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Scientists Made a List of Every Place Aliens Could Be Hiding

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Artistic depiction of a pulsar, which were thought to resemble an alien beacon when discovered in the 1960s.
Gif: NASA/Gizmodo

A team searching for signs of extraterrestrial intelligence has put together an Exotica Catalog, which lists every known type of celestial object, regardless of whether we understand it or not.

The Exotica Catalog was put together by researchers from Breakthrough Listen, a decade-long program seeking to find signs of intelligent life in the universe. It ambitiously includes “one of everything” in astronomy. Incredibly, the catalog lists over 700 distinct targets, ranging from asteroids, comets, and planets through to pulsars, galaxies, nebulae, and star clusters. The point of the catalog is to “expand the diversity of targets surveyed in the Search for Extraterrestrial Intelligence (SETI),” according to the team’s paper, but the compendium could be leveraged for other purposes.


“In the future, it might also benefit students or members of the public who are trying to get a big-picture overview of astronomy or are trying to decide what to study,” Brian Lacki, a Breakthrough Listen researcher from the Department of Astronomy at the University of California Berkeley and first author of the paper, wrote in an email to Gizmodo. “Some of the objects on the lists were things I didn’t learn about until grad school or later, or even until we were compiling the catalog.”

Lacki hopes the catalog will inspire scientists to explore some of the more esoteric or mysterious aspects of astronomy, but its primary purpose is to assist SETI researchers and astrobiologists.


Various classification schemes have been attempted before, but nothing on this scale. In total, Lacki and his colleagues identified 737 distinct astronomical objects, some very well understood and others in desperate need of scientific attention. While the catalog was being compiled, and as the number of targets grew and grew, Lacki became concerned with the ballooning number, simply for purposes of practicality.

“Fortunately, the team has been very supportive of the project even as that number grew,” he said. “What happens is that as you read more and more of the literature, and you learn more about all these different things, you find more and more that stands out just enough. So at times there was a feeling of ‘just one more.’”

Lacki likened the process to biology, in which “you have a few large groupings, but you can keep getting more detailed until you have millions of species.” Take planets, for example, of which there are several known types, including hot Jupiters, mini-Neptunes, super-Earths, and sub-brown dwarfs, just to name a few.

Fascinatingly, the new catalog also includes some technological objects in space, all of which belong to us. These include the Voyager probes, the New Horizons spacecraft, weather satellites, and even an automobile in the form of Elon Musk’s red Tesla Roadster. Kinda makes you wonder about all the weird stuff aliens might have tossed into space, some of which might even make it into this catalog some day.


The Exotica Catalog further signifies the ongoing shift away from traditional SETI strategies, in which scientists search for “familiar” alien signatures (such as radio emissions), and the shift toward Dysonian SETI, in which scientists look for extraterrestrial technosignatures, that is, signs of alien technology: stuff like Dyson shells (a star surrounded by solar panels), industrial waste, gigantic space habitats, beacons, and things we can’t even imagine.


A driving motivation for Lacki when compiling this catalog was the idea that some alien intelligences might be exceptionally different from us, and by consequence, we’re looking for them in the wrong places.

“For decades, people have been wondering, what if they’re actually machines, or do not use water, or are based on silicon or plasma or neutron star matter? Of course, that’s a common theme in science fiction, but those ideas have been spreading a bit into SETI as well. It’s exciting that we can begin to start answering that question,” said Lacki. “It may even be possible that there are things out there where it’s ambiguous if they are ‘intelligent’ but that are in some way just as interesting.”


This catalog should help in this regard, as it could flag potential targets of interest among Dysonian-minded alien hunters. What’s more, the compendium could also perform the opposite function, providing a natural explanation for something that appears to be artificial.

Lacki and his colleagues slotted astronomical objects into four major categories: Prototypes, Superlatives, Anomalies, and Controls (basically, boring reference points not expected to yield positive results).


Prototypes are stuff we’re super familiar with, like planets, galaxies, star clusters, neutron stars, black holes, and so on. Impressively, each individual item was assigned an archetypal example for reference. Superlatives serve as a kind of Guinness Book of World Records, providing a list of objects with the most extreme characteristics, such as the biggest planet, hottest star, or fastest-spinning pulsar. Anomalies are exactly as they sound: objects we don’t fully understand—things like variable stars (e.g. Boyajian’s Star), vanishing stars, fast radio bursts, and other unexplained phenomena.

Among the various anomalies listed, Lacki said NGC 247 is among his favorites. This object is a relatively nearby spiral galaxy that appears to have a hole in one side of its disk, with fewer stars than normal. Now, an idea put forth in SETI is that some advanced aliens can hop from star to star within their host galaxy, and they build Dyson spheres around each star to harvest as much energy as possible.


“That’s a giant shell or swarm that completely surrounds a sun, which would hide it from our view. So if we caught a galaxy midway through that process, or if they stopped before shrouding the entire galaxy, one expects to find a galaxy with a hole in it,” Lacki told Gizmodo. “That’s probably not what is going on with NGC 247—we expect the sunlight to heat the Dyson sphere until it glows in infrared light, and we don’t see that from the ‘hole’ as far as I know.”

Still, it’s definitely weird, and Lacki says little work has actually been done to fully explain this strange hole.


With this catalog, SETI scientists and astrobiologists finally have a handy reference guide to cross-check their findings and to hopefully inspire new avenues of research. Perhaps soon we’ll finally learn if someone’s out there.