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We have only one example of biology forming in the universe – life on Earth. But what if life can form in other ways? How do you look for alien life when you don’t know what alien life might look like?
These questions are preoccupying astrobiologists, who are scientists who look for life beyond Earth. Astrobiologists have attempted to come up with universal rules that govern the emergence of complex physical and biological systems both on Earth and beyond.
I’m an astronomer who has written extensively about astrobiology. Through my research, I’ve learned that the most abundant form of extraterrestrial life is likely to be microbial, since single cells can form more readily than large organisms. But just in case there’s advanced alien life out there, I’m on the international advisory council for the group designing messages to send to those civilizations.
Detecting life beyond Earth
Since the first discovery of an exoplanet in 1995, over 5,000 exoplanets, or planets orbiting other stars, have been found.
Many of these exoplanets are small and rocky, like Earth, and in the habitable zones of their stars. The habitable zone is the range of distances between the surface of a planet and the star it orbits that would allow the planet to have liquid water, and thus support life as we on Earth know it.
The sample of exoplanets detected so far projects 300 million potential biological experiments in our galaxy – or 300 million places, including exoplanets and other bodies such as moons, with suitable conditions for biology to arise.
The uncertainty for researchers starts with the definition of life. It feels like defining life should be easy, since we know life when we see it, whether it’s a flying bird or a microbe moving in a drop of water. But scientists don’t agree on a definition, and some think a comprehensive definition might not be possible.
NASA defines life as a “self-sustaining chemical reaction capable of Darwinian evolution.” That means organisms with a complex chemical system that evolve by adapting to their environment. Darwinian evolution says that the survival of an organism depends on its fitness in its environment.
The evolution of life on Earth has progressed over billions of years from single-celled organisms to large animals and other species, including humans.
Exoplanets are remote and hundreds of millions of times fainter than their parent stars, so studying them is challenging. Astronomers can inspect the atmospheres and surfaces of Earth-like exoplanets using a method called spectroscopy to look for chemical signatures of life.
Spectroscopy might detect signatures of oxygen in a planet’s atmosphere, which microbes called blue-green algae created by photosynthesis on Earth several billion years ago, or chlorophyll signatures, which indicate plant life.
NASA’s definition of life leads to some important but unanswered questions. Is Darwinian evolution universal? What chemical reactions can lead to biology off Earth?
Evolution and complexity
All life on Earth, from a fungal spore to a blue whale, evolved from a microbial last common ancestor about 4 billion years ago.
The same chemical processes are seen in all living organisms on Earth, and those processes might be universal. They also may be radically different elsewhere.
In October 2024, a diverse group of scientists gathered to think outside the box on evolution. They wanted to step back and explore what sort of processes created order in the universe – biological or not – to figure out how to study the emergence of life totally unlike life on Earth.
Two researchers present argued that complex systems of chemicals or minerals, when in environments that allow some configurations to persist better than others, evolve to store larger amounts of information. As time goes by, the system will grow more diverse and complex, gaining the functions needed for survival, through a kind of natural selection.
Doug Bowman, CC BY
They speculated that there might be a law to describe the evolution of a wide variety of physical systems. Biological evolution through natural selection would be just one example of this broader law.
In biology, information refers to the instructions stored in the sequence of nucleotides on a DNA molecule, which collectively make up an organism’s genome and dictate what the organism looks like and how it functions.
If you define complexity in terms of information theory, natural selection will cause a genome to grow more complex as it stores more information about its environment.
Complexity might be useful in measuring the boundary between life and nonlife.
However, it’s wrong to conclude that animals are more complex than microbes. Biological information increases with genome size, but evolutionary information density drops. Evolutionary information density is the fraction of functional genes within the genome, or the fraction of the total genetic material that expresses fitness for the environment.
Organisms that people think of as primitive, such as bacteria, have genomes with high information density and so appear better designed than the genomes of plants or animals.
A universal theory of life is still elusive. Such a theory would include the concepts of complexity and information storage, but it would not be tied to DNA or the particular kinds of cells we find in terrestrial biology.
Implications for the search for extraterrestial life
Researchers have explored alternatives to terrestrial biochemistry. All known living organisms, from bacteria to humans, contain water, and it is a solvent that is essential for life on Earth. A solvent is a liquid medium that facilitates chemical reactions from which life could emerge. But life could potentially emerge from other solvents, too.
Astrobiologists Willam Bains and Sara Seager have explored thousands of molecules that might be associated with life. Plausible solvents include sulfuric acid, ammonia, liquid carbon dioxide and even liquid sulfur.
Alien life might not be based on carbon, which forms the backbone of all life’s essential molecules – at least here on Earth. It might not even need a planet to survive.
