ASTR-2040 Final Project · Maya Hernandez-Diaz
Exploring what life is, what it needs, and where it might exist in a universe this vast.
Introduction
On a clear night, away from city lights, the number of visible stars can be striking and difficult to fully comprehend. The Milky Way alone has somewhere between 100 and 400 billion stars, and that’s just one galaxy out of about two trillion in the observable universe. When considering these numbers, it can seem highly unlikely that life exists solely on Earth, a single planet among an enormous number of stars and galaxies. However, despite this vastness, Earth remains the only location where the existence of life has been definitively confirmed. So the question is: are we really alone?
"The universe is under no obligation to make sense to you." — Neil deGrasse Tyson
That question is central to astrobiology, an interdisciplinary field that brings together astronomy, geology, chemistry, and biology to study the origin, evolution, and potential existence of life beyond Earth. Once considered speculative, it has become a major area of scientific research. This article explores what we currently know, including what life is, what it needs to survive, where it might exist in our solar system, and what the numbers suggest about other civilizations.
Section 01
Before asking whether life exists elsewhere, we need to pin down what life actually is. It turns out this question has many layers to it and can be surprisingly complicated to answer, especially when we start comparing very different kinds of entities that share some life-like traits. Stars are born, evolve, use energy, and eventually die, but we do not consider them alive. Mules are clearly living organisms, yet they cannot reproduce. Viruses can replicate, but only by relying on a host cell. A thermostat can respond to changes in its environment, and crystals form highly ordered structures. Each of these examples reflects a different aspect of what we associate with life, but none of them fully meets the criteria on its own.
Scientists generally point to six key properties that living organisms share: internal order and organization, the ability to use energy, reproduction, responsiveness to the environment, growth and development guided by genetic information, and the capacity for evolution over time. The challenge is that no single characteristic is sufficient on its own; rather, it is the combination of these traits that defines life.
Viruses
Viruses are the most abundant biological entities on Earth and have likely existed since the earliest cells. They meet some criteria for life, such as having an organized structure and the ability to evolve. However, they cannot reproduce, grow, or respond to their environment without relying on a host cell. This gray area has led to the development of astrovirology, a field that studies viruses as possible models for primitive or unconventional forms of life beyond Earth.
All known life on Earth is carbon-based and relies heavily on a small set of key elements. In living organisms, oxygen (65%), carbon (18.5%), hydrogen (9.5%), and nitrogen (3.3%) make up the vast majority of biological material. Carbon in particular plays a central role because of its exceptional versatility. It can form stable chains of varying lengths, branch into complex structures, create single and double bonds, and form rings. This flexibility allows for an almost limitless diversity of molecular structures.
From this chemical foundation, life is built around four major classes of biomolecules. Carbohydrates provide energy and structural support, lipids form cell membranes and store energy, proteins act as the primary drivers of biochemical processes, and nucleic acids, such as DNA and RNA, store and transmit genetic information. Despite the enormous diversity of life on Earth, all organisms rely on these same four molecular families, a consistency that strongly suggests a shared evolutionary origin.
"The universe is dissymmetric, and I am persuaded that life, as it is known to us, is a direct result of the dissymmetry of the universe." — Louis Pasteur
One of biology’s most interesting clues about our shared ancestry comes from a property called chirality. Many molecules can exist in two mirror-image forms, like left and right hands. When amino acids are created in a lab without any biological influence, they typically form an even 50/50 mix of left- and right-handed versions. But in living organisms, all amino acids are left-handed, while all sugars are right-handed, and this pattern holds true across every form of life on Earth.
This consistent bias reflects a deeper kind of asymmetry in nature, often referred to as dissymmetry, a lack of perfect mirror symmetry at the molecular level. It suggests that life did not simply arise from random chemistry, but was shaped by underlying directional preferences in how biological molecules form and interact.
If life had originated independently multiple times, we would expect to see a more random distribution of molecular handedness. Instead, the consistency points to a single shared origin for all life, known as LUCA, the Last Universal Common Ancestor. Even more intriguing, chiral molecules have also been found in interstellar space, suggesting that the chemistry underlying life may be more widespread and complex than we once thought.
