Study suggests alien hunters should look for city lights on ‘urban planets’

A new study suggests that scientists looking for signs of extraterrestrial technology should watch for city lights on exoplanets.

Technosignatures are manifestations of technology that can be detected by astronomical means. To date, the Search for Extraterrestrial Intelligence (SETI) has mostly focused on radio signals, but shorter wavelength optical light is also a potential technosignature.

For example, sharp-eyed aliens studying the night side of the Earth could determine our presence by the radiation of city lights, even though such radiation is relatively concentrated. And advanced civilizations on exoplanets may have built cities on the vastly larger surface of their planets.

“These more urbanized planets will have higher nighttime brightness from city lights and will be easier to spot,” the authors of the new study write.

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Renewed interest

The study, which was published online last month in the journal Acta Astronautica. (will open in a new tab)presents a set of findings from an August 2020 workshop sponsored by NASA and the Blue Marble Space Science Institute in Seattle.

The main messages of the article include the conclusion that the search for technosignatures can be included in the scientific rationale for most missions at no additional cost.

The study also urges the wider astronomical community to seriously consider the possibility of detecting technosignatures. Tools for searching for technosignatures may already be available, “but it will require the efforts of the entire community to start the search,” the document says.

“A logical extension of the search for extraterrestrial life using biosignatures is to search for evidence of the existence of extraterrestrial technologies,” he adds. “The idea of ​​searching for ‘technosignatures’ has been considered by astronomers for more than half a century, with initial efforts focused on the possibility of detecting extraterrestrial radio transmissions.”

Funding remains a limiting factor in the development of the science of technosignatures. But in recent years, both public and private funding organizations have shown increased interest in the search for technosignatures, the study says.

Atmospheres too

Electromagnetic signals are not the only possible technosignatures. The new study notes that SETI scientists can also target atmospheric technosignatures — gases produced artificially, either as a by-product of industrial civilization or for a specific purpose such as climate control.

An example of an atmospheric technosignature is nitrogen dioxide (NO2).

“NO2 production on Earth today includes biogenic and anthropogenic sources other than lightning,” the paper notes. “However, anthropogenic NO2 is three times the amount from non-human sources. The detection of high levels of NO2 above non-technological emission levels found on Earth could be a sign that there may be active industrial processes on the planet.”

Leonard David is the author of Moon Rush: The New Space Race published by National Geographic in May 2019. David, a longtime contributor to, has been writing about the space industry for over five decades. Follow us on Twitter @Spacedotcom (will open in a new tab) or on facebook (will open in a new tab).

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