Advanced forms of life on alien planets could be so strange that they’re unrecognizable. As astrobiologists try to detect life off Earth, they’ll need to be creative.
One strategy is to measure mineral signatures on the rocky surfaces of exoplanets, since mineral diversity tracks terrestrial biological evolution. As life evolved on Earth, it used and created minerals for exoskeletons and habitats. The hundred minerals present when life first formed have grown to about 5,000 today.
For example, zircons are simple silicate crystals that date back to the time before life started. A zircon found in Australia is the oldest known piece of Earth’s crust. But other minerals, such as apatite, a complex calcium phosphate mineral, are created by biology. Apatite is a primary ingredient in bones, teeth and fish scales.
Another strategy to finding life unlike that on Earth is to detect evidence of a civilization, such as artificial lights, or the industrial pollutant nitrogen dioxide in the atmosphere. These are examples of tracers of intelligent life called technosignatures.
It’s unclear how and when a first detection of life beyond Earth will happen. It might be within the solar system, or by sniffing exoplanet atmospheres, or by detecting artificial radio signals from a distant civilization.
The search is a twisting road, not a straightforward path. And that’s for life as we know it – for life as we don’t know it, all bets are off.
Chris Impey receives funding from the National Science Foundation and the Howard Hughes Medical Institute.
This article is republished from The Conversation under a Creative Commons license.
10 Best-Selling Science Fiction Books Worth Reading
Dune
Frank Herbert’s Dune is a classic science fiction novel that follows Paul Atreides after his family takes control of Arrakis, a desert planet whose spice is the most valuable resource in the universe. The story combines political struggle, ecology, religion, and warfare as rival powers contest the planet and Paul is drawn into a conflict that reshapes an interstellar civilization. It remains a foundational space opera known for its worldbuilding and long-running influence on the science fiction genre.
Foundation
Isaac Asimov’s Foundation centers on mathematician Hari Seldon, who uses psychohistory to forecast the collapse of a galactic empire and designs a plan to shorten the coming dark age. The narrative spans generations and focuses on institutions, strategy, and social forces rather than a single hero, making it a defining work of classic science fiction. Its episodic structure highlights how knowledge, politics, and economic pressures shape large-scale history.
Ender’s Game
Orson Scott Card’s Ender’s Game follows Andrew “Ender” Wiggin, a gifted child recruited into a military training program designed to prepare humanity for another alien war. The novel focuses on leadership, psychological pressure, and ethical tradeoffs as Ender is pushed through increasingly high-stakes simulations. Often discussed as military science fiction, it also examines how institutions manage talent, fear, and information under existential threat.
The Hitchhiker’s Guide to the Galaxy
Douglas Adams’s The Hitchhiker’s Guide to the Galaxy begins when Arthur Dent is swept off Earth moments before its destruction and launched into an absurd interstellar journey. Blending comedic science fiction with satire, the book uses space travel and alien societies to lampoon bureaucracy, technology, and human expectations. Beneath the humor, it offers a distinctive take on meaning, randomness, and survival in a vast and indifferent cosmos.
1984
George Orwell’s 1984 portrays a surveillance state where history is rewritten, language is controlled, and personal autonomy is systematically dismantled. The protagonist, Winston Smith, works within the machinery of propaganda while privately resisting its grip, which draws him into escalating danger. Frequently categorized as dystopian fiction with strong science fiction elements, the novel remains a reference point for discussions of authoritarianism, mass monitoring, and engineered reality.
Brave New World
Aldous Huxley’s Brave New World presents a society stabilized through engineered reproduction, social conditioning, and pleasure-based control rather than overt terror. The plot follows characters who begin to question the costs of comfort, predictability, and manufactured happiness, especially when confronted with perspectives that do not fit the system’s design. As a best-known dystopian science fiction book, it raises enduring questions about consumerism, identity, and the boundaries of freedom.
Fahrenheit 451
Ray Bradbury’s Fahrenheit 451 depicts a future where books are outlawed and “firemen” burn them to enforce social conformity. The protagonist, Guy Montag, begins as a loyal enforcer but grows increasingly uneasy as he encounters people who preserve ideas and memory at great personal risk. The novel is often read as dystopian science fiction that addresses censorship, media distraction, and the fragility of informed public life.
The War of the Worlds
H. G. Wells’s The War of the Worlds follows a narrator witnessing an alien invasion of England, as Martian technology overwhelms existing military and social structures. The story emphasizes panic, displacement, and the collapse of assumptions about human dominance, offering an early and influential depiction of extraterrestrial contact as catastrophe. It remains a cornerstone of invasion science fiction and helped set patterns still used in modern alien invasion stories.
