Since the dawn of the Space Age, agencies have relied on powerful arrays of communication antennas positioned worldwide to control, coordinate, and retrieve data from their missions. Today, NASA and its partner agencies rely on the Deep Space Network (DSN) to communicate with the many probes, orbiters, landers, and rovers they have operating beyond Earth. These signals also lead to "spillover," where radio signals reach far beyond robotic missions and propagate for light-years through space.
For those engaged in the Search for Extraterrestrial Intelligence (SETI), "spillover" from communications could be a promising technosignature. In a new study, researchers from Penn State University and NASA's Jet Propulsion Laboratory (JPL) analyzed when and where transmissions from the DSN would be most detectable outside of our Solar System. Their findings suggest that future SETI surveys should look for signs of radio communications along the ecliptic plane of other systems, emanating toward or away from the star, and toward any orbiting exoplanets.
The research was led by Pinchen Fan, a graduate student with the Eberly College of Science at Pennsylvania State University and the science principal investigator of the NASA grant supporting this research. She was joined by Jason Wright, a Professor of astronomy and astrophysics at Penn State and the leader of the Penn State Extraterrestrial Intelligence Center and a member of the Center for Exoplanets and Habitable Worlds, and T. Joseph W. Lazio, an Interplanetary Network Directorate Scientist at NASA's Jet Propulsion Laboratory (JPL).
For their study, the researchers analyzed logs from NASA's Deep Space Network (DSN), which they carefully matched with the locations of spacecraft in the Solar System to determine the timing and direction of radio communications. This included transmissions to space telescopes and interplanetary spacecraft rather than missions or satellites in Low-Earth Orbit (LEO), which are relatively low-power and would be difficult to detect beyond the Solar System. As Fan indicated in a Penn State press release:
Humans are predominantly communicating with the spacecraft and probes we have sent to study other planets like Mars. But a planet like Mars does not block the entire transmission, so a distant spacecraft or planet positioned along the path of these interplanetary communications could potentially detect the spillover; that would occur when Earth and another solar system planet align from their perspective. This suggests that we should look for alignment of planets outside of our solar system when searching for extraterrestrial communications.
While Roscosmos, the China National Space Agency, and other countries have deep-space networks, the team emphasized that the DSN was the best benchmark because NASA has led most deep-space missions to date. As a member of the Interplanetary Network Directorate, which oversees operations for the DSN, Joseph Lazio is intimately familiar with deep-space communications.
"NASA's Deep Space Network provides the crucial link between Earth and its interplanetary missions like the New Horizons spacecraft, which is now outbound from the Solar System, and the James Webb Space Telescope," he said. "It sends some of humanity's strongest and most persistent radio signals into space, and the public logs of its transmissions allowed our team to establish the temporal and spatial patterns of those transmissions for the past 20 years."
Their research revealed that deep space radio signals using the DSN were predominantly directed toward spacecraft near Mars. This is understandable, seeing as how so many missions are currently conducting astrobiology missions there. Other transmissions were directed towards other planets and the Sun-Earth Lagrange points, where NASA's James Webb Space Telescope, the ESA's Euclid, and other observatories are currently operating. They also found that most DSN communications occurred within 5 degrees of Earth's orbital plane. This was also expected since the Solar System is relatively flat and most of its planets orbit along the ecliptic plane.
Their results also showed that an average DSN transmission could be detected by instruments similar to ours about 23 light-years beyond our Solar System. In this volume of space, there are 19 systems with at least one confirmed exoplanet. Therefore, the team recommends that SETI efforts focus on these star systems, especially those whose planes are oriented edge-on with Earth. In addition, they recommend that SETI researchers look for alignments between exoplanets (or alignments with their host star) for signs of transmissions, which will likely be visible along the ecliptic plane. Said Fan:
Based on data from the last 20 years, we found that if an extraterrestrial intelligence were in a location that could observe the alignment of Earth and Mars, there's a 77% chance that they would be in the path of one of our transmissions — orders of magnitude more likely than being in a random position at a random time," said Fan. "If they could view an alignment with another solar-system planet, there is a 12% chance they would be in the path of our transmissions. When not observing a planet alignment, however, these chances are minuscule.
These findings could have significant implications for astrobiologists and SETI researchers. For instance, astronomers frequently study exoplanets during alignments with their parent star. This practice, known as the Transit Method (Transit Photometry), is how most exoplanets have been discovered to date. It consists of observing distant stars for periodic dips in luminosity, which may indicate a planet passing in front of them (transiting) relative to the observer. Sometimes, they are able to observe light passing through the exoplanet's atmosphere with spectrometers, which can provide data on its chemical composition.
However, Fan emphasized that there is still much to be learned about the orbital arrangements of other planetary systems. "[B}ecause we are only starting to detect a lot of exoplanets in the last decade or two, we do not know many systems with two or more transiting exoplanets," Fan added. "With the upcoming launch of NASA's Nancy Grace Roman Space Telescope, we expect to detect a hundred thousand previously undetected exoplanets, so our potential search area should increase greatly."
"Humans are pretty early in our spacefaring journey, and as we reach further into our solar system, our transmissions to other planets will only increase," said Prof. Wright. "Using our own deep space communications as a baseline, we quantified how future searchers for extraterrestrial intelligence could be improved by focusing on systems with particular orientations and planet alignments."
The researchers also found that the DSN transmission patterns could be applied to searches for directed-energy (DE) transmissions, similar to the interplanetary laser communication system NASA is currently testing. This echoes recommendations previously made by Prof. Philip Lubin, a professor of Physics at UC Santa Barbara and the head of its Experimental Cosmology Group. In his 2016 paper, "The Search for Directed Intelligence," he recommended that future SETI searches should look for indications of "spillover" from directed energy communications or propulsion.
However, the team also noted that lasers would have much less spillover than radio transmissions. Looking to the future, the team plans to study the systems located within a 23 light-year radius from our Sun and quantify how frequently they could have received signals coming from Earth. The authors presented their findings at the 2025 Penn State SETI Symposium, hosted by the Penn State Extraterrestrial Intelligence Center (PSEI), and a paper describing the research recently appeared in the Astrophysical Journal Letters.
Further Reading: Penn State University, Astrophysical Journal Letters