There is a theory currently in vogue in astrochemistry called “assembly theory”. He postulates that highly complex molecules – many acids, for example – could only come from living beings. Molecules are either part of living things or they are things that intelligent living things make.
If the assembly theory holds up, we could use it to search for extraterrestrials – scanning distant planets and moons for complex molecules that should be proof of living beings. It’s the latest idea from the creator of Assembly Theory, Leroy Cronin, a chemist from the University of Glasgow. “It’s a radical new approach,” Cronin told The Daily Beast.
But not all experts agree that it would work, at least not any time soon. To take chemical measurements of distant planets, scientists rely on spectroscopy. It is the process of interpreting a planet’s color palette to assess the possible mix of molecules in its atmosphere, land, and oceans.
Spectroscopy is not an exact science. That might leave astrochemists and alien-hunting astrobiologists guessing, for now. “There are a lot of uncertainties,” Dirk Schulze-Makuch, an astronomer at the Technical University of Berlin, told The Daily Beast.
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Scientists have been actively searching for signs of extraterrestrial life for at least a century. The search for extraterrestrial intelligence (SETI) accelerated in the 1950s and 1960s, with the advent of radio-based SETI. In SETI radio, scientists point sensitive radio receivers skyward and listen for faint signals that could be coming from an alien civilization.
In the decades since the broadcast of SETI radio, astronomers have expanded their research. Increasingly powerful telescopes allowed them to capture colorful spectroscopic images of planets and moons, and then interpret those colors to make educated guesses about the chemical makeup of the atmosphere. Some items could be prerequisites for life. Many astrobiologists agree that a planet should have carbon, hydrogen, nitrogen, oxygen, sulfur and phosphorus just to have a chance to support biological evolution.
Once life has evolved on a distant exoplanet, it could paint the planet into complex molecules mixing these and other elements. There could be chlorophyll, the substance that allows plants to absorb energy from light. It is made up of a family of molecules combining carbon, hydrogen, oxygen and magnesium which together give it a molecular mass of almost 900.
But chlorophyll isn’t the only complex molecule that could be a marker of life. According to a new peer-reviewed study by Cronin and his British and Spanish colleagues, most molecules with a molecular mass of at least 300 could be evidence of extraterrestrial microbes, or even intelligent extraterrestrials.
Cronin and his team came to this conclusion after analyzing 10,000 chemicals found here on Earth. “Most molecules greater than [a] molecular weight [or mass] of 300 [are] related to the existence of life on Earth,” they wrote.
These complex molecules make up our bodies, the waste products of our bodies, and even the chemicals we make. Pharmaceuticals, for example. “This is because complex molecules…are too complex to form by chance in detectable abundance, and therefore can only be made by the complex biochemical pathways found in biological cells,” Cronin and coauthors wrote.
In other words, if you find complex molecules on a distant planet or moon, then you’ve probably found life, Cronin and company claimed.
It’s an exciting prospect for scientists, but there’s a catch: Not everyone agrees on what “complex” means. Yes, a molecular weight of at least about 300 correlated with Cronin’s notion of complexity. But there are too many possible exceptions, including forms of chlorophyll, for mass alone to be the norm. “There are many competing notions of chemical complexity,” conceded Cronin and his team.
Cronin’s assembly theory solves this problem. The theory “estimates the complexity of a molecule by quantifying the minimum stresses required to construct an object from the building blocks”. Clearly, the theory asks how many times, at a minimum, a simple molecule would need to add an element or copy a part of itself in order to obtain a given structure.
Any molecule that requires 15 steps must reach a molecular mass of 300 or more and qualify as “complex,” according to Cronin. And if Earth chemicals are any guide, the widespread presence of such a complex molecule on an alien planet or moon is a strong sign of the presence of living things nearby.
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Niels Ligterink, a physicist at the University of Bern in Switzerland, told The Daily Beast he agreed with Cronin’s thinking. “In general, I would say that chemical complexity, in this case determined with assembly theory, is a good additional tool to search for life.”
The assembly theory avoids a big question in astrobiology, Ligterink added. Life on Earth contains DNA or RNA, the nucleic acids that carry genes. It’s not safe to assume that extraterrestrial life would share this basic structure, Ligterink said. “But we can be fairly certain that extraterrestrial life is also chemically complex.”
But applying Cronin’s theory to the everyday search for extraterrestrial life is easier said than done. How can a scientist sample molecules on an “exoplanet” light years away? They just can’t, not with today’s technology anyway. The best they can do is study an exoplanet with a powerful telescope – the new James Webb Space Telescope or the Vera Rubin Telescope in Chile, to name just two – and analyze the color palette spectroscopy .
You see, each element absorbs certain wavelengths of light and reflects others. The carbon absorbs a little purple and blue and a lot of orange, leaving behind a whole bunch of greens, reds and yellows. Nitrogen has almost the opposite light absorption pattern. Spectroscopy observes these wavelengths and helps determine what type of chemistry they correspond to. Certain mixtures of colors could indicate complex molecules combining various elements in complex ways.
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The challenge of spectroscopy is precision. Imagine the light pattern of each element as a fingerprint. Now imagine a million fingerprints smudged on top of each other. “The spectral signature…can rarely be uniquely assigned to a specific molecule,” Schulze-Makuch said.
We might need much better telescopes or probes to make Cronin’s assemblage theory work as an alien-hunting strategy among distant exoplanets and their moons. It might take some time.
But it’s possible the same theory could help scientists find evidence of extraterrestrial life in existing data from closer planets and moons. There’s tons of data from various Mars missions since NASA’s Viking probes first landed on the Red Planet in 1976.
The two Viking probes collected soil samples, boiled them and analyzed the gases that came out of them. The probes sent the data back to NASA. Reviewing the numbers, agency scientists Gil Levin concluded that the probes had found the first-ever chemical evidence of extraterrestrial microbes.
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Levin was ready to announce to the world that we would make first contact with microbial ET. But his NASA colleagues insisted he misinterpreted the data, a position the space agency has maintained for 47 years. Levin did not respond to a request for comment.
It’s worth reconsidering the Viking data as well as data from other past space missions, Cronin said. If there is evidence of complex molecules, there may be signs of life that scientists have overlooked. “It’s possible,” Cronin said.
In this way, assembly theory could help us make sense of pass searches for extraterrestrial life long before it helps coming research.
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