Neuromancer
William Gibson’s Neuromancer follows Case, a washed-up hacker hired for a high-risk job that pulls him into corporate intrigue, artificial intelligence, and a sprawling digital underworld. The book helped define cyberpunk, presenting a near-future vision shaped by networks, surveillance, and uneven power between individuals and institutions. Its language and concepts influenced later depictions of cyberspace, hacking culture, and the social impact of advanced computing.
The Martian
Andy Weir’s The Martian focuses on astronaut Mark Watney after a mission accident leaves him stranded on Mars with limited supplies and no immediate rescue plan. The narrative emphasizes problem-solving, engineering improvisation, and the logistical realities of survival in a hostile environment, making it a prominent example of hard science fiction for general readers. Alongside the technical challenges, the story highlights teamwork on Earth as agencies coordinate a difficult recovery effort.
10 Best-Selling Science Fiction Movies to Watch
Interstellar
In a near-future Earth facing ecological collapse, a former pilot is recruited for a high-risk space mission after researchers uncover a potential path to another star system. The story follows a small crew traveling through extreme environments while balancing engineering limits, human endurance, and the emotional cost of leaving family behind. The narrative blends space travel, survival, and speculation about time, gravity, and communication across vast distances in a grounded science fiction film framework.
Blade Runner 2049
Set in a bleak, corporate-dominated future, a replicant “blade runner” working for the police discovers evidence that could destabilize the boundary between humans and engineered life. His investigation turns into a search for hidden history, missing identities, and the ethical consequences of manufactured consciousness. The movie uses a cyberpunk aesthetic to explore artificial intelligence, memory, and state power while building a mystery that connects personal purpose to civilization-scale risk.
Arrival
When multiple alien craft appear around the world, a linguist is brought in to establish communication and interpret an unfamiliar language system. As global pressure escalates, the plot focuses on translating meaning across radically different assumptions about time, intent, and perception. The film treats alien contact as a problem of information, trust, and geopolitical fear rather than a simple battle scenario, making it a standout among best selling science fiction movies centered on first contact.
Inception
A specialist in illicit extraction enters targets’ dreams to steal or implant ideas, using layered environments where time and physics operate differently. The central job requires assembling a team to build a multi-level dream structure that can withstand psychological defenses and internal sabotage. While the movie functions as a heist narrative, it remains firmly within science fiction by treating consciousness as a manipulable system, raising questions about identity, memory integrity, and reality testing.
Edge of Tomorrow
During a war against an alien force, an inexperienced officer becomes trapped in a repeating day that resets after each death. The time loop forces him to learn battlefield tactics through relentless iteration, turning failure into training data. The plot pairs kinetic combat with a structured science fiction premise about causality, adaptation, and the cost of knowledge gained through repetition. It is often discussed as a time-loop benchmark within modern sci-fi movies.
Ex Machina
A young programmer is invited to a secluded research facility to evaluate a humanoid robot designed with advanced machine intelligence. The test becomes a tense psychological study as conversations reveal competing motives among creator, evaluator, and the synthetic subject. The film keeps its focus on language, behavior, and control, using a contained setting to examine artificial intelligence, consent, surveillance, and how people rationalize power when technology can convincingly mirror human emotion.
The Fifth Element
In a flamboyant future shaped by interplanetary travel, a cab driver is pulled into a crisis involving an ancient weapon and a looming cosmic threat. The story mixes action, comedy, and space opera elements while revolving around recovering four elemental artifacts and protecting a mysterious figure tied to humanity’s survival. Its worldbuilding emphasizes megacities, alien diplomacy, and high-tech logistics, making it a durable entry in the canon of popular science fiction film.
Terminator 2: Judgment Day
A boy and his mother are pursued by an advanced liquid-metal assassin, while a reprogrammed cyborg protector attempts to keep them alive. The plot centers on preventing a future dominated by autonomous machines by disrupting the chain of events that leads to mass automation-driven catastrophe. The film combines chase-driven suspense with science fiction themes about AI weaponization, time travel, and moral agency, balancing spectacle with character-driven stakes.
Minority Report
In a future where authorities arrest people before crimes occur, a top police officer becomes a suspect in a predicted murder and goes on the run. The story follows his attempt to challenge the reliability of predictive systems while uncovering institutional incentives to protect the program’s legitimacy. The movie uses near-future technology, biometric surveillance, and data-driven policing as its science fiction core, framing a debate about free will versus statistical determinism.
Total Recall (1990)
A construction worker seeking an artificial vacation memory experiences a mental break that may be either a malfunction or the resurfacing of a suppressed identity. His life quickly becomes a pursuit across Mars involving corporate control, political insurgency, and questions about what is real. The film blends espionage, off-world colonization, and identity instability, using its science fiction premise to keep viewers uncertain about whether events are authentic or engineered perception.