Section 02
Knowing what life is made of is only half the puzzle. The other half is understanding the environmental conditions that allow it to exist. Astrobiologists often frame this through what is sometimes called the Habitability Triangle, which describes three requirements that must be present at the same time.
Liquid water is the cornerstone. Water is an exceptional solvent that enables the chemistry of life. It helps stabilize large molecules, allows nutrients to move through cells, and participates directly in many biochemical reactions. Without it, the chemical processes we associate with life as we know it essentially come to a halt.
CHNOPS elements — Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, and Sulfur. These are the basic building blocks of biochemistry. Together, these six elements make up about 98% of all living matter. They are also among the most abundant elements in the universe, produced in the cores of massive stars and distributed through supernova explosions. This means that the raw ingredients for life are not rare, but widely available.
Energy is needed to power biological processes and maintain the organized complexity that life depends on. On Earth, much of this energy comes from sunlight through photosynthesis. However, ecosystems discovered around hydrothermal vents on the ocean floor show that life can also thrive without sunlight, powered instead by chemical energy from Earth’s interior. This finding significantly broadened our understanding of where life might be possible.
Key Concept · Plate Tectonics
Earth is the only planet in our solar system with active plate tectonics, and this appears to be important for long-term habitability. The movement of tectonic plates drives a CO₂ cycle that acts like a natural thermostat. When Earth warms, more CO₂ is removed from the atmosphere through weathering. When it cools, volcanic activity releases CO₂ back into the atmosphere, strengthening the greenhouse effect and helping stabilize temperatures.
This feedback system has kept Earth’s surface conditions stable enough for liquid water to exist for over four billion years. In contrast, Mars and Mercury cooled and became geologically inactive, while Venus likely lost its oceans to a runaway greenhouse effect. Earth’s stability raises an open question: was this balance luck, or a more common outcome on habitable worlds than we currently understand?
For most of Earth's history, the atmosphere contained essentially no free oxygen. The first life was anaerobic, thriving in an oxygen-free world. Then, around 2.7 billion years ago, a microscopic organism called cyanobacteria began producing oxygen as a byproduct of photosynthesis. For many anaerobic organisms, this was catastrophic, effectively a mass extinction caused by oxygen, which was toxic in their environment.
But oxygen also unlocked something unprecedented. It allowed for far more efficient energy production inside cells, making it possible for life to grow more complex. Over time, this shift supported the emergence of multicellular organisms and eventually led to plants, animals, and humans. The implication for astrobiology is significant. The long delay before oxygen accumulated on Earth suggests that complex life may be rare in the universe. Many planets, if they host life at all, may never progress beyond simple microbial ecosystems.
Section 03
Even if a planet contains all the necessary ingredients, life still has to emerge. The transition from chemistry to biology is one of the deepest unsolved problems in science, but we have strong clues.
In 1953, Stanley Miller and Harold Urey ran a landmark experiment now known as the Miller-Urey experiment. They filled a flask with water, ammonia, hydrogen, and methane to simulate early Earth’s atmosphere, then introduced electrical sparks to mimic lightning. Within days, amino acids began to form. This showed that the basic building blocks of life can emerge naturally from nonliving chemistry.
Key Experiment · Miller-Urey (1953)
Mix water, ammonia, hydrogen, and methane, then add electricity and wait. Amino acids begin to form. Later analysis of the original samples revealed that silicon from the glass apparatus, acting as a stand-in for rock, may have increased the yield. Since then, organic molecules have also been found in hydrothermal vents, as well as in asteroids and comets, suggesting that the ingredients for life are widespread throughout the universe.
The RNA World hypothesis is a leading explanation for the origin of life, proposing that RNA existed before DNA and played a central role in early biological systems. This model resolves the “chicken-and-egg” problem in molecular biology, since DNA requires proteins for replication, while protein synthesis depends on DNA. RNA can perform both roles because it is capable of storing genetic information and acting as a catalyst without proteins.
