites within a system [4].
Astrobiology pursues a broad definition of life within the context of life on Earth. The origins of this life, particularly the earliest evidence of life, is a core pursuit. This earliest evidence is also the oldest evidence for stable and habitable conditions on this planet. There are several Earth-based targets that are thought to preserve an environmental history associated with early Earth conditions. As a result of plate tectonics there is no Earth-based geological record that predates 4.0 Ga. However, there are older meteorites (specifically, those that formed from the differentiated parent bodies of the group 4 chondrites and similar ungrouped meteorites; e.g. H Chondrites) that preserve geochemical archives associated with the formation of bodies that accreted in the inner Solar System.
4.2. Key Concepts in Astrobiology
The field of astrobiology has grown tremendously since its inception. It has become increasingly multidisciplinary, incorporating multiple sciences at a plethora of different scales. As a result, new students interested in pursuing a career in astrobiology often wonder what exactly the field entails, and what the key concepts within astrobiology are [13].
Life is a highly complicated phenomenon. As a first step to understanding life, astrobiology considers life on Earth. Four general properties are required to characterize life. These include metabolism, replication, the presence of information, and vesicles or membranes. Any Earth-based biosystems lacking any of these four features is not classified as life. College courses and textbooks exist that present these basic ideas, and they remain valid as a starting point for exploring the phenomenon of life. However, there are fundamental difficulties with understanding the nature of life, even one Earth-based example. These include how to constrain the chemical nature of life, and the relationship between living systems and their environment.
Another essential aspect of life is its ability to evolve through natural selection. Natural selection upon refracted replicators, catalyzed by earthly minerals or deriding simple organic oligomers, could plausibly account for the origin of life at the very beginnings of Earth’s history. Regardless of the more or less likeliness of the various scenarios proposed for earth life’s origin, there is a wide ocean of worlds beyond Earth where a different chemistry may have been present, or where different planetary environments led to initially different, but nevertheless classical origins of life [4]. The chemical nature of life could differ from earthly biochemistry in almost every respect, and thus it is not constructive to try and devise a search for biosignatures based on Earth’s life.
5. Extremophiles: Life in Extreme Environments
The special microorganisms which are able to colonize severe conditions are called “extremophiles.” Almost all other organisms would perish in the severe conditions extremophiles thrive on. That would be the edge of temperature, hypersalinity, pH, pressure, dryness, and desiccation [14]. All three domains of life (Archea, Bacteria, and Eukarya) are among the extremophiles. Extremophiles have been found in a wide range of environments on Earth, wherever there is liquid water. Extremophiles thrive at 3 km depth under the surface, in nuclear reactors, hydrothermal vents and springs, acid mine drainages, acid rivers, and rivers with a temperature above 65 degrees Celsius. At the same time, extremophiles tolerate enormous toxicity. Most of the present technology on the detoxification of hazardous wastes is based on extremophilic biochemistry. Discovered inability of life to survive in unusual conditions that don’t exist on Earth (e.g. strong acid rivers) speaks against the possibility of extremophiles. The temperature limits compatible with the existence of life are imposed by the essential properties of chemical bonds at different temperatures. The average Earth temperature is 293 K (20 degrees Celsius). The upper limit for the life compatible with the presence of liquid water is thought to be below 110 degrees Celsius (183 K) [1]. Thermophiles are found in hot waters, geothermal areas, and sun-heated soils. Hyperthermophiles thrive between 80 and 122 degrees Celsius. Ice is a solid, crystalline, homogenous chemical state of water while liquid is an amorphous random state. Enormous structural changes occur with an increase in temperature. Life cannot exist in deep outer space and planetary interiors due to thermodynamic reasons. Thus, the temperature range within which life exists is commonly thought to be between 40 degrees Celsius and 100 degrees Celsius.
5.1. Types of Extremophiles
Recent discoveries, often landing on an astrobiologist’s desk, are evidence that water once flowed freely on the surface of Mars, molecules necessary for life have been found in the outside hot, cold, high-energy locations of the hot gas giant Jupiter, and elsewhere. Whether they can produce strings of atoms long enough to form life, and pondering the prospects for interstellar probes, credit for recent advances often passes over horizons of sunlight, doubt, and despair on the softly glowing seas of beach-ball sized planets forever hid from their parent stars buried in the solid ice of their shell [1]. It is indeed a celestial-crash-flame-burn-heated platinum case to be wontedly broken into pieces and zoomed through space on a glider to be eaten by a sun.
