Last year HarperCollins published Adam Frank’s The Little Book of Aliens. Frank is an accomplished astronomer, both in terms of research and in terms of public outreach. He is a Professor at the University of Rochester. He is the Principal Investigator on NASA’s first-ever grant to study “planetary technosignatures” — technosignatures being the technological markers produced by intelligent life (markers, possibly observable from great distances, that could provide to us on Earth evidence of intelligent life on distant planets). He writes books, is on NPR, is published in The Atlantic and the New York Times, etc. — the book’s “About the Author” discussion fills a page. Readers of The Little Book of Aliens will learn a lot about science from someone who both knows the subject and knows how to communicate.
And yet…
The Little Book of Aliens emphasizes that we are now poised to finally do real science regarding the question of life on other worlds. Because of advances in our technology, we are now going to get, as Frank puts it, “a true scientific view of if, where, and when extraterrestrial life exists.”* “Now, finally, we are on the road” to finding aliens, or to finding that we are alone.
I disagree. I think astronomy’s history says we not finally on the road. Rather, we are approaching the end of the road. The Little Book of Aliens reflects a long-standing trend in science writing for the public, one that hinges on bad history or merely ignorance of history. This trend is not serving science well. It is going to serve science even worse in the future. I think the scientific community needs to recognize that we are approaching the end of the road, and needs to prepare for that end. We risk reaching that end with a public that is angry and disillusioned with science. To know where you are on the road requires knowing where you came from — thus the importance of history.
One of the many cool things that Frank discusses is the idea of artifacts left by aliens visiting our solar system. He notes that the various pieces of equipment that we have placed on the moon since the 1960s “will stay intact and discoverable for possibly millions of years or more” thanks to the lack of weather on the moon to degrade them. He then asks, “if aliens passing through the solar system a long time ago left equipment on the Moon, would we still be able to find it?”
Yes, we should be able to. Therefore he says, noting the “kinds of searches that are just becoming possible”, that we should direct some of the effort we expend on exploring the solar system toward “looking for stuff left behind by aliens”. What are these kinds of searches? Using an AI to scan vast numbers of high-resolution images of the moon for anomalies is one example Frank gives. Looking for ancient alien equipment in those parts of the solar system where orbits can be stable for long periods of time is another.
Frank describes how he was first brought around to the idea of hunting for alien artifacts by a 2020 presentation on alien lurkers. Imagine that, in the future, with some future super-telescope, we discover a planet orbiting another star, presumably not too far from Earth; within the planet’s light are clear signs of life. Well, Frank says, as soon as we could develop the technology to send a probe there and study that planet more carefully, we would. Our probe, hanging out in that star system, observing that planet, would be a lurker. That 2020 presentation sold Frank on the logic of looking for alien artifacts.
And so, he writes,
No matter how you think about it, solar system SETI [Search for Extra-Terrestrial Intelligence] artifacts represent a viable stream of technosignature research. Thanks to rapid advances in our own technology (AI and interplanetary exploration), the time to begin these searches in earnest has finally arrived.
The idea is interesting and logical, as is so much of what Frank discusses. But the time has not “finally arrived”; these kinds of searches are not “just becoming possible” today. We have been able to search for alien artifacts within the solar system since the invention of the telescope.
Imagine that it is fifty thousand years ago. An advanced alien civilization sets up shop on the moon. They have noticed that Earth is home to an intelligent species that shows real promise (namely, us). The aliens build a huge observing station on the moon to watch us, with the idea of making contact at the right time. The station attracts some visitors. Before long, there is on the moon a whole complex of structures, transit corridors, water tanks, atmospheric domes — a small city; an alien eco-tourist trap built around the serious scientific effort to study these Earthlings.
But, in time it becomes clear that the Earthlings are not so promising after all. It becomes clear that they are not going to make much progress for a long time. Yes, the Earthlings make some nice cave paintings and so on, but the fact is, they advance little, even over a span of centuries. The tourism falls off. The alien funding agency pulls the plug on the observing station. By forty-nine thousand years ago the whole place is abandoned, a lunar alien ghost town, with only the built stuff and some abandoned equipment remaining, and with a lot of trash lying around.
