We may finally know how aliens are going to find us

A new study from Penn State University and NASA Jet Propulsion Laboratory looks at how extraterrestrial observers might detect signals coming from Earth. The research, published August 21 in Astrophysical Journal Letters and presented at the 2025 Penn State SETI Symposium, focuses on the patterns of human deep space transmissions and how they could guide the search for extraterrestrial intelligence (SETI), the scientific effort to detect evidence of advanced alien civilizations through signs such as artificial radio or laser signals known as “technosignatures.”

SETI often involves scanning the skies for electromagnetic signals — waves of energy that include radio waves and travel across space — that might come from alien civilizations. To improve this search, scientists sometimes use our own transmissions as a model. Previous work suggested that the most noticeable technosignatures from Earth are signals sent through deep-space networks and interplanetary radar. This study builds on that idea by analyzing 20 years of logs from NASA Deep Space Network (DSN), a global system of giant antennas that communicates with spacecraft far beyond Earth.

The researchers found that most signals were sent along the ecliptic plane — the flat region around the Sun where most planets orbit — and often toward other planets. “Humans are predominantly communicating with the spacecraft and probes we have sent to study other planets like Mars,” said Pinchen Fan, graduate student at Penn State and lead 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.”

The study shows that if an extraterrestrial species were watching during an Earth-Mars conjunction — an alignment in which planets appear lined up from an observer’s perspective — there was a 77 percent chance they would catch one of our transmissions. That is a 400,000-fold increase compared to a random observer at a random time. Other planetary alignments gave a 12 percent chance. Outside of these alignments, the odds were much smaller.

“NASA's Deep Space Network provides the crucial link between Earth and its interplanetary missions like the New Horizons spacecraft and the James Webb Space Telescope,” said Joseph Lazio, project scientist at JPL. “It sends some of humanity's strongest and most persistent radio signals into space.”

The team also calculated that an average DSN transmission could be detected up to 23 light-years away — a light-year being the distance light travels in one year — with telescopes similar to ours. This means nearby solar systems that are oriented edge-on to Earth’s orbital plane are especially promising targets for SETI. Edge-on systems are planetary systems viewed sideways from Earth, making planetary crossings easier to observe. The study suggests that searches should prioritize these systems and time observations with exoplanetary conjunctions or planet–planet occultations, events in which one celestial body blocks another from view, since these conditions greatly improve the chance of intercepting signals.

Fan explained, “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.” Astronomers already study exoplanets — planets orbiting stars beyond our Solar System — during transits, when a planet crosses in front of its star and slightly dims the star’s light. With the upcoming launch of Nancy Grace Roman Space Telescope, scientists expect to discover hundreds of thousands of new exoplanets, which will expand the search area.

Jason Wright, professor of astronomy and astrophysics at Penn State, added, “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. 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 study also noted that while radio signals are the most common, future searches could consider laser transmissions. NASA is testing interplanetary laser communication, and extraterrestrial civilizations might use lasers instead of radio waves, though lasers produce less spillover.

Source: Penn State University, IOP Science

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