In the last decade, we have devised amazing instruments to glare unflinchingly at the stars and discovered that other planets are common around them. These exoplanet discoveries have thrown gasoline on the fire of the astrobiology field, where scientists seek to explore whether life might exist beyond Earth. But they have also fueled SETI, or the search for extraterrestrial intelligence. If life does evolve on other worlds, then we may very well find more than just biosignatures like oxygen.
We might find technosignatures, too. These are things like radio signals, or even megastructures; that is, artificial objects on a gigantic scale such as hypothesized star-sized supercomputers. Now, Supercluster reported in an article this week, NASA has quietly begun to fund the search for such alien megastructures for the first time in the agency's history.
Searches for biosignatures have been well funded for the last few decades. In contrast, technosignature funding has had an up and down history. In 1993, NASA famously killed a search program intended to find microwave signals that were artificial in origin. “That had a chilling effect for a couple decades,” said Steve Croft, a radio astronomer and a leader of the Breakthrough Listen project at the Berkeley SETI Research Center. But there are signs of a thaw.
Since the end of 2019, NASA has awarded four grants to fund searching for technosignatures. In November 2020, NASA awarded a grant to Ann Marie Cody of the NASA Ames Research Center and Croft to survey the whole sky for anomalous objects that transit across stars. It is possible, however uncertain, that they and their collaborators will find artificial alien megastructures.
Cody studies natural causes of dimming, which make stars vary in brightness. These can range from objects in orbit, like exoplanets or exocomets, to sources inherent to the star, like sun spots. In general, dimming is a key investigative tool for discovering new exoplanets or other objects that "transit," or pass in front of, stars. But dimming should also provide a way to identify the presence of alien megastructures, which might block out starlight in transit. With their collaborators, the pair will create the first large scale survey for transiting technosignatures. It is work that is pioneering on both a technical and organizational level.
“I’m thrilled to see NASA taking the hunt seriously,” said David Kipping, an astronomer at Columbia University, to Motherboard in an email. Kipping was not involved in the work, and has performed his own transiting technosignature research.
Croft pointed to several reasons why NASA is beginning to shift its stance towards funding. In 2015, the private investor Yuri Milner decided to fund Breakthrough Listen, providing $100 million dollars for technosignature research over ten years. Croft has been part of the project since its inception. The funding has already enabled a great deal of work to be done and landed papers in top astronomy journals. “It changed the perception," Croft said. "This is serious science that we should be doing."
Spurred by new interest in Congress, NASA held a Technosignatures Workshop in September 2018. “A bunch of people came to that," said Croft. "Folks from NASA were there, and they were basically saying: Okay, we're being told to do this by Congress. How should we do this? What should we fund? What should this entail?”
Croft thinks the explosion in exoplanet astronomy has also provided impetus for new SETI funding. Through the use of space telescopes like Kepler and now TESS, astronomers have come to believe that as many as one in five stars has a planet that is potentially habitable. As a result, research in astrobiology has taken off. Already, NASA spends billions of dollars a year on astrobiology, for things like Mars rovers, which help investigate whether there might have been life on Mars.
“We think we’re part of astrobiology,” Croft said. After all, SETI is about looking for signs of technology, but there can be no technology without life evolving to build it, first. “Technosignature searches are astrobiology,” he reiterated. Indeed, it would seem that if NASA continues embracing the mission of finding biological life on other planets, then it must come to terms with the possibility of finding distant, advanced civilizations.
Recent SETI efforts have been aided by the fact that they dovetail neatly with mainstream astronomy. The Kepler and TESS space telescopes, which were launched to find exoplanets, also make ideal instruments for looking for transiting technosignatures. In particular, Cody and Croft's new project will analyze the TESS dataset.
TESS was launched in 2018 to collect the patterns of light variation from across 85 percent of the sky. It measures the “light curves” of stars, which indicate the way their light dims, for example when an exoplanet passes in front of them.
Light curves have already revealed mysteries besides exoplanets. In 2015, astronomers and citizen scientists found that a star now known as Tabby’s star, or Boyajian’s star, exhibited unusual dimming, losing as much as 22 percent of total brightness. Since then, astronomers have come up with several natural explanations, including possibilities like the collision of exoplanets, which would create planet-sized shrouds of debris.
The strategy for the team's survey is to “search for the weird,” Croft said–simply, light curves that lack good explanations. The difficulty is in the sheer size of the data. The team is developing machine learning pipelines to automatically classify the millions of light variations that TESS will pick up. The aim is to create algorithms good enough to sort most of these into known causes, such as transiting exoplanets, allowing the researchers to focus on the cases that truly stand out.
The survey will provide a first estimate of how many transiting technosignatures might exist in the galaxy. Researchers had only begun studying the idea of technosignature transit in the last decade, and little is currently known. “Relatively little has been done in the sense of a systematic, observational survey,” Kipping told me. “There have been efforts to look for weird signals, including by my own team, but the few anomalies known, such as Boyajian’s star and the Random Transiter, do not present clear slam dunk detections of alien technology.”
Of course, there are other sorts of technosignatures, too. In 2019, NASA awarded a different search grant to Adam Frank of the University of Rochester and collaborators. They are studying what telescopes should pick up if they observed an exoplanet with polluting inhabitants, or inhabitants who covered their surface with solar panels. This was the first non-radio technosignature search project that NASA funded, Frank said in an interview.
Frank noted that he and his collaborators’ project picks up directly from work on biosignatures. In early work from the 1970s, Carl Sagan noted that astronomers could use telescopes to look for oxygen in planetary atmospheres. Oxygen could signal the presence of a biosphere, and hence act as a biosignature. But when researchers investigated the science more carefully, they realized there were other ways for a planet to maintain oxygen besides having life. Researchers went on to develop a sophisticated idea of the sort of oxygen signature that would be a dead giveaway. Frank and colleagues are aiming to do the same thing for technosignatures.
“What you want is a library of spectral signatures that observers, when they take data, can go back and say: Is anything we’re seeing not expected? Is anything indicative of a technosignature?” Frank said.
Researchers like Kipping have already modeled what an observation of a true, transiting technosignature might look like. It could be created, for example, by a star-shrouding megastructure that had a shape like a lampshade, with slits cut out. These slits could be purposefully crafted to dim the star in a pattern analogous to the digits of pi, which would be clearly unnatural.
Croft and his team plan to run this sort of test signature through their models in a process called “signal injection” to make sure that the pipeline flags them correctly. “If we don't detect them, then maybe the pipeline is just not sensitive to that data, or maybe we need to tweak the pipeline in some way to get it to detect that, and then we can be happy,” Croft said. In a collaboration with a high school teacher, he has already shown that their software can correctly flag the distant radio signal of the Voyager 1 spacecraft as an anomalous technosignature in background radio data.
Based on the history of prior searches, it is unlikely that the current survey will find anything unequivocally alien. However, it is likely to turn up intriguing phenomena of a natural origin. “It will be exciting to see what we find, even if just astrophysical oddities,” Cody told Motherboard in an email.
Finding nothing alien is also useful for other reasons. Once the researchers have ruled out one sort of searching, then they will know where to look next–as long as NASA keeps funding them. Based on the evidence of the last year, and the winning nature of current proposals, that looks increasingly likely to happen.