To date, astronomers have discovered 4,164 extrasolar planets in 3,085 star systems, with another 5,347 awaiting confirmation. With this many planets available for study, researchers have been able to apply new constraints on how likely habitable planets are. In fact, the latest estimates say there could be 6 billion in the Milky Way alone! Understandably, these discoveries have renewed interest in the Search for Extraterrestrial Intelligence (SETI).
But whereas the search for habitable planets is focused on finding evidence of biological processes (aka. “biosignatures”), SETI has historically been focused on evidence of technological activity – aka. “technosignatures.” With a grant from NASA, researchers from the Harvard-Smithsonian Center for Astrophysics (CfA) and the University of Rochester are gearing up for a new study that will look for different kinds of potential technosignatures.
Traditionally, technosignature surveys have involved radio telescopes searching the galaxy for evidence of artificial radio signals, since radio waves are a proven means of communication. The new study, “Characterizing Atmospheric Technosignatures,” will be looking for evidence of two other time-tested and validated forms of technological activity – solar panels and pollutants.
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As with all SETI research, this study proceeds with the notion that while life may appear in many forms, scientific principles (like those governing technology usage) are universal. Ergo, if a technology works for humanity, other intelligence species are likely to find uses for it as well. As Avi Loeb, the Frank B. Baird Jr. Professor of Science at Harvard University, said in a CfA press release:
“Technosignatures relate to signatures of advanced alien technologies similar to, or perhaps more sophisticated than, what we possess. Such signatures might include industrial pollution of atmospheres, city lights, photovoltaic cells (solar panels), megastructures, or swarms of satellites.”
Solar panels are a rapidly-growing means of generating renewable energy, and it stands to reason that other advanced civilizations at a post-industrial level of development would see it as a viable alternative to produce energy. Locating technosignatures has always been a challenge for SETI researchers, both in terms of knowing where to look as well as knowing what to look for.
However, when it comes to their study, Leob and his colleagues have some working parameters. Their work builds on previous studies, particularly a 2014 study (“Detecting Industrial Pollution in the Atmospheres of Earth-like Exoplanets“) that Leob co-authored and supervised. For this study, the team sought to identify anthropogenic pollutants as potential technosignatures, focusing on specific chlorofluorocarbons (CFCs).
Specifically, the team focused on tetrafluoromethane (CF4) and trichlorofluoromethane (CCl3F), which are the easiest chlorofluorocarbons (CFCs) tp detect because their absorption features are within the spectral range of the upcoming James Webb Space Telescope (JWST) – which is scheduled to launch sometime next year.
When searching for biosignatures, scientists often look for chemicals in planetary atmospheres that are associated with life. In recent years, our instruments and methods have improved vastly, to the point where telescopes are able to obtain spectra directly from exoplanet atmospheres – especially when next-generation telescopes like the Extremely Large Telescope (ELT) and Nancy Grace Roman space telescope are operational.
“We have come a long way toward understanding how we might detect life on other worlds from the gases present in those worlds’ atmospheres,” said Jason Wright, an professor of astronomy and physics at Penn State University. In addition to finding chemicals produced naturally by life (like methane), astronomers could also find chemicals and gases that are produced artificially (i.e. through industrial processes). Said Loeb:
“We pollute Earth’s atmosphere with our industrial activity. If another civilization had been doing it for much longer than we have, then their planet’s atmosphere might show detectable signs of artificially produced molecules that nature is very unlikely to produce spontaneously, such as chlorofluorocarbons (CFCs).”
And then there are the signatures produced by the use of solar panels. In this case, previous work conducted by Loeb and Manasavi Lingam (an Assistant Professor of astronomy at the Florida Institute of Technology) comes into play. As Avi Loeb explained to Universe Today via email:
“Solar panels absorb light up to a certain wavelength and reflect the rest. This leads to a spectral edge in their reflected light, which is distinguishable from the ‘red spectral edge’ produced by vegetation. By observing the starlight reflected off the surface of a planet, you can tell whether it is coated with Solar panels.
Such panels would be particularly useful for tidally-locked planets that show the same side to the host star at all times. A civilization might want to put Solar panels on the permanent dayside of the planet and use the power harvested there to illuminate and heat the cold nightside.”
In the past, the search for technosignatures has always been difficult because scientists have not known where to direct their searches. This, in turn, has made it difficult for researchers to obtain grants and a footing in mainstream astronomy. However, the surge in exoplanetary discoveries – including ones that orbit within their stars’ habitable zones and have atmospheric water vapor – has revitalized the search for intelligent life.
Said Adam Frank, a professor of physics and astronomy at the University of Rochester and the primary recipient of the grant:
“The Search for Extraterrestrial Intelligence (SETI) has always faced the challenge of figuring out where to look. Which stars do you point your telescope at and look for signals? Now we know where to look. We have thousands of exoplanets including planets in the habitable zone where life can form. The game has changed.“
Loeb, Frank, and Wright were joined by Mansavi Lingam of the Florida Institute of Technology, and Jacob Haqq-Misra of Blue Marble Space. In addition to looking for signs of solar panel usage and pollutants, the team also hopes to search for swarms of satellites around planets and stars (aka. Clarke Belts), megastructures, and signs of artificial gravity.
The signals these would produce are detectable with standard ground-based and space telescopes, according to Loeb. Given time, he and his colleagues hope to produce the first entries in what will eventually become an online technosignatures library.
Many of the issues raised in their study (as well as the question of whether or not the Solar System was recently visited by an extraterrestrial probe) will also be addressed in Prof. Loeb’s upcoming book, “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth.”
Further Reading: CfA