The search for extraterrestrial intelligence should not exclude our own planet.

a black dot against a grey background with two white lines on either side
A widely circulated clip from a video shot in 2015 by a Navy aircraft showing an unidentified aerial object.
U.S. Department of Defense / Public Domain

Since the early 1960s, SETI (the Search for Extraterrestrial Intelligence) has largely focused on detecting radio messages. Arguably this hunt has only begun. But how do we seek new civilizations that might not be beaming intentional signals our way?

A recent buzzword in SETI circles is technosignatures (S&T: Aug. 2022, p. 34). The term broadens the notion of biosignatures — signs of biological activity — to search for technological activity. This allows us to avoid having to define “intelligence,” to tacitly acknowledge both that intelligence may not be biological and that some very intelligent creatures (think cetaceans on Earth) might not have technology, and to simply focus on observable technological signatures (radio signals being one small subset of these). The technosignatures approach opens SETI to searching for anything from unintended flashes from spacecraft-propelling lasers to vast engineering projects, such as Dyson spheres or other “alien megastructures” built by those seeking to rearrange their planetary systems to harvest maximum energy from their star. 

If we’re open to technosignatures elsewhere in the galaxy, what about within the Sun’s orbital realm? Our infant civilization has already launched five spacecraft that will wander the galaxy, and we’re trying to figure out how to send tiny spacecraft to nearby exoplanets. Imagine what an aged ET civilization might have achieved and what machines might have entered our solar system over billions of years. A recently published white paper by a group of astronomers (including me) argues that as we explore the solar system, we should be on the lookout for artifacts (

This was highlighted when ‘Oumuamua, the first-ever interstellar visitor found in our solar system, exhibited a peculiar shape and acceleration. Could it be an artifact? Almost surely not — astronomers have found more prosaic and more likely explanations. Yet there’s value in such a false alarm, which invites us to ask, “Why not?”

Just as the galaxy doesn’t stop inside the Oort Cloud of comets, the solar system doesn’t stop at Earth’s upper atmosphere. This brings us to the possibility of finding ET technology in our skies.

Admittedly, when it comes to unidentified anomalous phenomena, or UAPs, astronomers often reflexively dismiss the subject, explaining to people we meet that we’re scientists, not UFO enthusiasts. There are many explanations — from lost balloons to internal camera reflections — for why people may see stuff in the air, and most of those explanations seem more likely than visitors from beyond our planet. Much obvious bullpucky also surrounds this topic, and anyone in astrobiology or SETI is so used to being bombarded with unhinged claims (you should see my inbox on any random week) that it’s easiest to dismiss all of it. 

But this response is intellectually lazy, and we back it up with rationalizations as to why we don’t have to worry about it, e.g., aliens would never spend all the energy needed to actually visit Earth; they’d surely just stay home and send messages instead. Yet we must admit that, given the unpredictability of advanced ET culture and technology, we have no idea what they’d value or be capable of.

We also don’t really know what we should be searching for. And astronomy, which assumes an objectivity and consistency to natural phenomena, is ill-equipped to study something that might be aware of and deliberately avoiding or confusing our observations, or even studying us. Nevertheless, there’s a logical continuity between extrasolar technosignatures, solar system SETI, and possible extraterrestrial UAPs. If we admit the worth of one, we cannot dismiss the others.

It’s a tricky balance, especially with a topic that’s so emotionally loaded for so many. But we need to follow the data wherever they lead and be open to all possibilities. Otherwise we might miss something really important.

This article originally appeared in print in the May 2023 issue of Sky & Telescope. Subscribe to Sky & Telescope.


Image of Rod


February 17, 2023 at 8:29 am

"We also don’t really know what we should be searching for."

An interesting report. Copernicus, Tycho Brahe, Galileo, Kepler, and Newton I suspect took a different approach to the heliocentric solar system astronomy vs. geocentric. Back then, science was taught to provide necessary demonstration from nature for the paradigm to be accepted as true.

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Tom Hoffelder

February 17, 2023 at 9:23 pm

In my humble opinion, which many of course are not willing to accept, because it is perhaps too frightening, I truly believe we are alone, at least in this corner of the Milky Way. I have always felt that way and then the book, Rare Earth: Why Complex Life is Uncommon in the Universe, came out in 2000. Being a nobody, it is always nice to see my gut feelings being reinforced by professionals, in this case Peter D. Ward, a paleontologist and Donald Brownlee, an astronomer. I urge you to please read it, to get an idea of why we really might be alone. Not only does it present so many unique situations regarding the Earth, but it covers other things, like mass extinctions (every time there was one, our ancestor at the time had to survive) and how difficult it is to get from basic life, bacteria and archea (something similar to bacteria), which is probably everywhere, upgraded to complex life, meaning a single cell creature that can evolve to higher levels. Based on the extent of life here, we are in a perfect Goldy Locks zone, and yet the appearance of a single cell animal happened ONLY ONCE in the long history of the Earth, because every living complex celled being today has evolved from that one chance occurrence! We are a fluke of the universe; we have no right to be here.

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February 18, 2023 at 1:11 pm

Hi David, fascinating to see the new NASA team get involved in UAPs. Just a point you made about astronomers in that Prof. Peter Sturrock of Stanford Uni. surveyed astronomers in 1975 about UAPs. From the NY Times link ... "He reported that 1,356 replied, of whom 23 percent said “certainly” U.F.O.'s merit such study. Another 30 percent said “probably” they do, 27 percent said “possibly” and the rest said probably not or certainly not. Of those who replied, 2.5 percent requested anonymity."
So that's a pretty good interest. This has surely increased after the revelations of the last few years, the new US legislation and the stigma on discussing this lifting.

Will you be investigating also the STS-48 shuttle objects seen on September 15 1991 from the shuttle? I actually bought the original VHS tape of Prof. Jack Kasher analysis when it came out! He provided several separate proofs that the objects had to be far away and they were also above the Earth's airglow. Tremendous observed accelerations which correspond to at least one of the well known "five observables" for UAPs we've heard of over the last few years.
Links are here of that analysis by Prof. Kasher from the NICAP organisation.

I studied astrophysics at university then went on to get a Masters in particle physics. Lapsed I'm afraid but still keenly interested! Regards.

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