Exoplanets were first confirmed in 1992. Before that, it was easy to simply mock the search for the flying saucers and the little green men. After that, the obvious question became: If planets, why not habitable planets? If inhabited, why not by intelligent life forms? It was the naysayers who had more to prove.

More recently, astrobiologists looking for signals from intelligent extraterrestrials (technosignatures) have started to doubt that the old standby, radio, is the best choice, as science writer Corey S. Powell reports,

‘I was never a big fan of what might be called “beacon SETI”,’ the astrophysicist Adam Frank from the University of Rochester tells me. ‘The idea is that you’re waiting for somebody to send you a message with radio, but I thought, maybe nobody wants to do that.’ Frank is one of the leading researchers embracing a different approach, one that focuses on the hunt for ‘technosignatures’: evidence of any kind of alien technology that modifies its environment in detectable ways.

Corey S Powell, “The search for alien tech” at Aeon (October 25, 2021)

For one thing, why radio? Suppose an intelligent caterpillar decided that the best way to communicate with humans is to chew leaves in a pattern that is deemed significant among intelligent caterpillars. Surely, the human maintenance crew, raking huge piles of leaves into the trucks headed for the compost dump, would notice. Right? Similarly, what if extraterrestrial intelligences are mainly communicating by photonics (light signals)? As Powell says, “The shift from SETI to technosignature is an intellectual sea change in thinking about what extraterrestrials could be, and about how they might reveal themselves to us.”

What really sparked the interest in technosignatures was the discovery of hundreds of exoplanets. Researchers soon began looking for “biosignatures” on planets outside our solar system — patterns of gases emitted by life forms, which would provide circumstantial evidence for life. Similarly, some occurrences might be patterns imposed by an intelligent agent (though not necessarily the ones we would use). Spurred in part by the work of SETI astronomer Jill Tarter, NASA held a workshop on technosignatures in 2018, with the promise of funding for promising proposals:

Two decades of exoplanet research had also established that the search for technosignatures, while challenging, is clearly possible. Some exoplanets are aligned so that they appear to pass directly in front of their stars as seen from Earth. When that happens, starlight streams through the planet’s atmosphere and a small fraction of that light gets absorbed by the gases there, indicating activity and chemistry on the surface. In other cases, it is possible to pick up a smidgeon of starshine reflected directly off the planet’s surface. By examining these exceedingly subtle effects, researchers have already identified sodium, water, carbon dioxide and other molecules – even oxygen and methane, though not together – on these far-off worlds. Artificial compounds might be rarer and harder to find than natural ones, but in principle the detection process is the same.

Corey S Powell, “The search for alien tech” at Aeon (October 25, 2021)

Frank is now part of a NASA-funded group looking for solar panels and chlorofluorocarbons (industrial activity) on exoplanets, spurred by papers by Harvard astronomer Avi Loeb. Of course, it may be argued that ETs could be using quite different technologies from us. But Frank would not see that as a deterrent to looking for intelligent activity whose signals we can pick up.

A recent article in Nature typefies the change in overall attitude. Following up on what turned out to be a false alarm about a 2019 radio signal (“alien beacon”) from the vicinity of Alpha Centauri, the prominent science publication did not mock or chide the astronomers for even supposing it could be a signal. Quite the opposite, it was seen as an opportunity to refine the methods:

“It’s really valuable for us to have these dry runs,” says Jason Wright, an astronomer at Pennsylvania State University in University Park. “We need these candidate signals so we can learn how we will deal with them — how to prove they are extraterrestrial or human-made.”

Alexandra Witze, “Mysterious ‘alien beacon’ was false alarm” at Nature (October 25, 2021)

Noting that searches for ET signals have become much more sophisticated, Witze reports that one of the astronomy papers discussing the signal (BLC1 or Breakthrough Listen Candidate 1) offers a checklist for determining whether a signal could have an ET origin or not. Here it is, “Creation of a technosignature verification framework,” and it’s open access.

Overall, it does seem like a “sea change” in attitudes.

You may also wish to read: Harvard astronomer: Advanced aliens engineered the Big Bang. Avi Loeb writes in Scientific American that when we humans are sufficiently advanced, we will create other universes as well. Avi Loeb’s hypothesis is not logically stranger than the many hypotheses that attempt to account for the Big Bang without underlying information/intelligence.