Are we alone in the universe? For centuries, humans have speculated about the existence of
extraterrestrial life. We imagine strange and colorful life on distant planets.
We tell stories about alien civilizations pursuing interplanetary dominance. But what are the
chances of extraterrestrial organisms existing somewhere in our galaxy? If aliens do exist,
why haven’t we encountered them before? The Milky Way galaxy contains between
100 and 400 billion stars. Around over 3000 of those stars, humans have discovered
extrasolar planets, or exoplanets, which may be capable of supporting life. To date,
scientists have detected more than 4,900 exoplanets in our galaxy alone.
With roughly 200 billion galaxies in the universe, there could be trillions of potentially habitable
planets out there. What are the chances that one of those planets supports life?
According to the mediocrity principle, the evolution of our solar system, and the properties
that eventually led to the evolution of life, may not be unusual in the universe. If the mediocrity
principle is true, alien life probably exists, in some form, elsewhere in our galaxy. However,
humans have not discovered any concrete evidence of extraterrestrial organisms. We don’t know which
planets they live on or what shape they take — even their chemical composition remains a mystery.
On Earth, carbon is the primary building block of all living organisms, constituting
the majority of biomass on our planet. Alien life may not share the same chemical structure.
In science fiction, writers have explored the possibility of silicon-based lifeforms, which may
behave and evolve entirely unlike life on Earth. Much of our understanding of extraterrestrial life
revolves around a basic assumption that alien organisms share certain traits with organisms
on Earth. Those traits include chemical structures and biological imperatives,
like survival, competition, and procreation. Given these assumptions, scientists believe
extraterrestrial life forms may resemble some of the earliest microorganisms found on Earth.
About 3.7 billion years ago, single-celled microbes, called prokaryotes, first evolved
on our planet. Some of the earliest organisms gathered at the bottom of the ocean, where they
survived on mineral-rich, superheated seawater. Roughly one billion years later, prokaryotes
evolved into more complicated eukaryotes. Another billion years passed before eukaryotes developed
into multicellular organisms, which eventually exploded into diverse kingdoms of complex life.
Complex organisms represent billions of years of evolution. Assuming life is consistent
throughout the universe, alien microbes may follow a similar evolutionary path.
Over time, these microbes may expand into a diverse ecosystem of complex organisms,
which could look radically different from life on our planet.
An alien ecosystem may even foster an intelligent species, which could develop into an advanced
civilization. They may study the cosmos, build advanced technology, and eventually explore space
beyond their solar system. If the evolution of intelligent life is not unusual in the universe,
humans should have contacted one or more of these interstellar civilizations.
Humans have launched several unmanned ambassadors to the outer boundary of our solar system.
We’ve sent radio waves like the Arecibo message and the Cosmic Call to distant star clusters.
Our reach may be small, and our technology may be underdeveloped; but if humans can
send messages and probes into space, why hasn’t another intelligent species answered our calls?
Italian physicist Enrico Fermi asked a similar question.
If the mediocrity principle is true, and there is a high probability an alien species exists in our
galaxy, why haven’t we heard from them? This famous contradiction, named the Fermi Paradox,
has sparked a myriad of radical theories, which may explain why
humans seem to be alone in the universe. One extraordinary hypothesis imagines our
solar system as a protected wilderness area — a zoo organized by an advanced alien civilization.
Another hypothesis claims every intelligent species in our galaxy was destroyed before
escaping their home planets. Stranger still, an alien species may exist outside of our perception,
perhaps living on a plane undetectable to humans. But these hypotheses are highly speculative. There
are far more likely explanations behind the apparent absence of extraterrestrial life.
The Milky Way Galaxy is incredibly vast, and we have only explored a very small piece of it.
An intelligent species may be living on the other side of our galaxy, on an exoplanet
thousands of light years from Earth. Perhaps this civilization sent a signal into space hundreds
or even thousands of years ago, and humans are still waiting for the message to arrive.
While possible, none of these hypotheses properly explain the Fermi Paradox.
If at least one alien civilization possessed the technology to send interstellar messages, by now,
that species should have developed interstellar transportation and traveled to our solar system.
In response to the Fermi Paradox, an economist named Robin Hanson proposed a theory called the
Great Filter. According to Hanson, one stage in the long evolutionary journey of intelligent life
is so rare or unlikely that no species has successfully achieved interstellar colonization.
Potentially, humans have not encountered aliens, because the Great Filter limits
the spread of intelligent life throughout our galaxy. If Hanson’s theory is correct,
does that mean humans are the first or only intelligent species to exist?
Even if humans are the only civilization to exist at this time, other civilizations may have existed
millions of years ago. It’s possible an intelligent, alien species lived in our
galaxy before the evolution of complex life on Earth. This species may have discovered tools,
built structures, and colonized their home planet. Perhaps they set their sights on the stars, but,
at some point, their civilization was annihilated. An entire empire may have risen and fallen while
life on Earth was primitive and microscopic. As a result, our civilizations did not overlap
in space or time — a likely possibility considering the age and size of our galaxy.
Homo Sapiens have walked the Earth for roughly 300,000 years. That’s a long time for humanity but
negligible in relation to planetary and galactic timescales. The likelihood of two technological
civilizations evolving at the same time, in the same galaxy, may be exceptionally low;
but this is one of many hypotheses in a diverse and largely abstract field of study.
Ultimately, humans know very few concrete truths about the creatures that may or may not live
among the stars. All it takes is one discovery to change our understanding of extraterrestrial
life. It may not happen in our lifetimes, but, someday, humans may prove whether or not
we are alone in the universe. Until then, the search for extraterrestrial life will continue.