Could We Detect Advanced Civilisations by their Industrial Pollution? Probably Not.

The hunt for aliens goes hand in hand with the hunt for habitable planets. Astronomers are on the hunt for exoplanets with atmospheric chemicals that could be a sign of an advanced civilisation. These chemicals, known as technosignatures are found on Earth and are the result of burning fossil fuels. A team of researchers have been exploring Polycyclic Aromatic Hydrocarbons and whether they could detect them.

Over the decades, researchers have developed a number of different ways to hunt for advanced civilisations. From scanning stars for abnormal radio signals or laser pulses to searching for evidence of water the techniques have so far returned no positive results. Initiatives like SETI (Search for Extraterrestrial Intelligence) have used some of the world’s most powerful radio telescopes to listen for signals. At the same time, the habitable zones of exoplanets have been probed for signs of water suggesting life may exist. 

A team of researchers led by Dwaipayan Dubey explored the viability of using Polycyclic Aromatic Hydrocarbons (PAH) as an alternate way to continue the search. PAH’s sprung into the headlines when they were detected inside a Martian Meteorite. Their discovery received a lot of attention since the hydrocarbons are known to be the byproduct of life and finding them buried in Martian meteorites suggested some form of life at some point in the history of Mars. The debate is still continuing but the team believe searching for the hydrocarbon in planetary atmospheres could give away advanced civilisations.

There are sources of PAH’s in space such as the interstellar medium but they are mostly associated with activities from biological beings. The team focus their attention on hydrocarbons that have available absorption cross sections in the atmosphere of exoplanets like Earth. An absorption cross section is a measure of the probability of an absorption process such as particle scattering being detected by the 8m Habitable Worlds Observatory. The chosen chemicals are Naphthalene, Anthracene, Phenanthrene, and Pyrene. 

Drawing on evidence from Earth-based PAH concentrations the team knew that they have declined a little since the industrial revolution. Learning from this they ran simulations across a range of concentrations hoping they could prove the detection capabilities of an Earth-like civilisation. Telescope architecture was also explored in the paper and, whilst large mirrors help improve resolution and light gathering capability the result was less positive. 

The analysis relied upon a large telescope mirror being able to resolve detail in the spectral signature of four molecules. They found however that telescopes with 6m, 8m or 10m aperture would have an insufficient signal to noise ratio to be able to resolve the necessary details. The final conclusion of the team was that the detection of PAH signatures between 0.2 and 0.515?m using large Earth-based telescope is infeasible. 

This is a great example of a piece of work that doesn’t yield a positive outcome however a negative result in scientific research is also valuable. Further research and lab based measurements are now needed to help improve the detectability of the molecules and perhaps help us to find our first comic neighbour. 

Source : Polycyclic Aromatic Hydrocarbons as an Extraterrestrial Atmospheric Technosignatures

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