Why it's so hard to look for evidence of aliens

The perennial question on the existence of aliens has been circulating anew. Some evidence suggesting there is life outside of Earth, including from the Perseverance rover on Mars, have been found, but many unanswered questions remain. If aliens really exist, how could we find them?

What does the evidence suggest?

Other than various claimed UFO sightings and some evidence of life-building compounds on extraterrestrial bodies, there is no solid evidence of aliens, at least not as they exist in the popular imagination. However, this doesn't mean that they don't exist.

The discovery of China's "spy balloon" drew attention to a number of other unidentified objects in the sky, sparking conspiracy theories and rumors about aliens, per The Associated Press. White House press secretary Karine Jean-Pierre even commented on the rumors, emphasizing "there is no — again no indication — of aliens or extraterrestrial activity with these recent takedowns."

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While there is no evidence of actual life other than on Earth, recent samples from the Perseverance rover on Mars could contain evidence of biosignatures, or features that indicate that life could have been present in the past. "It's an amazing mission," remarked Jorge Vago, part of the European Space Agency's ExoMars project. "If we find super interesting stuff that's suggestive of a possible biological origin, I would expect we may want to have another sample return mission and bring back samples from the subsurface."

How could we find aliens?

Some researchers argue that we are closer than we think to finding evidence of extraterrestrial life. This is largely because of the discovery of a number of exoplanets located in habitable zones or in the right proximity from a star to host life, according to Space.com. "What we do not know is if these terrestrial planets have atmospheres and what these atmospheres are made of," explained astrophysicist Sasha Quanz. "We need to investigate the atmospheres of these planets. We need an observational approach that would allow us to take pictures of these planets."

However, being located in a habitable zone doesn't necessarily mean life will form. For example, Mars and Venus are both considered to be located in the habitable zone of the sun, but neither have been proven to have hosted life, LiveScience writes. Research published in January posited that the habitable zone may not be the end all be all, suggesting that life itself may have influenced the habitability of the planet. Essentially, "once life gets started on a planet, it really doesn't want to go away," and so a "life-altered planet then becomes much more habitable than it was before."

Finding and proving the existence of aliens could require rethinking much of what we know about life itself. "Life as we do know it is really just a specific instance of one kind of biology that could exist," according to Sarah Scoles, who writes for Scientific American. "We really only think about life as life. Biology as biology." Instead, conditions that could be considered harsh from our standpoint may be able to host other forms of life.

"How do we contend with the truly alien?" asked Sarah Johnson, an associate professor at Georgetown University. "It's one of the biggest challenges we have, like imagining a color we've never seen."

Will we ever actually find them?

The image of aliens as terrifying, socially advanced monsters is mostly a product of science fiction. In reality, we don't know whether life exists, let alone intelligent life. "I'm truly agnostic about whether there's even anything to find," commented Jason Wright, a professor of astronomy and astrophysics at Penn State to The New York Times. He added that biosignatures are very difficult to detect. "We have a much bigger search radius for technology ... But also, perhaps complex life that builds technology is itself extremely rare, even when life forms."

Scientists have long asked why, even though there are so many stars, we have yet to come in contact with another civilization. This question is known as the Fermi Paradox and the main theories are that life is rare, or that those civilizations do not want to make themselves known. However, another theory is that no life form has survived long enough to have the technology to contact other civilizations. "Do technical civilizations tend to destroy themselves shortly after they become capable of interstellar radio communications?" asked Rebecca Charbonneau, a science historian at the National Radio Astronomy Observatory.

On the other hand, "It may also be that really successful technological societies at some point become hard to detect," NASA administrator Michael New told the Times, "because they're living in more or less equilibrium with their planet." Either way, the search continues in hopes of discovering whether we are alone in the universe.