UNIVERSITY PARK, Pa. — If an extraterrestrial intelligence were looking for signs of human communications, when and where should they look? In a new study, researchers at Penn State and NASA's Jet Propulsion Laboratory in Southern California analyzed when and where human deep space transmissions would be most detectable by an observer outside our solar system and suggest that the patterns they see could be used to guide our own search for extraterrestrial intelligence (SETI).
"Humans are predominantly communicating with the spacecraft and probes we have sent to study other planets like Mars," said Pinchen Fan, graduate student in astronomy and astrophysics in the Penn State Eberly College of Science, science principal investigator of the NASA grant supporting this research and first author of the paper. "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."
A paper describing the research appears today (Aug 21) in the Astrophysical Journal Letters , and the authors present their findings today at the 2025 Penn State SETI Symposium , hosted by the Penn State Extraterrestrial Intelligence Center .
"SETI researchers often search the universe for signs of past or present technology, called technosignatures, as evidence of intelligent life," Fan said. "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."
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. The research team 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.
"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," said Joseph Lazio, project scientist at JPL and an author of the paper. "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 this study, the researchers 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.
The researchers 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."
To improve our own search for technosignatures, the researchers said, humans should look for alignment of exoplanets — planets outside our solar system — 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, the researchers explained. 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 research team also calculated that an average DSN transmission could be detected about 23 light-years away using telescopes like ours. Focusing efforts, they said, 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. The team now plans to identify these systems and quantify how frequently they could have received signals coming from Earth.
The DSN transmission patterns found also could be applied to searches for laser transmissions from exoplanets, the researchers said, though they noted that lasers would have much less spillover than radio transmission. NASA is testing its interplanetary laser communication system, and extraterrestrial civilizations may opt to use lasers instead of radio waves.
"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 Jason Wright, professor of astronomy and astrophysics in the Penn State Eberly College of Science, director of the Penn State Extraterrestrial Intelligence Center, and an author of the paper. "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."
Funding from the NASA Exoplanets Research Grant Program and the Penn State Extraterrestrial Intelligence Center supported this work. Computations for this research were performed on the Penn State Institute for Computational and Data Sciences Roar supercomputer.
