Editor’s Note: For the remainder of 2025, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded articles published in AAS journals this year. The usual posting schedule will resume January 2nd.
Earth Detecting Earth: At What Distance Could Earth’s Constellation of Technosignatures Be Detected with Present-day Technology?
Published February 2025
Main takeaway:
In a collaborative effort between the Search for Extraterrestrial Intelligence (SETI) Institute and the Penn State Extraterrestrial Intelligence Center, Sofia Sheikh (SETI Institute) and team turned inward to expand the search for intelligent life in the universe. Right here on Earth, humans have created a laboratory of technological signatures that can be detected from space — if another Earth-like civilization is out there, could they detect us? Using theoretical modeling, the authors determined the reaches of various human technosignatures such as radio transmissions, optical and infrared emission, satellites, and other probes sent into space. They found humanity’s strongest signals, radio transmissions, could be detected as far as 12,000 light-years away.
Why it’s interesting:
Scientists search for intelligent civilizations by hunting for signs of technology — signals or patterns that natural phenomena cannot explain. While we cannot expect other intelligent life to look exactly like Earth’s, many SETI projects have surveyed the skies for hypothetical signals far exceeding humanity’s own technological advancements. Taking a step back, Sheikh and collaborators considered where human technology is currently and what present-day instrumentation could pick up on if we were to search for ourselves. This “Earth detecting Earth” paradigm recenters the search for intelligent life and provides a multiwavelength framework for understanding the detectability of technology on far-away worlds.
Maximum distances that Earth’s technosignatures could be detected by current technology. Click to enlarge. [Sheikh et al 2025]
Earth’s constellation of technosignatures:
What exactly are Earth’s many technosignatures, and how far could our current technology detect them? The most prominent and farthest detectable signal comes from radio transmissions. As the authors noted, these signals come from multiple sources: targeted radar transmissions used to characterize planets and asteroids, radio transmissions used to communicate between space probes and ground stations (e.g., space telescopes, Mars rovers, etc.), and radio leakage from Earth’s communication systems like cell towers and broadcasting stations. Additionally, there are atmospheric technosignatures from air-polluting compounds, optical and infrared emission from cities, targeted lasers from telescopes, and interplanetary and interstellar probes sent into space. All of these combined create a constellation of technosignatures detectable across a range of distances from Earth. This study underscores the importance of assessing Earth’s technology and detection capabilities, and repeating this type of study as technological advancements continue will enhance the search for intelligent life.
Astronomers from around the world were fascinated last year when 3I/ATLAS, the third interstellar object ever observed passing through our Solar System, was discovered.
The SETI Institute was one of the many organizations that examined the unusual interstellar object, conducting a comprehensive search for any signs that it might be producing technological signals.
Using the Allen Telescope Array (ATA) at the Hat Creek Radio Observatory in Northern California, SETI researchers scanned a broad range of radio frequencies in search of potential evidence of extraterrestrial technology associated with 3I/ATLAS. The data that came back was clear: no signs of artificial signals. As most astronomers expected, 3I/ATLAS is most likely a natural comet.
“We searched for narrowband signals across a broad range of spectrum: 1–10 GHz. We can do this efficiently due to the ATA’s ability to record and search billions of channels at a time,” says Dr. Sofia Sheik, lead author of a new paper describing the SETI Institute’s work, in an email to The Debrief.
The interstellar comet 3I/ATLAS (Image Credit: International Gemini Observatory/NOIRLab/NSF/AURA/Shadow the Scientist)
Observations of interstellar objects have become more common in recent years, due to advances in telescopic technologies available to astronomers. Discovered last July, 3I/ATLAS is only the third confirmed interstellar visitor to enter our Solar System, preceded by 1I/ʻOumuamua in 2017, and 2I/Borisov in 2019.
These rare discoveries help astronomers understand the materials in our cosmic backyard. By analyzing the composition of these objects, scientists can gain a deeper understanding of how planetary systems develop and evolve throughout the galaxy.
The Research
The majority of recent astronomical observations indicate that 3I/ATLAS is a natural object, albeit one that exhibited some unusual qualities. However, the possibility remains that we may one day find a technological artifact from an intelligent civilization; it is equally possible that at some point, aliens may also discover a spacecraft of Earthly origin.
