In a new study, a team of astronomers from Penn State and NASA’s Jet Propulsion Laboratory analyzed when and where human deep space transmissions would be most detectable by an alien observer outside our Solar System and suggest that the patterns they see could be used to guide our own search for extraterrestrial intelligence (SETI).
Analyzing Deep Space Network uplink transmission logs over the last 20 years, Fan et al. found that these emissions were predominantly directed along the ecliptic plane, toward or directly away from the Sun, and toward other planets. Image credit: Gemini AI.
“Humans are predominantly communicating with the spacecraft and probes we have sent to study other planets like Mars,” said Penn State graduate student Pinchen Fan.
“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.”
“SETI researchers often search the Universe for signs of past or present technology, called technosignatures, as evidence of intelligent life.”
“Considering the direction and frequency of our most common signals gives insights into where we should be looking to improve our chances of detecting alien technosignatures.”
In the study, the researchers analyzed logs from NASA’s Deep Space Network (DSN), a system of ground-based facilities that permits two-way radio communications with human-made objects in space, acting as a relay to send commands to spacecraft and receive information they send back.
They carefully matched up DSN logs with information about spacecraft locations to determine the timing and directionality of radio communications from Earth.
Although several countries have their own deep-space networks, the researchers said that the NASA-run DSN should be representative of the types of communications coming from Earth, in part because NASA has led most deep-space missions to date.
“DSN provides the crucial link between Earth and its interplanetary missions like NASA’s New Horizons spacecraft, which is now outbound from the Solar System, and the NASA/ESA/CSA James Webb Space Telescope,” said Dr. Joseph Lazio, an astronomer at NASA’s Jet Propulsion Laboratory.
“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.”
For the study, the scientists focused on transmissions to deep space, including transmissions to telescopes in space as well as interplanetary spacecraft, instead of transmissions intended for spacecraft or satellites in low-Earth orbit, which are relatively low power and would be difficult to detect from a distance.
They found that deep space radio signals were predominantly directed toward spacecraft near Mars.
Other common transmissions were directed toward other planets and to telescopes at Sun-Earth Lagrange points — points in space where the gravity of the Sun and Earth keep the telescopes in a relatively fixed position as viewed from Earth.
”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,” Fan said.
“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.”
According to the team, to improve our own search for technosignatures, humans should look for alignment of exoplanets or at least when exoplanets align with their host star.
Astronomers frequently study exoplanets during alignment with their host star. In fact, most of the currently known exoplanets were detected by looking for the darkening of a star when a planet crosses in front of, or transits, its host star from Earth’s line of sight.
“However, because 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 said.
“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.”
Because our Solar System is fairly flat with most planets orbiting on the same plane, the majority of DSN transmissions occurred within 5 degrees of Earth’s orbital plane.
If the Solar System were a dinner plate with all the planets and objects sitting on that plate, human transmissions tended to follow along the plate’s surface, rather than shooting out into space at a stark angle.
The authors also calculated that an average DSN transmission could be detected about 23 light-years away using telescopes like ours.
“Focusing efforts on solar systems that are within 23 light-years and especially whose plane is oriented with its edge toward Earth could improve our search for extraterrestrial intelligence,” they said.
The team now plans to identify these systems and quantify how frequently they could have received signals coming from Earth.
“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 Penn State Professor Jason 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 team’s paper was published online today in the Astrophysical Journal Letters.
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Pinchen Fan et al. 2025. Detecting Extraterrestrial Civilizations that Employ an Earth-level Deep Space Network. ApJL 990, L1; doi: 10.3847/2041-8213/adf6b0