Life on Earth is fragile but desert-adapted leaves have been found in the wind-swept dust. Living spores of something have been found in the stratosphere. Extremophiles squeezing out the last atoms of energy from crudely fissioning rocks in the peppery rim of the Hclab crater have also been pictured. Clearly life can exist in conditions that were thought to be unthinkable earlier yet nothing happened for millions of years [15]. It has been also recently pictured blooming avalanche huge mats of microbes at depths where only inorganics were expected to survive. In good agreement with thermodynamic aspirations of an ocean world Darwinian meteorologists also envisage planetary snowball regimes with thick ice caps growing and collapsing repeatedly over gigayear timescales. It was discovered that moving giants migrate just enough to maintain habitable zones.
All above, together with the primordial conditions, seem to suggest that cold Neptunes would be a cradle of life. Of many doubts remaining about the very origins, type, and emergence of life there is certainty that if something appeared it was most likely simple, searching, bartering, and moving from planetesimal to planetesimal on a swarm-of-comets timescale. Something hybrid appearing on Earth-like planets in or near the hydrogen burning zone, in the co-density or co-thermal, fashion, shielding planetesimals with body masses concentrated near 1:1 in snaking mean motion resonances, seems to be possible.
5.2. Implications for the Search for Extraterrestrial Life
Although the search for extraterrestrial life is still very much in its infancy, extremophiles have provided clues as to what should be looked for and where. The most important lessons learnt to date are the following: planets can be transformed to seemingly extreme and lifeless environments by very common processes (so negative analogies should be screened); morphologically simple species able to colonize such extreme environments are likely to be successful; and the probability of survival, resilience, and adaptability of life increases with the extremity of the conditions [14]. Consequently, the emphasis in astrobiology has shifted from perennial negative analogies, such as those in Swifts Gulliver’s Travels, to positive ones based on extremophiles. On Earth, life has thrived on the edge of temperature, hypersalinity, pH, dryness, and desiccation. All domains of life (Eukarya, Bacteria, and Archae) are among the extremophiles, and life has been found at 3 km depth, in polar ice, very salty, and very acidic environments. Extending the search for life beyond Earth to similar extreme conditions within Mars and oceanic icy worlds of the outer solar system seems reasonable. This raises the question of relevance that extremophiles have to the possible existence of life in extreme conditions beyond our planet.
What is the nature of the extreme environments inhabitable by life? Life on Earth is based on the chemistry of carbon in water. Concentrations of dissolved chemical compounds and known salts define the lower threshold, whereas the upper temperature limit is imposed by the properties of chemical bonds at different temperatures. Covalent bonds should be sufficiently stable to allow the existence of large macromolecules (amino acids, nucleotides, and sugars). Non-covalent bonds (H2, ionic, hydrophobic, and van der Waals bonds) should be sufficiently labile to allow fast, specific, and reversible interactions. Reversible manifestations of temperature depend on the bonding characteristics of the chemical species that provide the buffering/damping of the temperature effects for each specific case. This chemical framing will mainly define the limits compatible with the existence of life.
6. Mars: Past, Present, and Future Exploration
Mars, the fourth planet from the sun, has long captivated human imagination. The Red Planet’s resemblance to Earth, along with its proximity, raises the fundamental question of whether Mars could one day be a second home for humanity [16]. Mars rovers have been exploring and sending data back to Earth for decades now, with major milestones. Each mission encountered its fare share of triumph and challenge. On February 2021, the Jezero crater was the target site for NASA’s Perseverance rover as it landed on Mars, collecting rocks in search of signs of ancient life. With the Curiosity rover exploring the Gale crater, there is much to learn about Mars’ past, present, and future [17]. The knowledge that many of the conditions for past life may be obtained from ancient rocks within a relatively small region of the Jezero crater motivates the exploration of this site with careful sample collection. A great deal of knowledge was gained about how water environments could have formed in and escaped the Jezero crater as well as what kinds of rocks and situational contexts within those rocks might preserve promising biosignatures to look for in the samples collected. This knowledge, along with what is known about the crater’s geography and earth’s ancient analogues, converged constructively on a story about how Jezero crater may have once hosted life.