“Eventually, our own Voyager spacecraft will be extraterrestrial artifacts in other stellar systems,” Sheik told The Debrief. “Given that, it is important that we understand the natural distribution of interstellar objects so that we will be able to identify any anomalies that could one day be signs of an artificial interstellar object.”
The SETI research team studied 3I/ATLAS for over seven hours using the ATA, searching radio frequencies from 1 to 9 gigahertz. This range is especially important because it can detect narrowband radio signals, which are rarely produced by natural space phenomena and are often considered possible signs of technological activity.
Researchers found 74 million narrowband signals, but after filtering out human interference and motion-matched sources, only about 200 remained. These were later confirmed to come entirely from Earth-based technology or satellites.
“To be clear, we did not detect any radio signals from 3I/ATLAS. We ruled out the presence of signals down to power levels comparable with everyday appliances: for example, we definitely would have picked up a cell tower or a radio station equivalent had it been on the surface of 3I/ATLAS,” Sheik said.
In other words, no technosignatures were detected. The SETI observation ruled out signals stronger than approximately 10 to 110 watts across the observed frequencies (comparable to the power consumption of a household appliance).
NASA diagram showing the trajectory of the interstellar object 3I/ATLAS (Image Credit: NASA).
The Origins of 3I/ATLAS
For a time, there was a considerable amount of speculation over whether 3I/ATLAS could represent something more than just a natural object. Among the proponents of such ideas, theoretical physicist Avi Loeb emerged as one of the most visible supporters of the possibility that the interstellar object displayed anomalous qualities that could point to technological origins.
Last July, shortly after the object’s discovery, Loeb predicted on his Medium page that 3I/ATLAS may be one of two things: a comet possessing a smaller core with a radius less than 400 meters, or “an object that favored a radial orbit into the inner solar system,” meaning that it may also be a larger object compared to its predecessors. Indeed, further observations of the object eventually suggested that 3I/ATLAS appears to be significantly larger than ‘Oumuamua or Borisov was.
Loeb also suggested in a podcast interview that 3I/ATLAS might not be following a random trajectory, but rather that its current motion could be understood as “directed” toward the inner solar system.
“That obviously opens the door for it being technological, because who would target the inner solar system?” said Loeb during an interview.
In an email to The Debrief, Loeb shared his thoughts on the recent SETI findings.
“Radio communication is an old technology, which was recently accompanied by optical laser communication in NASA’s Artemis II mission to the Moon,” Loeb said. “Lasers might replace radio communication in our future space missions, and there is no guarantee that they would operate at the frequencies scanned in this study.”
“To save power, a technological transmitter might beam its signal in the direction of the receiver (rather than Earth) and might only transmit sporadically,” Loeb added.
“Interestingly, the anomalous ‘Wow! Signal’ detected on August 15, 1977, came from a direction which is separated by only 9 degrees from the arrival direction of 3I/ATLAS into the inner Solar System, but a similar signal was never detected again,” Loeb told The Debrief.
The Search for Interstellar Objects
For Sheik and the team, while 3I/ATLAS did not display any obvious signatures that could be associated with technosignatures, it still provided a significant learning opportunity.
“The results from 3I/ATLAS show how realistic it is to detect a signal with the technology we have today,” said co-author Valeria Garcia Lopez. “That is why it is important to keep searching for technosignatures, even from objects we might not expect to have signals.”
Additionally, this project highlighted the ATA’s ability to respond quickly to new discoveries. Observations began less than 24 hours after the discovery of 3I/ATLAS was announced, showcasing the telescope’s flexibility and preparedness.
“One of the ATA’s advantages is being able to get quickly on-source to follow up on new and exciting discoveries, which was showcased in the 3I/ATLAS observations,” Sheik told The Debrief. “3I/ATLAS was announced on July 1st, and the ATA began observations on July 3rd.”
With ongoing advancements in technology, future observations of similar interstellar objects should become more frequent.
“For sources that might be changing quickly, this kind of rapid follow-up is essential to get early data and context for later observations with other observatories,” Sheik added.
Chrissy Newton is a PR professional and the founder of VOCAB Communications. She currently appears on The Discovery Channel and Max and hosts the Rebelliously Curious podcast, which can be found on YouTube and on all audio podcast streaming platforms. Follow her on X: @ChrissyNewton, Instagram: @BeingChrissyNewton, and chrissynewton.com. To contact Chrissy with a story, please email chrissy @ thedebrief.org.
