NASA scientists have detailed a genius way to spot signs of thriving alien civilizations on distant planets.
In a paper this week, they suggested that greenhouse gases on other worlds could provide proof that we're not alone in the universe.
That's because industrial-scale farming carried out by little green men would release plumes of emissions into the atmosphere.
Just as they do on Earth, the gases would change the makeup of the planet's air, acting as a detectable signature of life.
These "technosignatures" could be picked up by powerful telescopes here on Earth and help to guide our search for ET.
It's a fresh take on the search for so-called biosignatures – natural substances that scientists look for as indicators of life.
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"The search for technosignatures is a continuation of the search for biosignatures," researchers wrote in their paper.
"It includes the idea of looking for spectral evidence of technology in the atmospheres of exoplanets."
The team, which included astronomers at Nasa's Goddard Space Flight Center, investigated whether nitrogen-containing gases found on Earth could act as effective technosignatures on other planets.
They reasoned that, in order for plants to grow on other worlds, they would likely need water and carbon cycles, as well as the presence of nitrogen in the soil.
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Any emerging civilisations would presumably work to farm crops by applying fertilizer – such as manure – to the soil.
On Earth, the use of such fertilisers has raised the level of chemicals such as ammonia and nitrous oxide in the atmosphere.
The presence of these gases on alien worlds could, therefore, indicate that alien farmers are working below.
"The combined presence of these gases in the atmosphere of a habitable planet could serve as a remotely detectable spectral signature of technology," researchers wrote.
To test their theory, the team calculated how detectable Earth's nitrogen-containing gases would be from afar.
They concluded that present-day levels of ammonia and nitrous oxide would be difficult to detect.
However, were our planet's population were to jump to at least 30billion, the resulting rise in greenhouse gases would be high enough for telescopes to spot.
It's possible that scientists could look for technosignatures as they expand their search for potential alien worlds.
Of course, the presence of greenhouse gases would not prove that alien life is present somewhere – they could always be the byproduct of the planet's natural processes.
Scientists have previously proposed scouring distant planets for non-natural waste products such as chlorofluorocarbons (CFCs), which can persist in the atmosphere for tens of thousands of years.
Famed astrophysicist Avi Loeb of Harvard University has suggested light pollution as a possible sign of technological civilisations.
A number of powerful new telescopes, including the recently launched James Webb space telescope, are training their sights on exoplanets in the hope of spotting elusive biosignatures.
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They include oxygen, which is produced by photosynthesis carried out by plants and certain microbes.
The research was published in the pre-print journal Arxiv.
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