The Mars Sample return (MSR) mission is a follow-up to the Perseverance rover and aims to tell this story in greater detail. The missions will investigate how the features and environmental conditions within these rocks change across the landscape in the crater, the geological context of rocks that may contain ancient Martian life, and the details of rock textures and chemistry using helicopters to climb hills and orbiters to support rovers. This will provide important context to the samples collected by Perseverance and search for features that look similar to biosignatures on Earth. These features would be of great astrobiological interest, and with plans to send life detection instruments, they may ultimately be the target of a future sample return. Sample return may play an important role in ensuring that the most promising biosignatures from Jezero crater do not go uninvestigated. Global Mars exploration will teach how landed missions can work together by collecting data to inform one another and how to operate distributed networks of spacecraft with different capabilities and designs.
6.1. Mars Missions and Discoveries
Mars has captivated humanity’s interest for centuries, but only in the last few decades have we begun to uncover its secrets. Several missions to Mars are in progress or planned, and important discoveries have recently been made. These discoveries have advanced understanding of the geology, climate, atmosphere, and history of Mars.
The Viking missions of the 1970s were the first probes to Mars. They sent back some remarkable pictures, as well as the first estimates of surface and atmospheric properties. A low resolution albedo map and high resolution false color pictures were generated from Viking Orbiter images. The extensive Viking lander data set has returned information on landing site temperatures and dust storm activities. It also contributed evidence for the possible existence of H2O (in the form of water ice). A search for clues about the past ocean was channelized because of the two general categories the models could be grouped into [17]. Only the younger martian geological and climatic history is still poorly understood.
The more recent Mars Global Surveyor (MGS), Mars Odyssey, and Mars Express missions have added greatly to the understanding of Mars. The principal results from these missions are presented. One major emphasis of the Mars program is finding indications of past or present life. Active research is underway to understand if life ever arose on Mars and at present whether it has survived [16].
6.2. Potential for Past Life on Mars
Mars is an intriguing place to search for life beyond Earth for many reasons: it has been geologically active, probably had a thicker atmosphere, a global magnetic field, and at least some surface water. Methane has been detected in the atmosphere [17] , several places with unusual surface features have suggested a recent water activity, and several minerals have been identified in situ to point to the previous or even current existence of liquid water. Cyano-bacteria and under 1 μm biofilms discovered in terrestrial hyper alkaline lakes showed similar behavior, growth and SHD signatures as the Martian clays. However, the existing photos and images of alleged formations do not exclusively point to biogenic processes. Such formations can also be made by geological processes of abiotically active clays [16].
Strongly compacted clays with icons of planets and moon formations, spheroidal and elliptical shapes, hypothesized habitats of inhabitable planets can be formed under certain similar conditions of mining and weathering. Compacting clay minerals triggers dramatic rock forming processes as shrinkage of 80% volume and 50% diameter. Just a stabling wetting interval enlarges the crack formation. The oval crystals can merge or vanish the dry fracturing under stable wetting and drying cycles. Thus, more simplistic tasks should be undertaken first in Mars exploration focused on exploring abiotic chemistry before extra-complex biogenic hypotheses.
7. Europa and Enceladus: Ocean Worlds in the Outer Solar System
The outer solar system is an intriguing region that challenges our understanding of where and how life could arise. Only a few ice-covered moons have been detected so far, which strongly resemble a recently proposed class of ocean worlds that contain a thick, near-surface ocean covered by an icy shell. Europa was the first discovered ocean world, and it has since generated great interest due to its large global ocean, geologically young ice shell, and compelling evidence for a recent plume. Enceladus may have a deeper subsurface ocean and cryovolcanic jets with a unique plume composition, including microcrystalline ice crystals and organic molecules. Typically, the search for biosignatures is conducted by trying to detect the end products or remnant signatures of microbial metabolism. Life that arose in hydrothermal systems in the absence of Hadean oceans may possess biosignatures that differ from common terrestrial life, representing a pressing need for additional laboratory experiments and models involving prebiotic chemistry under hydrothermal conditions [18].
