Human radio signals sent to spacecraft near Mars (lower left) and other planets spill over. An alien intelligence could detect this during planetary alignments, suggesting a new way to search for extraterrestrial intelligence. Credit: Zayna Sheikh, CC BY-SA
- A new SETI methodology focuses on detecting the “spillover” from directed interstellar communications, mirroring how we communicate with our own spacecraft, rather than relying on broad, speculative searches.
- This approach leverages the predictable alignment of transmissions within a planetary system’s orbital plane, estimating a 77% chance of detecting a similar civilization’s signal during Earth-Mars alignments.
- The study calculates a detection limit of approximately 23 light-years for a civilization with comparable technology, utilizing the Deep Space Network data and Green Bank Telescope capabilities as a reference.
- The method’s practicality was demonstrated via a null result search of the TRAPPIST-1 system, during predicted planet-planet occultations, with future exoplanet discoveries expected to significantly expand the number of suitable targets.
A new study suggests a novel approach in the long-running scientific endeavor to find intelligent life beyond Earth. Instead of casting a wide, speculative net across the cosmos, researchers from Penn State and NASA’s Jet Propulsion Laboratory propose a more targeted strategy: listen for alien civilizations in the same way they might be listening for us. By analyzing our deep space communications, they’ve identified specific patterns that could aid in the search for extraterrestrial intelligence (SETI). Their paper, to be published in The Astrophysical Journal Letters, argues that the key may lie in looking for the “spillover” from routine interplanetary conversations.
What is SETI?
The search for extraterrestrial intelligence, or SETI, is a scientific field dedicated to finding evidence of technological life elsewhere in the universe. It’s a quest that began in 1960 when astronomer Frank Drake pointed a radio telescope at two nearby Sun-like stars, hoping to catch a stray signal. While that initial search, Project Ozma, came up empty, it ignited a global effort that has continued for over six decades. SETI operates on the assumption that if other intelligent species exist, they might be using technology, like radio waves, that we could detect.
Early SETI efforts were sporadic and limited by the available equipment. The field gained momentum with projects like the Big Ear telescope at Ohio State University, which in 1977 detected the famous “Wow!” signal — a powerful, narrowband radio signal that remains an unexplained candidate for an extraterrestrial transmission. Over the years, the search for these “technosignatures” has become more sophisticated. Beyond listening for radio signals or looking for laser pulses, modern SETI also considers searching for signs of massive-scale engineering, such as the waste heat from a Dyson sphere built to enclose a star, or even industrial pollutants in the atmosphere of an exoplanet. The rise of powerful computers has allowed for the automated analysis of vast amounts of data to sift through the cosmic noise.
Studying our cosmic footprint
The new method, proposed by a team led by Penn State graduate student Pinchen Fan, flips the traditional SETI script. Instead of guessing where aliens might broadcast from, it starts by asking: if a distant civilization were looking for us, what would they see? The logic is that by understanding our own transmission patterns, we can develop a targeted search for other civilizations behaving similarly. To establish that baseline, the researchers analyzed 20 years of data from NASA’s Deep Space Network (DSN). The DSN is our planetary switchboard, a global system of antennas that sends commands to and receives data from our interplanetary spacecraft. The study found that our transmissions are not randomly distributed; they are highly directed and predictable.
Humanity’s strongest and most persistent signals are beamed toward our robotic explorers. We are constantly talking to spacecraft at Mars, Jupiter, and beyond. Because our solar system is fairly flat, with most planets having formed from the same spinning disk of gas and dust, our transmissions tend to be aligned with this plane. As the researchers noted, the vast majority of our deep space communications are aimed within 5 degrees of Earth’s orbital plane. These radio transmissions are powerful and focused, but they aren’t perfectly contained. As a signal travels millions of miles to a probe, it spreads out, and a significant amount “spills over” into the space behind the target.
This creates a predictable technosignature for any distant observers who happen to be aligned with us and our target planet. The study quantifies this dramatically: if an extraterrestrial intelligence were in a location to observe an alignment of Earth and Mars, there’s a 77 percent chance they would be in the path of one of our transmissions. As Fan noted in the Aug. 21 press release, this is “orders of magnitude more likely than being in a random position at a random time.” During alignments with other solar system planets, the chance is reduced to 12 percent, but during times of no alignment, the chances are “minuscule.” This makes these predictable alignments the most likely times for civilization on Earth to be detected.
The planetary alignment strategy
If an alien civilization explores space in a similar fashion, their communication patterns might mirror our own. They, too, would likely be sending focused transmissions to their spacecraft. Think of our solar system as a dinner plate, with the planets orbiting on its surface because they all formed from that initial flat disk. Our deep-space signals largely travel along the surface of this plate. To intercept a similar signal from another civilization, we would need to be looking at their dinner plate from the side. The edges of our plate would need to align with the edges of their plate.
The most common method for discovering exoplanets is the transit method, which spots the slight dimming of a star as a planet crosses in front of it. This can only happen if we are viewing that system’s dinner plate along its edge. Therefore, the transit method naturally selects for systems that are already in the geometric alignment needed for us to observe their planets lining up.
The team also calculated a practical distance limit based on the distance at which our spill-over signal can be detected; an alien network equivalent to our own could be detected by a facility like the Green Bank Telescope from about 23 light-years away, adding another crucial filter to the search.
The strategy in action: a search of TRAPPIST-1
Researchers have already applied this technique to one of the closest and best known systems that meets those criteria, TRAPPIST-1. The system has seven transiting rocky exoplanets packed in closely to their host star, making it a perfect laboratory for this technique. In a study published in late 2024, a team led by Nick Tusay, a graduate student at Penn State, used the Allen Telescope Array to conduct a 28-hour radio technosignature search of TRAPPIST-1 with this planetary alignment technique.
The team specifically observed the system during seven predicted “planet-planet occultations,” or PPOs — the precise windows when one planet passed in front of another from our vantage point. After carefully filtering millions of potential signals, the researchers reported a null result: they found no evidence of signals of nonhuman origin.
While not the detection SETI scientists hope for, the result demonstrated the practicality of the method.
And while our catalog of systems that meet the stringent criteria for this method is currently rather small, that is poised to change dramatically with the upcoming launch of NASA’s Nancy Grace Roman Space Telescope. That telescope is expected to discover 100,000 new exoplanets, opening up a world of possibilities for this technique.