And so the ghost town remains, for almost fifty millennia, until the early seventeenth century. Then the telescope gang shows up and starts observing the moon. Galileo sees craters. Fr. Christoph Scheiner (S.J.) makes this map of the moon in 1614:
And in 1651, Fr. Giovanni Battista Riccioli (S.J.) makes the map of the moon seen below, giving lunar features the names we still use today (when Apollo 11 reported “Tranquility Base here; the Eagle has landed” — the area of the moon called “Tranquility” had been so named by Riccioli).
Well, that big, sprawling alien research station/tourist trap ghost town would be visible by telescope. Someone would see it, eventually, and say, “what in the world is that?” We have been searching for alien artifacts on the moon for centuries, whether we understand that or not.
Riccioli understood this in some way. He studied the moon carefully enough to be convinced that the moon had never been home to anyone. Do you know what he had inscribed on the top of his map? “No men dwell on the moon — no souls travel there.”
Not everyone saw things as Riccioli did. William Herschel (the first astronomer to discover a new planet — Uranus) thought he could observe lunar forests. In his notes from May 28, 1776 he discussed a thing he saw when studying “Mare humorum, and this I now believe to be a forest”. He believed that “there must be inhabitants on the Moon of some kind or another”.*
Of course, Riccioli was right; Herschel was wrong. As telescopes got better and better, and then as we began sending probes to the moon, the upper limit on the size of any possible alien artifacts existing on the moon that we had not happened upon (even accidentally) got smaller and smaller.
Likewise for alien probes lurking in orbit somewhere. If the aliens had constructed a vast space station (now long-abandoned) in which to lurk, bigger than the asteroid Ceres, then we might suppose that Fr. Giuseppe Piazzi would have discovered it instead of Ceres in 1801. If Piazzi did not discover it, then someone else would have, in the searches for asteroids that followed. If the station was smaller than Ceres, maybe it would have taken longer for it to be discovered.
Once discovered, eventually some astronomer would have been trying to determine the characteristics of this “asteroid” for his or her thesis project, and would have noticed there was something weird about it. And then this weird asteroid would have attracted further study, revealing further weirdness. Eventually it would have been visited by a NASA probe, or imaged by Hubble or Webb, and by now we would know what it was. Every country in the world that could would be sending robots to crawl all over it (and probably working hard on trying to send people to it). We would not be all excited about getting a cup of dirt from the asteroid Bennu (sorry Br. Bob Macke!).
So what kinds of alien artifact searches “are just becoming possible” today? What sort of artifact searches can begin in earnest thanks to “rapid advances in our own technology”? The last of them, that’s what. We have the ability now to find small stuff on the moon — things like the lower portion of the Apollo 11 lander that is still there. Frank notes that an AI search of high resolution lunar images had “no problem picking out the lander”. Once someone gets funding to do an AI search of all the lunar imagery we have, then we are going to find out if there are any alien artifacts of the size of the stuff we have put on the moon.
And if there are not, then the story of looking for alien stuff on the moon, a story that began over four centuries ago with the invention of the telescope, is going to be over. No men on the moon; no ancient cities on the moon; no forests on the moon; no alien devices of the size of our devices on the moon. What will we do then? — assume that aliens sent little tennis-ball-sized probes to the moon to lurk on us, and search for them? No, the idea of alien artifacts on the moon is going to be finished, one way or another. We are not finally on the road; we are approaching the end of the road.
We need to tell the public that. We need to explain that, while artifact searches are a cool idea, and worth pursuing, the fact is that we’ve found nothing so far, despite four centuries of data. Our remaining chance for success here depends on aliens being like us — building stuff the size we would build, and nothing larger. What will people think if we reach the end of the road, having found nothing, and without having explained this? We scientists must recognize that we are approaching the end of the road, ponder what that means, and share that with the public. It will not serve science well to keep talking as though we are finally beginning.
*Adam Frank quotations here are from pages xiv-xv and 148-150 of The Little Book of Aliens (New York: HarperCollins, 2023). Quotations from William Herschel are from Michael J. Crowe, The Extraterrestrial Life Debate 1750-1900: The Idea of a Plurality of Worlds from Kant to Lowell (Cambridge: Cambridge University Press, 1986), page 63.