Mars and the icy ocean moons around Jupiter and Saturn (in particular Europa and Enceladus) are prime goals in the astrobiological search for life within the Solar System. The rationale for expecting life on other Solar System bodies is simply the simultaneous presence of carbon molecules, an energy source, bio-essential elements and liquid water, either during the past or present day, for instance continuously on Europa and Enceladus [16]. Galileo and Cassini have provided important information regarding the Jupiter and Saturn system and, in particular, the icy moons orbiting these giant gas planets, which are believed to have liquid water–ocean beneath the icy crust. Specifically, the Jovian moon Europa and the Saturn moon Enceladus show cryo-plate tectonics and cryo-volcanism suggesting an interaction between the surface and postulated subsurface oceans. The Cassini spacecraft has observed gigantic jets of cryo-geysers (plumes) consisting of salt, water, and gases, such as CO2, N2 and methane (CH4) that could serve as major components to form and sustain microbial life.
7.1. Subsurface Oceans and Potential for Life
Moons are prime destinations for the exploration of life beyond the bounds of Earth. Two icy ocean moons with embarked missions are Jupiter’s Europa (in 2025 by NASA’s Europa Clipper spacecraft) and Saturn’s ocean moon Enceladus (in 2029 by the NASA/ESA/ASI Enceladus Life Finder spacecraft), which also has active cryovolcanic plumes, some of which are imagined to even penetrate deep into the ocean, up to 23 km depth [16]. Both moons are bathed in a radiation environment expected to be detrimental for life as we know it; nonetheless, subsurface oceans kept warm under ice shells are considered to have a climate favorable for life [18]. At the time of writing, no life detection missions have been flown to moons besides Earth, but recent developments in the exploration of icy moons and ongoing engineering efforts for future missions are elucidated.
Photometric and spectroscopic data suggest the presence of a water ocean beneath the icy crust of Europa. Europa has been recognized as a prime destination for astrobiology since the discovery of its surface features, which could be compatible with a liquid water ocean beneath the icy surface. Later discoveries by Galileo of the plumes originating from the putative ocean expelled by the Europan icy crust and containing simple salts, organics, and biogenic gases made Europa even more interesting with respect to the search for extraterrestrial life within the Solar System. However, geophysical modeling indicates that harsh Europa’s radiation environment, which generates oxidants, could just imperil astrobiological explorations there. Future missions to Europa are required to sample materials from beneath the icy crust or plumes rather than exploring the potentially biogenic surface materials directly.
8. SETI: The Search for Extraterrestrial Intelligence
The search for extraterrestrial intelligence (SETI) is perhaps the most romantic and captivating of scientific pursuits. Humans have wondered whether they are alone in this universe for millennia, and the scientific study of the question has been an organized effort for almost half a century. In the past, interest often derived from mythological writings or questionable relics, while today interest is driven by encouraging scientific developments, both in the knowledge of life-giving environments on other worlds and the engineering capabilities of detection technologies. Its common misconception is that SETI charges off madly with a few radio telescopes pointed at random stars, hoping to serendipitously catch a noise, a fantasy portrayed in Hollywood films. In truth, SETI is a balanced, sober effort set within very clear boundaries, many arising out of the same contemplative philosophizing that inspired its founders [19]. Humans wonder if technologically-driven civilizations exist elsewhere in the universe for many reasons. Many simply want to know, driven by a philosophical desire to understand humankind’s place in larger things. Others regard the search as essential to understanding the creation and destiny of the universe. Some entertain hope that a technologically-driven civilization elsewhere may wish to make itself known, or that it even has the ability to affect the future of mankind, for good or ill. Regardless of the underlying motive, an elaborate program beginning with observational surveys, directed investigations, and laboratory modeling of the signals has been set up to attempt to answer this fundamental question. However, given that little needs to be known to initiate the program and that the first conclusive proof of existence (or non-existence) cannot be dispensed with independently of the search being undertaken, the same program set at its initiation by its founders today remains essentially unchanged [6].
