If you hear “extraterrestrial,” what do you picture? Google it, and chances are you’ll find at least one visualization of a humanoid alien, maybe even waving at the camera.
The idea that extraterrestrials, specifically intelligent ones, might exist has long captured our fascination. Its longevity across the ages indicates a burning question: Is something or someone else out there?
The word itself is quite simple. “Extra” means outside of, and “terrestrial” means the Earth. Add that together, and it means something from outside of our Earth. Because it’s so general, “extraterrestrial” can describe other cosmic objects beyond our immediate backyard, such as space dust or meteorites.
In the context of life, “extraterrestrial” would mean an organism originating and capable of surviving outside of Earth.
“It’s a broad term that brackets a whole range of methods for looking for life in different locations and different environments,” says Chris Impey, a university distinguished astronomy professor at the University of Arizona.
To seek out and unravel life’s origins is not merely a philosophical conundrum, but a scientific pursuit, too.
The existence of extraterrestrial life, commonly referred to as “aliens,” is a topic of great interest and speculation. Scientists have been actively searching for signs of extraterrestrial life through programs like the Search for Extraterrestrial Intelligence (SETI) Institute.
While there is no conclusive evidence yet confirming the existence of extraterrestrial life, scientists continue to search for signs of life beyond our planet.
“We should not assume there’s anything notable or particular, in a cosmic sense, about our situation, or the Earth, or the sun, or any part of our proximate universe,” Impey says. “The final frontier of that would be to decide that biology is not special to Earth.”
What and where is life, then? Budding biology fields, planet science, and even the search through the skies for messages are all part of one cosmic attempt to open one more window into our universe.
Astrobiology is a multidisciplinary scientific field that explores the origin, evolution, distribution, and potential existence of life in the universe. It’s just as it sounds, a combination of astronomy and biology.
Astrobiology is concerned with some of the deepest questions humanity can pose, says NASA astrobiologist Lynn J. Rothschild. There are three to be exact: Where do we come from? Where are we going? Are we alone?
What set the actual spark for the birth of astrobiology as a field, Rothschild says, was the Mars meteorite ALH84001, discovered in Antarctica’s icy fields in 1984.
While experts determined there was no sign of life on Mars in the rock, it ignited a fervor for the possibility of extraterrestrial life. Exhilarating detections of the first exoplanets in the 1990s, alongside a growing understanding of microbiology and extremophiles, had generated scientific buzz.
“We already had the planets going on, we had molecular biology, and so on. Then, all of a sudden, bam! Here’s this really exciting potential for microbial life on Mars right in our labs,” Rothschild says.
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The field of exoplanet science is also a big player in the hunt for life. Sara Seager, an MIT professor and one of the early pioneers of exoplanet study, has watched the field grow first-hand.
Exoplanets are planets that exist outside our solar system. Exoplanets can vary in size and composition, and most are tethered to host stars. All sorts of wacky worlds exist, but the search is on to find another Earth-like planet that could host life.
There are billions of stars in the galaxy, and according to NASA, astronomers have confirmed over 5,500 exoplanets.
Studying exoplanets can give us a broader understanding of our universe. Scientists can look at what’s called the habitable zone, where planets are the ideal distance from their host stars to have liquid water on their surfaces. Seager is particularly concerned with finding another Earth.
“My desire, like a growing number of people now, is to find signs of life out there,” Seager says. “It just may be easier if we also know that the planet is somewhat like ours.”
Life can leave unusual fingerprints, or biosignatures, on a planet’s atmosphere, just like it does on Earth. Our anomaly is oxygen, which is present only because living organisms consistently produce it.
“What would happen is we find a gas that doesn’t belong. It’s in huge quantities. Perhaps it’s very reactive. It doesn’t make sense with what else is there [in the atmosphere],” Seager says. “And then we have to go through all the scenarios about what it could be.”
Answers on life may lie within our own solar system, too. Mars comes to mind first – a cold, desert planet with ancient lakebeds that may have once supported life. Venus, though, is another candidate.
Seager, who’s now researching the potential habitability of Venus’ sulfuric acid clouds (by testing the building blocks of DNA), was part of the team that first reported the discovery of phosphine in its boiling atmosphere. It’s deadly to Earth’s organisms, but a potential harbinger of life on Venus.
The news sparked a significant debate that continues now, but it’s put Venus back on the radar. Douglas Caldwell, who was the instrument scientist on the 2009 Kepler mission, sees the Venus debate as a sort of prelude.
“When we start to get data back from JWST or the next big telescope, there’s going to be a huge debate to convince ourselves and other scientists and the rest of the world that this actually is or is not an indication of life,” Caldwell says.
The SETI Institute began with a NASA-funded project on the search for extraterrestrial intelligence and eventually grew into the nonprofit research organization it is now. The institute conducts research in a variety of fields related to astrobiology, astronomy, and space science, with a primary focus on detecting signals or signs of intelligent life in the universe. Caldwell is a research astrophysicist at the SETI Institute.
In search of extraterrestrial intelligence, there are those who are trying to detect signals, be it through radio waves or lasers. These efforts abound at the Allen Telescope Array, which scans the skies at a wide range of frequencies.
“A lot of [SETI scientists] are people like me, scientists doing other studies that are related to trying to understand the idea of life out there,” Caldwell says, who’s currently supporting the exoplanet hunter TESS. “Not necessarily explicitly searching for intelligent life, but people here are doing that too.”
SETI (not to be confused with the institution, but the pursuit itself) is a bit of a wild card. Like life, there’s no standard definition for intelligence. Impey points out elephants are intelligent species on Earth, but they’re not preoccupied with alien signals like we are.
The pursuit of life is one of the most complicated scientific problems, and it requires scientists from all corners to join forces. Just one detection of life, no matter how marvelous or novel, wouldn’t be enough to provide a definitive answer.
“It’s not necessarily like we’ll see life that’s waving at us and it’s obvious,” Caldwell says. “It’s gonna be more like, ‘Okay, this is weird, and life seems to explain it. But we’re not certain.’”
And scientists can’t be certain without multiple sets of improved data and multiple instances of life. That’s all work that would involve investing in entire new paradigms of technology and experimentation, Seager says.
To her, that’s why searching for extraterrestrial life matters – even if the nearest microbe is light years away.
“It makes us think outside the box. We have to think really, really hard about brand new ideas,” Seager says. “Once in a while, this kind of brand-new thinking does lead to something big.”