A trendy new chemical theory for alien lairs

Photo illustration by Erin O’Flynn/The Daily Beast/Ken Crawford/Wikimedia Commons

There is a theory that is currently in vogue in astrochemistry called “Assembly Theory”. It assumes that highly complex molecules – for example many acids – can only come from living beings. The molecules are either part of living beings or they are made by intelligent beings.

If the assembly theory holds, we could use it to search for extraterrestrials – scanning distant planets and moons for complex molecules that can should be evidence of living beings. This is the latest idea from the founder of assembly theory, University of Glasgow chemist Leroy Cronin. “This is a radically new approach,” Cronin told The Daily Beast.

But not every expert agrees it would work — at least not any time soon. To get chemical readings from distant planets, scientists rely on spectroscopy. This 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 keep alien-hunting astrochemists and 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. At Radio SETI, scientists aim sensitive radio receivers into the sky and listen for faint signals that may be coming from an extraterrestrial civilization.

In the decades after radio SETI caught on, astronomers widened their search. Increasingly powerful telescopes allowed them to take colorful spectroscopic images of planets and moons, and then interpret those colors to make educated guesses about the chemical composition of the atmosphere. Certain elements could be prerequisites for life. Many astrobiologists agree that a planet should have carbon, hydrogen, nitrogen, oxygen, sulfur, and phosphorus just to have one chance supporting biological evolution.

Once life has evolved on a faraway 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 that combine carbon, hydrogen, oxygen, and magnesium, giving it a combined molecular weight of almost 900.

But chlorophyll isn’t the only complex molecule that could be a marker for life. According to a new peer-reviewed study by Cronin and his UK and Spanish colleagues most Molecules with a molecular mass of at least 300 could be evidence of extraterrestrial microbes or even intelligent aliens.

Cronin and his team came to this conclusion after analyzing 10,000 chemicals found here on Earth. “Most molecules are larger than [a] molecular weight [or mass] from 300 [are] linked to the existence of life on earth,” they wrote.

These complex molecules make up our bodies, our body’s waste products, and even the chemicals we make. Medicines for example. “That’s because complex molecules … are too complex to form randomly in detectable quantities, and thus can only be made through the complex biochemical pathways found in biological cells,” Cronin and his co-authors wrote.

In other words, if you find complex molecules on a distant planet or moon, you’ve probably found life, Cronin and co claimed.

That’s an exciting prospect for scientists, but there’s a catch: Not everyone agrees on what “complex” means. Yes, a molecular weight of approximately at least 300 correlated with Cronin’s concept of complexity. But there are too many possible exceptions, including forms of chlorophyll, for mass alone to be the standard. “There are many competing notions of chemical complexity,” Cronin and his team acknowledged.

Cronin’s Assembly Theory addresses this issue. The theory “estimates the complexity of a molecule by quantifying the minimal constraints required to construct an object from the building blocks.” In plain language, the theory asks how often at least a simply Molecule would have to add an element or copy part of itself to achieve a specific structure.

Any molecule that requires 15 steps should reach a molecular mass of 300 or more and be considered “complex,” according to Cronin. And if Earth chemicals are any indication, the widespread presence of such a complex molecule on an alien planet or moon is a strong sign of nearby creatures.

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Niels Ligterink, a physicist at the University of Bern in Switzerland, told The Daily Beast he agreed with Cronin’s reasoning. “In general I would say that the chemical complexity, in this case determined with assembly theory, is a good additional tool to search for life.”

The assembly theory helps sidestep a big question in astrobiology, Ligterink added. Life on Earth has 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 day-to-day search for extraterrestrial life is easier said than done. How can a scientist study molecules on an “exoplanet” light years away? They just can’t – at least not with today’s technology. The best they can do is survey 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 through spectroscopy.

You see, each element absorbs certain wavelengths of light and reflects others. Carbon absorbs some violet and blue and a lot of orange, leaving a whole range of greens, reds and yellows. Nitrogen practically has the Opposite light absorption pattern. Spectroscopy observes these wavelengths and helps determine what type of chemistry they correspond to. Certain color mixtures could indicate complex molecules that combine different elements in intricate ways.

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The challenge in spectroscopy is precision. Think of each element’s light pattern as a fingerprint. Now imagine a million fingerprints smeared on top of each other. “The spectral signature … can rarely be clearly assigned to a specific molecule,” said Schulze-Makuch.

We may need much better telescopes or probes for Cronin’s assembly theory to work as a strategy for hunting aliens between distant exoplanets and their moons. This can take a while.

But it’s possible that the same theory could help scientists find evidence of extraterrestrial life in existing data from nearby planets and moons. There has been a plethora of data from various missions to Mars since NASA’s Viking probes first landed on the red planet in 1976.

The two Viking probes took soil samples, boiled them and analyzed the escaping gases. The probes beamed the data back to NASA. Summarizing the numbers, agency scientist 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 were making first contact with microbial ET, but his NASA colleagues insisted he misinterpreted the data — a position the space agency has held for 47 years. Levin did not respond to a request for comment.

It’s worth considering the Viking data, as well as data from other previous space missions, Cronin said. If there is evidence of complex molecules, there may be signs of life that scientists have missed. “It’s possible,” Cronin said.

In this way, assembly theory could help us make sense of it Past looks for extraterrestrial life long before it cooperates Future seeks.

Read more at The Daily Beast.

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