8.1. History of SETI
The search for extraterrestrial intelligence (SETI) is the scientific effort to locate evidence for extraterrestrial civilizations by searching for purposely transmitted, information-carrying signals [19]. The limits of detecting extraterrestrial intelligence (ETI) civilizations have been considered in common scientific contexts. Questions have been posed regarding the existence of ETI civilizations, addressing their abundance, representations, expected lifetimes, distances, and communication possibilities. Paralleling contemporary astronomical searches for planets around other stars, much thinking has gone into what ETI signals might be expected, what to look for, and how to look. The history of SETI is one of continually expanding scientific efforts, engines for pursuing the detection of ETI signals, and attempts at making a signal detection seem reasonable [6].
SETI has experienced an ebb and flow of credibility, scientific leadership, and prestige. The interest of scientists in the activity has had rich parallels in the interest of society, culture, and mass media. This coincidence augurs well for the future of the activity because it encourages the increased involvement of scientists in both the SETI search and the public discussion of results, present or absent. Several major sponsored initiatives in the quest for ETI, unobtrusive to participate in but ambitious in scope, are underway to conduct the search on a worldwide basis with built-in longevity, stability, and funding.
8.2. Current SETI Projects
The Search for Technosignatures: 20 Years On and Miles to Go examines the 20-year timeline of the scientific field of searching for technosignatures, which are scientific evidence for an active technological extraterrestrial civilization. It discusses the past perspectives on the field and what actually occurred in the last 20 years, emphasizing the growth of the field in both instrumentation and analyses, but also in outreach and educational efforts. It notes that in 20 years, much has been achieved, but also that there are still mountains to be climbed in terms of first detection. Nevertheless, excitement for the upcoming detection of extraterrestrial intelligence is burgeoning as the search is becoming ever more sophisticated [6].
The Limits of Detecting Extraterrestrial Civilizations examines the unique issues that arise in the search for extraterrestrial intelligence (SETI) and attempts at finding limits of possible communication between extraterrestrial intelligence (ETI) and human civilizations, as well as benchmarks for expected signals. There is a need for scientists to propose and prioritize possible signals [19].
9. Ethical and Philosophical Implications of Discovering Extraterrestrial Life
The potential discovery of extraterrestrial life has various implications that have been explored by scientists and philosophy scholars. Of particular concern are ethical questions about how humanity should conduct itself upon encountering other forms of life [20]. These questions are more pronounced when the other life forms discovered are intelligent, and thus able to communicate. Moreover, the very nature of the other life forms becomes relevant, so that concerns differ for single-celled organisms, plants, animals, and intelligent beings. Similarly, the character of the contact matters, such as whether it would be direct and close, but undetectable, or indirect and far away [21]. In many cases, the implications are difficult to predict beforehand, as humanity has never encountered beings from outside Earth and previous experiences have all been terrestrial. There are also reasons to believe that intelligent life elsewhere is either very rare or that it does not attempt to communicate. Each of these potentially divergent and extreme recommendations warrants further investigation.
The implications are likely to differ in important ways based on the parameters of the contact. The most widely discussed scenario involves the detection of radio signals emanating from a distant star system. Such a discovery would suggest that the source is either technologically sophisticated and highly capable, or alternatively, massive and so out of control that it unintentionally transmits visible signals. In the latter case, there would be little to fear. In the former case, much depends on the particulars of the signals, such as the likely motives of the intelligent beings who transmitted them, the nature of the existence patterns and the character of the grant itself.
10. Conclusion and Future Directions
The search for extraterrestrial life is among the most profound and exciting questions in science. Finding evidence for life beyond Earth would have profound implications for humanity’s understanding of its place in the Universe. The driving questions are: Is there life elsewhere? If so, what form does it take? Is it similar to life on Earth, or more exotic? Understanding the origin, evolution and distribution of life is a key to addressing these fundamental questions. Seeking evidence for habitable environments on other worlds, or for life beyond Earth, is a central focus of a broad range of scientific disciplines [4].