This idea was supported by the discovery of catalytic RNA molecules, by University of Colorado Boulder scientist Thomas Cech, who received the 1989 Nobel Prize in Chemistry. His work showed that RNA can function enzymatically, making it plausible that early life could operate without complex protein machinery.
According to the hypothesis, life began with simple organic molecules that formed RNA strands, possibly on mineral surfaces that helped organize chemical reactions. Some RNA molecules gained catalytic and self-replicating abilities, eventually becoming enclosed in lipid membranes to form primitive proto-cells. Over time, RNA-based life evolved, with DNA later emerging as a more stable genetic material due to its double-stranded structure and improved error correction. DNA-based organisms eventually replaced RNA-based systems, forming the basis of all modern life.
Section 04
In 1961, astronomer Frank Drake proposed a way to organize our ignorance. Instead of asking whether intelligent life exists on other planets, he broke the question into measurable components. The result was the Drake Equation, not a formula designed to produce a definitive answer, but a framework for identifying what we still need to figure out.
With optimistic assumptions (fl = fi = fc = 1, L = 10,000 years), the equation yields N ≈ 100,000 civilizations currently detectable in the Milky Way. Under more conservative assumptions, N approaches 1, and that is us. The honest answer is we genuinely do not know the values of the key biological and technological terms in the equation, meaning we have no real data to estimate how often life begins, how often intelligence emerges, how often civilizations develop detectable technology, or how long they tend to last.
SETI · Listening for a Signal
If intelligent civilizations exist and have developed technology, they might be broadcasting signals, intentionally or not. SETI (Search for Extraterrestrial Intelligence) is the organized effort to detect such signals. The Breakthrough Listen project uses the Green Bank Telescope in West Virginia to scan the closest million stars for unusual radio and laser emissions. It is essentially a large-scale attempt to find patterns in cosmic noise that do not look natural. We have also been broadcasting our own presence for about a century through radio and television signals. As a result, a sphere of “human noise” roughly 200 light-years in radius is now expanding outward through the galaxy.
What we do know is that planet formation is extremely common. NASA’s Kepler and TESS missions have shown that virtually every star hosts at least one planet. This strongly suggests that the obstacle must lie in fl, fi, fc, or L. In other words, the limiting step is not the existence of planets, but what happens after they form. Either life rarely starts at all, intelligence rarely evolves from life, technological civilizations rarely develop in a detectable way, or civilizations do not last very long once they become capable of being seen across interstellar distances.
Section 05
We do not need to travel to another star to search for life. Several worlds right here in our solar system are strong candidates, and missions are already actively targeting them. These environments are not Earth-like on the surface, but they may contain the basic ingredients for life in hidden or subsurface habitats, especially where liquid water could exist beneath ice or rock.
Planet · Inner Solar System
Mars once had liquid water on its surface, as shown by ancient river valleys and lake beds. Today it is cold and dry, but liquid water likely still exists underground. Of roughly 61,000 meteorites found on Earth, 224 are confirmed to be from Mars, ejected during asteroid impacts. If life ever existed on Mars, it may have survived underground. And here's a twist: Earth microbes have almost certainly traveled to Mars via meteorites, so finding life there wouldn't automatically mean it originated there independently.
Moon · Jupiter System
Beneath Europa’s icy crust lies a liquid water ocean, possibly containing twice the volume of all Earth’s oceans combined. Tidal heating from Jupiter’s gravity keeps this ocean from freezing solid. It likely interacts with a rocky seafloor, which could create hydrothermal vents similar to those that support thriving ecosystems on Earth. NASA’s Europa Clipper mission, launched in 2024, is currently en route to study the moon in detail. If anywhere in the solar system besides Earth could harbor life, Europa is one of the most promising candidates.
