Alien biology: How will astronomers actually know if they've found extraterrestrial life?

There are no set standards for extraterrestrial life, so will scientists even recognize it when they see it?

By Nicole Karlis

Senior Writer

Published July 20, 2023 5:30AM (EDT)

Microbes in space, illustration (Getty Images/MARK GARLICK/SCIENCE PHOTO LIBRARY)
Microbes in space, illustration (Getty Images/MARK GARLICK/SCIENCE PHOTO LIBRARY)

While nobody really knows what an 'alien' looks like, Hollywood films and sci-fi novels often depict aliens as scary, supernatural beings. Typically they have a reverse teardrop-shaped head, buggy eyes and slimy, grey skin. They're usually more intelligent than humans, perhaps even be immortal. Popular culture makes it seem as if one day Earthlings will find out extraterrestrial life exists when we encounter such a being, but the reality is likely going to be very different from what our imaginations suggest.

As U.S. lawmakers push to declassify UFO-related records, and the National Aeronautics and Space Administration (NASA) searches for extraterrestrial life on exoplanets via the James Webb Space Telescope, how scientists will know that they've found extraterrestrial life — and what it will even look like — is a question on many folks' minds.

On Earth, the field of biology has core characteristics that define life. It must be made of multiple cells, respond to stimuli, be able to reproduce, adapt, grow, use a source of energy for metabolic activities and be capable of homeostasis. However, even with those tenets, life can look differently for each living being and species. The NASA definition of life in its search is "a self-sustaining chemical system capable of Darwinian evolution." It's likely intentionally vague, as the space agency points out, there is no "universally accepted definition of life."

Jason Wright, a professor of astronomy and astrophysics at Pennsylvania State University, told Salon that as it stands, there are no set standards to define extraterrestrial life in astronomy.

"What are the standards for saying that we have found microbial life in the solar system, what is it going to take?" Wright asked, adding that standards are being set as science progresses and it's unlikely there will be a "eureka moment" when scientists do find it. "It's probably going to be a scenario where it's an accumulation of evidence over time, of ambiguous evidence, until all of the evidence that comes together is overwhelming."

"How can we even define what the standards will be before we know what evidence we're even talking about?"

Wright used the example of when scientists announced that they detected the chemical phosphine (PH3), a molecule composed of phosphorus and hydrogen, in the upper atmosphere of Venus – and how that appeared to be a "promising" sign of life. But nobody knows for certain yet if life exists on Venus. If phosphine is the first step to discovering life on Venus, it's going to be a long time before we do, Wright said, including starting and completing many of the planned missions to Venus.  

"This is going to be a long road," Wright elaborated. "And that's something that the community is still struggling with — how can we even define what the standards will be before we know what evidence we're even talking about?"

How astronomers search for extraterrestrial life

Generally, astronomers and astrophysicists search for extraterrestrial life by looking for biosignatures — such as water, oxygen or chlorophyll — on other planets. That seems like a sound strategy in general. After all, life has existed on Earth for around 3.7 billion years, while industrial civilization has only existed for 200. However, let's say there is compelling evidence that biosignatures found on an exoplanet suggests life exists there. Would scientists be able to scientifically confirm life exists from Earth, or have an idea of what it looks like?

"Secrecy is neither a possibility nor a policy."

Unfortunately, Wright said it will largely depend on the quality of evidence. These questions are difficult to answer because astronomers are still trying to define standards that will define extraterrestrial life. Currently, in the search for extraterrestrial intelligence (SETI) community, there is a protocol for possible detection called the Declaration of Principles Concerning the Conduct of the Search for Extraterrestrial Intelligence. In the protocol, in the case of a compelling signal detection, astronomers are advised to verify that it's truly an extraterrestrial source, announce its discovery to the world, and refrain from replying without first seeking international consultation.

As SETI senior astronomer Seth Shostak wrote, despite conspiratorial thinking, such news would not be buried by the government or media.        

"Rest assured that the first thing anyone would do upon detecting a tantalizing signal is to contact people at other observatories to request help in confirming the discovery," Shostak wrote. "Lots of people would know. Secrecy is neither a possibility nor a policy."

In an interview with Salon, Shostak said he doesn't think it's problematic that there are no set standards for extraterrestrial life.

"I think most scientists would say, we're fairly confident that we could recognize that this is a deliberate signal and not just nature," Shostak said.

As Salon previously reported, some astronomers do believe we have been searching for alien life by following the wrong clues, in part due to the power of our telescopes.

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"The mainstream astronomical search for extraterrestrial life focuses on the telltale signatures of primitive forms of life because microbes emerged soon after the Earth cooled, whereas humans emerged merely in the last 0.1% of Earth's history," said Avi Loeb, Director of the Institute for Theory and Computation at Harvard University, via email. "But we now know that most stars in the Milky Way galaxy formed billions of years before the Sun and the billions of habitable planets around them are well ahead of the history of life on Earth."

The idea of searching for life via technosignatures — such as radio waves, industrial pollution, light pollution, or anything that would suggest advanced technology is being used — is becoming more accepted.

"If we search our cosmic backyard we might find technologically-manufactured objects from our cosmic neighborhood, akin to tennis balls thrown by a neighbor," Loeb said.

Wright said astronomy has put a lot of effort into finding life via biosignatures, but he believes extraterrestrial life could be found if more of an emphasis was placed on technosignatures.

"If we get a powerful radio signal from space, there's no way that can be natural, there's no arguing about ambiguity."

"If you were looking at the solar system from a nearby star, trying to prove it had life in the solar system, you'd be hard pressed to show that Earth had life — that would be a gigantic space telescope level thing, but it might be easier to show that it has technology because you could detect the radio waves," Wright said. "If we get a powerful radio signal from space, there's no way that can be natural, there's no arguing about ambiguity like phosphine on Venus, there's only technology that can create certain kinds of radio emission."

Let's say astronomers are confident they've come across life: is there any way intelligent life could look like the way culture depicts aliens? It's actually possible. In 2018, astrobiologists published a study in the International Journal of Astrobiology showing for the first time that our evolutionary theory can be used to support alien predictions, such as how they look. Surprisingly, they landed on the conclusion that they would look a lot like humans.

"By predicting that aliens undergone major transitions - which is how complexity has arisen in species on earth, we can say that there is a level of predictability to evolution that would cause them to look like us," Sam Levin, a researcher in Oxford's Department of Zoology and the study's lead author, said in a press release. "Like humans, we predict that they are made-up of a hierarchy of entities, which all cooperate to produce an alien."

By Nicole Karlis

Nicole Karlis is a senior writer at Salon, specializing in health and science. Tweet her @nicolekarlis.

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