If you look up at the stars tonight, you aren't just looking at light. You are looking at a math problem. There are roughly 200 billion stars in our galaxy alone. Most of those have planets. Many of those planets sit in the "Goldilocks zone" where water doesn't boil or freeze. So, what is an extraterrestrial, exactly?
Basically, it’s any life form that didn't start here on Earth. That’s it. It sounds simple until you realize we aren't just talking about little green men in silver saucers. We’re talking about everything from microscopic sludge on a moon of Jupiter to a civilization so advanced they use entire suns for batteries. Honestly, the popular image of "aliens" is probably the least likely version of what’s actually out there.
We have zero confirmed proof of life beyond our atmosphere. None. But the search has shifted from science fiction to a serious, high-stakes branch of biology and physics.
The broad definition: It's not just "Grey" guys
When people ask "what is an extraterrestrial," they usually picture something with two eyes and a forehead. That's "anthropocentrism"—our habit of assuming everything important has to look like us.
In reality, an extraterrestrial could be a bacterium living in the subsurface oceans of Enceladus. It could be a virus-like strand of RNA drifting in the clouds of Venus. Scientists like those at the SETI Institute (Search for Extraterrestrial Intelligence) divide the hunt into two buckets: biosignatures and technosignatures.
Biosignatures are the chemical farts of life. Think methane or oxygen levels that shouldn't exist unless something is breathing or eating. Technosignatures are the big stuff. Radio bursts. Laser pulses. Massive structures like a Dyson Sphere. If we find a weird radio signal, we’ve found an ET, even if we never see a "body."
Why we look at water first
Water is the universal solvent. It dissolves nutrients and allows the chemistry of life to happen. This is why NASA follows the water. When we talk about what is an extraterrestrial, we are usually talking about carbon-based chemistry because carbon is the most social element on the periodic table. It loves to bond.
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But what if life is silicon-based? Or what if it lives in liquid methane? On Saturn’s moon Titan, it’s so cold that water is basically rock, but there are lakes of liquid ethane and methane. If something swims there, it wouldn't just be an "alien"—it would be a life form with a biology we can’t even fully simulate yet.
The Fermi Paradox: If they exist, where is everybody?
Physicist Enrico Fermi famously asked this over lunch in 1950. The logic is simple: The universe is old. Really old. If interstellar travel is possible, even at slow speeds, a civilization should have colonized the galaxy by now. Yet, we see nothing. Silence.
This leads to some pretty dark theories about what an extraterrestrial might actually be.
- The Great Filter: This is the idea that some wall exists that prevents life from becoming an interstellar empire. Maybe it’s nuclear war. Maybe it’s climate change. Or maybe it’s just the jump from single-celled life to multi-celled life. If the filter is ahead of us, we’re in trouble. If it’s behind us, we’re the lucky ones.
- The Dark Forest Theory: Popularized by Liu Cixin’s sci-fi, but based on real game theory. The idea is that the universe is a dark forest filled with armed hunters. Everyone is quiet because if you reveal your location, someone else might view you as a threat and wipe you out.
- The Zoo Hypothesis: They know we’re here. They’re just watching us like we watch zebras at the park. We aren't worth talking to yet.
Maybe the "what" of an extraterrestrial is just too different for us to notice. If a civilization evolved to live inside a computer, would they even care about radio waves? Probably not.
What is an extraterrestrial in the eyes of the law?
Believe it or not, we have rules for this. The Outer Space Treaty of 1967, which most nations signed, governs how we handle "out there."
One of the biggest concerns is planetary protection. If we send a rover to Mars and it has Earth bacteria on it, and then we find life, how do we know we didn't just find our own stowaways? This is why NASA’s Jet Propulsion Laboratory has "clean rooms." They are terrified of accidental contamination.
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If we actually met an ET tomorrow, there is no "official" global protocol. There's a document from the International Academy of Astronautics (IAA) that says we should tell the UN first and not send a reply until the world agrees on what to say. But let’s be real: someone would post it on X (formerly Twitter) within thirty seconds.
Extremophiles: The aliens living next door
To understand what an extraterrestrial could be, scientists look at the weirdest places on Earth. We find life in boiling volcanic vents. We find it in the frozen dry valleys of Antarctica. We’ve even found bacteria that can survive the vacuum of space on the outside of the International Space Station.
These are called extremophiles.
- Tardigrades: These "water bears" can survive radiation that would kill a human instantly.
- Deinococcus radiodurans: A bacterium nicknamed "Conan the Bacterium" because it can survive in the cooling water of nuclear reactors.
If life on Earth can handle these brutal conditions, then the definition of what is an extraterrestrial doesn't have to be limited to "Earth-like" planets. Mars might look like a desert, but a few miles underground, it might be a paradise for bacteria.
The UAP shift: From tin foil hats to the Pentagon
For decades, talking about aliens was a career-killer for scientists and pilots. That changed around 2017. The New York Times published videos of "Unidentified Aerial Phenomena" (UAP) tracked by Navy jets.
The Pentagon now has an office called AARO (All-domain Anomaly Resolution Office). They aren't saying it's aliens. In fact, they’re very careful to say there’s no "verifiable evidence" of extraterrestrial technology. But the fact that the government is even looking marks a massive shift.
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When we ask what is an extraterrestrial today, we are increasingly looking at data from high-end sensors, not just grainy photos from a gas station in New Mexico. We’re looking at objects that move without visible wings or engines. Are they ETs? Or are they secret Chinese drones? Or just weird weather? The point is, the question is now being asked by people in suits, not just people in tinfoil hats.
The Drake Equation
$N = R^* \cdot f_p \cdot n_e \cdot f_l \cdot f_i \cdot f_c \cdot L$
This formula, created by Frank Drake in 1961, isn't meant to give a "final answer." It’s a way to organize our ignorance. It looks at the rate of star formation, the fraction of those with planets, and the length of time a civilization lasts. Even with conservative numbers, the equation suggests that thousands of civilizations should exist.
What happens if we actually find them?
The "Post-Detection" era would be chaos.
First, there’s the Ontological Shock. Our religions, our philosophy, and our sense of being "special" would be shattered. If an extraterrestrial exists, then "humanity" is just one of many experiments.
Second, there’s the communication gap. If we find a signal from 500 light-years away, a "hello" takes 500 years to get there and another 500 to come back. We wouldn't be having a conversation. We’d be reading a very old book.
Practical steps for the curious
If you want to move beyond the movies and understand the reality of the search, here is how you actually keep up with the science:
- Follow the JWST (James Webb Space Telescope) data. It’s currently looking at the atmospheres of planets in the TRAPPIST-1 system. If we find oxygen and methane together there, that’s the "smoking gun" for life.
- Check the NASA Exoplanet Archive. There are over 5,000 confirmed planets outside our solar system. You can see which ones are rocky and which are gas giants.
- Support Citizen Science. Projects like "Planet Hunters" let you look at real satellite data to find dips in star brightness that might indicate a planet (or a structure).
- Distinguish between "Ufology" and "Astrobiology." One is about sightings and stories; the other is about chemistry, physics, and peer-reviewed evidence. Both are interesting, but only one is building the tools to actually solve the mystery.
The hunt for what is an extraterrestrial is really a hunt for our own place in the universe. Whether we find a neighbor or find out we’re totally alone, the answer will be the biggest discovery in human history. Until then, we keep the telescopes pointed up.