Moon · Saturn System
Enceladus actively vents plumes, jets of water vapor and ice erupting from its south pole, into space. NASA’s Cassini spacecraft flew through these plumes and detected water, organic compounds, silica particles indicating hot water–rock interaction, and even molecular hydrogen, a chemical energy source that supports life on Earth’s ocean floors. Enceladus may have all three corners of the habitability triangle: liquid water, CHNOPS elements, and chemical energy.
Moon · Saturn System
Titan is a wild card. It has a thick nitrogen atmosphere and stable rivers and lakes, but they are filled with liquid methane and ethane rather than water. It is far too cold for water-based life as we know it, but some scientists speculate that an entirely different form of biochemistry could exist, using liquid methane as a solvent. Titan pushes us to think beyond Earth-centric definitions of life. NASA’s Dragonfly rotorcraft is scheduled to land there in 2034 to investigate its chemistry and habitability potential.
Section 06
In 1950, physicist Enrico Fermi sat down to lunch with colleagues and asked a deceptively simple question: if the galaxy is billions of years old and potentially teeming with civilizations, why have we not heard from any of them? This is the Fermi Paradox, the tension between the high probability of extraterrestrial life and the complete lack of evidence for it.
"The universe is a pretty big place. If it's just us, seems like an awful waste of space." - Carl Sagan
There are many proposed explanations, though none are confirmed. One possibility is that civilizations routinely destroy themselves before developing the ability to communicate across interstellar distances, an idea often called the Great Filter hypothesis. Another is that the distances are simply too vast and signals too faint to detect. Perhaps intelligent life is far rarer than we assume, since evolution on Earth took over four billion years and required an extraordinary chain of unlikely events. Or, more unsettlingly, the Great Filter may still lie ahead of us rather than behind.
The James Webb Space Telescope is already revolutionizing the search. By analyzing starlight filtered through exoplanet atmospheres, a method known as transmission spectroscopy, JWST can detect the chemical signatures of gases such as water, methane, oxygen, ozone, and carbon dioxide. The simultaneous presence of oxygen and methane, for example, would be a strong biosignature. These gases react with each other quickly, so finding both in the same atmosphere suggests something must be continuously replenishing them. We have not detected such a signal yet, but we are now capable of searching in ways that were previously impossible.
One of the most important recent discoveries is Henneguya salminicola, a tiny ten-celled parasite and the first known animal that does not require oxygen to survive. It has completely lost the genes responsible for aerobic respiration. This discovery highlights how much metabolic diversity already exists on Earth. Life elsewhere could be even more unfamiliar, chemically or structurally different enough that we might not recognize it immediately. The search for life in the universe may ultimately require us to rethink how we define “life.”
Conclusion
So what do we truly know about the search for life? We know that the building blocks of life form naturally through chemistry, something we have demonstrated in a lab and observed in organic molecules found on asteroids and in interstellar space. Planet formation is common, and nearly every star we have examined appears to host planets. Liquid water exists beneath the ice of Europa and Enceladus. Life on Earth has also proven extraordinarily resilient, surviving in environments once thought impossible, including boiling hydrothermal vents, the freezing vacuum of space, and regions more than two miles underground.
And yet, the transition from chemistry to biology is only known to have happened once, and it took billions of years. Complex, oxygen-breathing life emerged even later. The universe has been running this experiment for 13.8 billion years, and the only confirmed result so far is us.
"Two possibilities exist: Either we are alone in the Universe or we are not. Both are equally terrifying." - Arthur C. Clarke
The honest scientific answer to “are we alone?” is that we don't know yet. But for the first time in human history, we are genuinely equipped to begin finding out. Space telescopes are scanning exoplanet atmospheres for biosignatures. Spacecrafts are orbiting and landing on moons that may support life. The unknown terms in the Drake Equation may not remain unknown forever.
The most profound insight from the search for life in space is not about aliens at all, but instead about life here on Earth. Understanding what made Earth habitable, how life began, and what keeps it going is essential for preserving the only confirmed example of life in the universe. Although there is much to discover beyond Earth, we would do just as well to focus on the unknowns here at home.
Sources & Further Reading