The question of extraterrestrial life is not a new one; nevertheless, we still seek an answer [22]. Indications are on man’s part that life must exist somewhere in the universe, outside of our planet. The first aspects to investigate are the conditions for life. If there are habitats for life beyond Earth, evidence for life in the form of bio-signatures should also be found here. Many definitions of life exist that focus on characteristics of Earth life. When there is no prior knowledge about life, habitability becomes a necessary and sufficient condition to experimentally exclude a location as a target for astrobiological studies. Understanding the origin, evolution and distribution of life is a key to addressing these profound questions.
References:
[1] M. Popovic, “Wanted Dead or Alive Extraterrestrial Life Forms (Thermodynamic criterion for life is a growing open system that performs self-assembly processes),” 2018. [PDF]
[2] D. Slade, A. Price, R. Hamp, and N. Ramkissoon, “Biosignatures in the solar system,” 2018. [PDF]
[3] D. Scott and A. Frolop, “The search for life and a new logic,” 2020. [PDF]
[4] C. Giri, T. Jia, H. James Cleaves, T. Usui et al., “Life Detection Technologies for the Next Two Decades,” 2018. [PDF]
[5] A. Loeb and M. Zaldarriaga, “Eavesdropping on Radio Broadcasts from Galactic Civilizations with Upcoming Observatories for Redshifted 21cm Radiation,” 2006. [PDF]
[6] C. D. Tremblay, S. Shynu Varghese, J. Hickish, P. Demorest et al., “COSMIC: An Ethernet-based Commensal, Multimode Digital Backend on the Karl G. Jansky Very Large Array for the Search for Extraterrestrial Intelligence,” 2023. [PDF]
[7] J. L. Nadeau, M. Bedrossian, and C. A. Lindensmith, “Imaging technologies and strategies for detection of extant extraterrestrial microorganisms,” 2018. [PDF]
[8] J. Horner and B. W. Jones, “Determining Habitability: Which exoEarths should we search for life?,” 2010. [PDF]
[9] D. M. Gelino, V. S. Airapetian, D. C. Catling, N. Narita et al., “Life Beyond the Solar System: Remotely Detectable Biosignatures,” 2018. [PDF]
[10] B. Cervetti and J. McCall, “Defining the Circumstellar Habitable Zone,” 2015. [PDF]
[11] J. Dunnuckb and J. M. Jenkins, “The Little Photometer That Could: Technical Challenges and Science Results from the Kepler Mission,” 2011. [PDF]
[12] N. M. Batalha, “Exploring Exoplanet Populations with NASA’s Kepler Mission,” 2014. [PDF]
[13] H. B. Smith and C. Mathis, “The Futility of Exoplanet Biosignatures,” 2022. [PDF]
[14] S. Dejan B., F. Oliver, D. Aleksandra V., Čajko Kristina O. et al., “Extremophiles: Link between earth and astrobiology,” 2008. [PDF]
[15] D. Schulze-Makuch, A. Schulze-Makuch, and J. M. Houtkooper, “The Physical, Chemical and Physiological Limits of Life,” 2015. ncbi.nlm.nih.gov
[16] J. P. de Vera, M. Baque, D. Billi, U. Böttger et al., “The search for Life on Mars and the Solar System – Strategies, Logostics and Infrastructures,” 2018. [PDF]
[17] H. G. Changela, E. Chatzitheodoridis, A. Antunes, D. Beaty et al., “Mars: new insights and unresolved questions,” 2021. [PDF]
[18] D. Deamer and B. Damer, “Can Life Begin on Enceladus? A Perspective from Hydrothermal Chemistry,” 2017. ncbi.nlm.nih.gov
[19] I. George, X. Chen, and L. R. Varshney, “Limits of Detecting Extraterrestrial Civilizations,” 2021. [PDF]
[20] S. D. Baum, J. D. Haqq-Misra, and S. D. Domagal-Goldman, “Would contact with extraterrestrials benefit or harm humanity? A scenario analysis,” 2011. [PDF]
[21] F. Neukart, “Toward the Stars: Technological, Ethical, and Sociopolitical Dimensions of Interstellar Exploration,” 2024. [PDF]
[22] O. Lee Wilkins, “On the possibility of extraterrestrial life,” 1967. [PDF]