Researchers from the University of Pennsylvania and NASA’s Jet Propulsion Laboratory have found that extraterrestrial civilizations may be able to detect the radio signals humanity uses to communicate with its spacecraft. The most likely moment for such “eavesdropping” is when Earth and Mars line up with an observer’s viewpoint. The study, published in The Astrophysical Journal Letters on August 21, 2025, points to where and when signs of alien intelligence could be detected.

How Could Aliens Hear Us?

Humanity actively uses radio signals to maintain contact with spacecraft exploring other planets, especially Mars, as well as probes located at the Lagrange points of the Solar System. These signals, scientists discovered, can “leak” into space and be picked up by extraterrestrial observers.

“Humans mainly communicate with spacecraft sent to explore other planets, like Mars. But the planet doesn’t fully block the signal, and its leakage could be detected by an external observer,” explained study author Pinchen Fan, a PhD student at the University of Pennsylvania.

Researchers analyzed 20 years of data from NASA’s Deep Space Network (DSN), a global system of antennas that supports communication with interplanetary missions. Most of the signals are directed toward Mars, making it the key point for possible interception.

When Are We Easiest to “Eavesdrop” On?

Modeling showed that the probability of detecting Earth’s radio signals is highest when Earth and Mars line up with a hypothetical observer. In such configurations, signals aimed at Martian rovers or orbiters could be intercepted.

“According to calculations, if an alien civilization observes Earth and Mars during their alignment, in 77% of cases they would fall into the ‘beam’ of one of our signals. For other planets, the probability is about 12%. Outside of such alignments, the chance of interception is almost negligible,” the study notes.

DSN radio signals can be picked up from as far as 23 light-years away, provided aliens have telescopes comparable to those on Earth. Particularly promising are star systems where planetary orbits are oriented ‘edge-on’ relative to Earth, which increases the chances of catching these signals.

Where to Search for Aliens?

The study offers a new approach to the search for extraterrestrial civilizations. If Earth’s radio signals are most noticeable during certain alignments, then signals from other civilizations likely follow similar principles.

“We’ve shown that our own communications can tell us where to best search for technosignatures — signs of extraterrestrial activity,” said University of Pennsylvania professor Jason Wright.

Scientists recommend that SETI (Search for Extraterrestrial Intelligence) efforts focus on star systems within 23 light-years, where planetary alignments are visible from Earth. Systems such as Proxima Centauri or Barnard’s Star could be priority targets for radio telescopes.

Why Does This Matter?

With each year, humanity increases the volume of radio transmissions into space, raising the likelihood of Earth being detected by other civilizations. This discovery not only points to where aliens might be found but also underscores that our activity in space makes us increasingly noticeable.

“The authors stress that as exploration of the Solar System expands, the volume of such transmissions will only grow. This means the likelihood of being noticed in the Universe rises with each passing year,” the study says.

However, this also raises questions: should humanity be so “loud” in space? Some scientists, including Stephen Hawking, warned that active broadcasting might attract the attention of potentially hostile civilizations.

In Brief…

A study by the University of Pennsylvania and NASA shows that the radio signals Earth uses to communicate with spacecraft — especially those on Mars — can be intercepted by alien civilizations within 23 light-years, provided they observe during planetary alignments. The probability of detection reaches 77% under certain configurations. Published in The Astrophysical Journal Letters, the research suggests using this knowledge to search for technosignatures in star systems with favorable orbital orientations. The findings highlight that as space activity grows, humanity becomes more noticeable in the cosmos — a development that brings both opportunities for contact and potential risks.