Extraterrestrial Life: What are the Chances?

Let's get out of the medical laboratory for a second to explore something called ASTROBIOLOGY, that is, the study of life in the universe outside of Earth - Literally, STAR (astro) STUDY OF LIFE (biology).

The chances that life exists outside of Earth are widely considered by scientists to be highly probable, though definitive evidence has not yet been discovered. This belief stems from a convergence of astrophysical, chemical, and biological reasoning that points to life being a natural consequence of the conditions that seem to be common across the universe.

The upcoming movie, Project Hail Mary (based on the excellent book by Andy Weir) explores the possibility of life outside of our solar system and what it might look like and a science teacher's unlikely trip to the cosmos on a long-shot mission to save the earth.

Here, we describe how thought has evolved as humans have explored the possibility of extraterrestrial life and what is being done today to answer the question: "Are we alone?"

Aliens: an imminent threat to human existence, or just fun guys?

The Immensity of the Universe

The observable universe contains over two trillion galaxies, each with millions to billions of stars, many of which are orbited by planets. Current estimates suggest there may be more than 10²⁴ planets in the observable universe. Even if life were an extraordinarily rare phenomenon—occurring on only one in a billion planets—that would still leave billions of worlds potentially harboring life.

The Drake Equation and Probabilistic Arguments

In 1960, Frank Drake was working at the National Radio Astronomy Observatory (NRAO) in Green Bank, West Virginia. There, he conducted the first modern SETI experiment, known as Project Ozma, in which he aimed a radio telescope at two nearby Sun-like stars—Tau Ceti and Epsilon Eridani—to listen for possible signals from intelligent alien life. While the project detected no signals, it ignited interest in the scientific community and laid the groundwork for what was to come. Later that year, Drake was asked to host a small, confidential conference at Green Bank to discuss the future of SETI. The meeting was funded by the National Academy of Sciences and convened in 1961. It brought together a dozen leading thinkers from various disciplines—astronomy, biology, chemistry, engineering, and more—including notable names like Carl Sagan, Melvin Calvin, and Philip Morrison.

As Drake prepared for the meeting, he faced a problem. The question of how to detect intelligent life beyond Earth was vast, complex, and overwhelming. There was no clear framework for how to talk about it in a structured way. So Drake decided to create a kind of roadmap—an equation—to help guide the discussion and quantify the problem.

The Drake Equation is a probabilistic framework used to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way. While many of its variables remain uncertain, improvements in astronomy have allowed better estimates of factors like the rate of star formation and the fraction of stars with planets. When reasonable values are plugged into the equation, the result typically suggests that life is likely to have emerged elsewhere.

The equation Drake wrote on the chalkboard for the opening of the Green Bank meeting was:

N = R* × fₚ × nₑ × fₗ × fᵢ × f_c × L

Each term in the equation represented a factor necessary to estimate the number (N) of communicative extraterrestrial civilizations in the Milky Way galaxy. The variables were:

R*: The average rate of star formation per year in our galaxy

fₚ: The fraction of those stars that have planetary systems

nₑ: The average number of planets that could support life per star with planets

fₗ: The fraction of those planets where life actually develops

fᵢ: The fraction of planets with life where intelligent life emerges

f_c: The fraction of intelligent species that develop technology to communicate across interstellar distances

L: The length of time such civilizations release detectable signals into space

So, while this isn't an exact science, the Drake Equation provides a ballpark, back-of-the envelope calculation for our chances of civilizations advanced enough to communicate.

There is one important factor that would guide the amount of time that one of these civilizations is communicative: That is the amount of time before that civilization possibly collapses and becomes non-communicative.

The future is bright.

Enrico Fermi and the Fermi Paradox: Where Is Everybody?

Enrico Fermi was an Italian-American physicist and Nobel laureate, best known for his work on nuclear physics and the development of the first nuclear reactor. He was a brilliant theoretical and experimental physicist, often blending deep mathematical insight with practical engineering skill. Fermi played a central role in the Manhattan Project and is considered one of the founding figures of modern particle physics.

The Fermi Paradox emerged informally from a casual lunchtime conversation in 1950 at Los Alamos National Laboratory. Fermi and his colleagues—among them Edward Teller, Emil Konopinski, and Herbert York—were discussing recent UFO sightings and the possibility of intelligent life elsewhere in the universe. Suddenly, Fermi famously asked, “Where is everybody?” What he meant was this: if the universe is so vast and old, and if intelligent life is likely to have emerged elsewhere, then why haven’t we seen any evidence of it—no spaceships, no signals, no signs at all?

This paradox highlights the contradiction between the high probability of extraterrestrial civilizations (given the size and age of the universe) and the lack of observational evidence. While Fermi never formally published this idea, the paradox has since become a central question in astrobiology and the search for extraterrestrial intelligence.

SETI and the Search for Technosignatures

In addition to the search for microbial life and biosignatures, several projects are focused on detecting technosignatures—indications of intelligent life, such as artificial radio signals or laser transmissions.

The most well-known of these efforts is the Breakthrough Listen project, launched in 2015 with $100 million in funding from Yuri Milner. This initiative uses some of the world’s most powerful radio telescopes—including the Green Bank Telescope in the U.S. and the Parkes Telescope in Australia—to scan the skies for narrowband radio signals that might indicate advanced extraterrestrial technology.

Breakthrough Listen is notable for its breadth: it is targeting over a million stars, the center of the Milky Way, and 100 nearby galaxies, covering a vast region of the sky across multiple frequency bands. The data from this project are also being made public, allowing scientists around the world to join the search.

Meanwhile, the SETI Institute continues its decades-long work in signal detection and astrobiology research. It operates the Allen Telescope Array, a facility dedicated solely to the continuous monitoring of radio signals from deep space.

Other new approaches to technosignature detection include searching for infrared waste heat, transiting megastructures, or unusual light curves that could indicate large-scale engineering projects.

Safety first when travelling through the cosmos.

Focusing on Microbial Life Beyond Earth

The search for extraterrestrial microbes focuses on identifying simple life forms—like bacteria or archaea—beyond Earth, especially in environments that could support or preserve microbial life. This effort centers on planetary bodies within our solar system and hinges on the idea that microbial life, being the earliest and most resilient form of life on Earth, is the most likely to exist elsewhere. Mars is the primary target. Rovers like Perseverance and Curiosity explore ancient lakebeds and river deltas for signs of past or present microbial life. Perseverance is collecting rock and soil samples that may eventually be returned to Earth through the Mars Sample Return mission, where scientists will look for biosignatures—chemical or structural traces of life.

Beyond Mars, Europa and Enceladus, icy moons of Jupiter and Saturn, are prime candidates due to their subsurface oceans. Missions like Europa Clipper and proposed Enceladus flybys aim to study these moons’ plumes and ice shells for organic molecules and potential microbial ecosystems similar to those found in Earth's deep-sea hydrothermal vents.

Researchers also study meteorites, simulate alien environments in labs, and monitor extremophiles on Earth to understand how life could survive on other worlds. Though no microbial life has been confirmed beyond Earth, the search continues using advanced tools in astrobiology, chemistry, and planetary geology.