Satan's rebellion has led humanity to believe in false origins of life, such as abiogenesis and panspermia, instead of a divine Creator. I critique the scientific community's efforts to explain life without acknowledging supernatural creation, asserting the biblical viewpoint that true life and intelligence can only stem from God.
The Search for Extra-Terrestrial Intelligence (SETI) program has yielded no evidence of intelligent life beyond Earth, raising questions about life’s uniqueness to our planet. Critics claim SETI lacks empirical support and resembles ufology. The author argues that life is a divine creation, asserting evolution cannot account for life's origin or complexity.
Earth has many unique features for a planet, such as a magnetic field, a large moon, and plate tectonics. It’s also the only planet we know of that harbors life. These facts form the basis of the Rare Earth hypothesis, which posits that we haven’t found aliens because other planets in the Galaxy probably don’t have all the right conditions for life.
Another characteristic of Earth is that about 30% of its surface is land and about 70% is ocean. Recently, Columbia University Assistant Professor David Kipping investigated whether the proportion of Earth’s surface covered by dry land versus ocean, or its land fraction, is another reason Earth is habitable not only for simple single-celled organisms, but also for intelligent species like humans.
To test this hypothesis, Kipping created 4 statistical models of planets with different land fractions that intelligent aliens could potentially evolve on. First, he created an equation to describe the likelihood that a planet in its star’s habitable zone has a particular land fraction, known as a probability distribution. Kipping weighted this probability distribution toward the extreme ends, making it more likely that a planet would be covered by a single huge landmass or a single vast ocean than by a mix of both, as on Earth.
Kipping then incorporated this land fraction probability distribution into his statistical models to calculate the probability that a random planet will have that land fraction and host intelligent life. The 4 scenarios Kipping tested were: 1) that intelligent life is more likely to emerge on land-dominated planets, 2) that it’s more likely to emerge on ocean-dominated planets, 3) that it’s more likely to emerge on planets with roughly equal amounts of land and ocean, and 4) that its emergence is independent of a planet’s land fraction.
As a first step in determining the kinds of planets intelligent aliens would tend to emerge on, Kipping used each model to predict the probability that intelligent life would emerge on a planet with the same land fraction as Earth. He then compared these probabilities by calculating the ratios between each value. Because Earth is the only known planet with intelligent life, a model that predicted a greater probability for humanity’s existence on Earth would be more likely to reflect reality.
Kipping considered it strong evidence that a given model was more realistic than another if the ratio between 2 of them was greater than 10, meaning one model was 10 times more likely to predict the existence of Earth and humanity. Kipping found that no comparison of any 2 models passed this threshold. However, the models assuming that intelligent life prefers ocean-dominated planets or planets with a land-ocean balance were 2.5 and 3 times more likely to predict the existence of humanity than the model assuming that intelligent life prefers land-dominated planets. Additionally, the model assuming that intelligent life prefers a land-ocean balance was always more likely to predict humanity than any other model, though marginally.
Kipping also addressed whether finding more planets with intelligent life would affect which model was deemed most realistic, for example, if scientists discovered conclusive evidence of life on Mars in its distant past. Here, Kipping identified 2 complications. First, it’s uncertain how much of Mars’s surface was once covered by water – some estimate it had a land fraction as high as 81%, while others estimate it was as low as 25%. Second, proving that Mars once had life would not prove it once had intelligent life.
Regardless, Kipping reran the models assuming that ancient Mars had a land fraction comparable to Earth’s. Adding this second data point produced ratios similar to those in the earlier Earth-only calculations, meaning it still didn’t make any single model 10 times more likely to predict the existence of humans and Martians, respectively.
Kipping then took the 10-times threshold and reversed the calculations to find what conditions would exceed it. In doing so, he calculated that astronomers would need to find 14 other planets with intelligent life and known land fractions to robustly determine whether intelligent life is more likely to occur on desert planets, ocean planets, balanced planets, or without bias.
Kipping concluded that he can’t yet definitively state whether there is something special about Earth’s land fraction when it comes to producing intelligent species. However, Earth’s existence would suggest that intelligent life is unlikely to favor extreme desert planets, so the Milky Way probably isn’t filled with Tatooines and Jakkus. And while his analysis doesn’t debunk the Rare Earth hypothesis, it does undermine the argument that Earth’s ocean size explains why Earth is rare.
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