A shadow, a signal, and the search: How scientists are using Earth’s shadow to find aliens

For decades, astronomers have searched the skies for alien radio signals but with little success. A new study argues the problem might not be absence of signals but timing and direction.

The research, published in The Astrophysical Journal Letters on 21 August, examined 20 years of logs from NASA’s Deep Space Network (DSN), a global array of radio antennas that track spacecraft beyond low Earth orbit. Most transmissions, the study found, are aimed at Mars missions and telescopes near Sun-Earth Lagrange points.

That targeting has an unintended effect. From the perspective of a distant star system, Earth and Mars occasionally align. In those moments, our radio signals travel straight through space along the same path. According to the study, this creates a 77 per cent chance that an alien civilisation with the right instruments could intercept our transmissions.

“These are humanity’s strongest, most persistent radio signals,” said NASA JPL’s Joseph Lazio.

Turning the search around

The team behind the research argue that astronomers should flip this logic. If our signals are strongest during alignments, then alien signals may follow the same pattern.

“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,” said Pinchen Fan, an astronomer at Pennsylvania State University and lead author of the study.

The DSN does more than talk to Mars rovers. It maintains contact with the James Webb Space Telescope, New Horizons, the twin Voyager probes and many other spacecraft scattered across the Solar System. Its transmitters are so powerful that stray radio waves leak far beyond their intended targets. Fan and her colleagues estimate that aliens 23 light years away, if they had instruments comparable to ours, could detect those stray transmissions. But they would need to be listening at the right place and the right time.

Eavesdropping works both ways

This cuts both ways. If aliens are running their own versions of the DSN, they too would beam powerful signals toward planetary probes and telescopes. Those transmissions would not scatter randomly into the universe but travel along the flat disk of their star system, much as ours do.

“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,” Fan said. “Orders of magnitude more likely than being in a random position at a random time.”

By contrast, alignments between Earth and Jupiter only offer about a 12 per cent chance of intercepting stray radio waves, since fewer missions are sent to the outer planets.

Astronomers on Earth face the same challenge as their hypothetical alien counterparts. Unless we have an edge-on view of a star system, where planets can be seen crossing in front of their star, we may never pick up their strongest signals.

The role of future telescopes

At present, identifying the right alignments is difficult. Only a handful of star systems are known to host multiple planets that transit their star. TRAPPIST-1, with its seven visible worlds, is a rare case.

This may change with new technology. “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,” Fan said.

If astronomers can predict when two planets in a distant system will align, they can time their searches for stray signals more precisely. The best chance, researchers argue, is that we may overhear something akin to alien mission control speaking to its version of a Mars rover or Voyager probe.

Searching for artefacts in Earth’s shadow

While one team looks outward, another is focusing closer to home. A separate study published in Monthly Notices of the Royal Astronomical Society explores how to detect potential extraterrestrial probes already within our Solar System.

The challenge is clutter. Modern skies are filled with thousands of satellites and millions of fragments of debris, most reflecting sunlight and complicating the search for unknown objects. Beatriz Villarroel of Stockholm University led a team that used Earth’s shadow as a natural filter.

Every night, Earth casts a cone-shaped shadow into space, blocking direct sunlight and eliminating reflections from most satellites. This creates a ‘clean’ zone where unexpected light sources could be spotted more easily.

A mystery object that defied explanation

The team analysed over 200,000 images from the Zwicky Transient Facility in California, focusing only on frames taken within Earth’s shadow. Their automated system, called NEOrion, detected thousands of candidates. Most turned out to be meteors, aircraft or known asteroids.

Yet one case stood out. The system flagged an uncatalogued object moving much faster than typical asteroids and not listed in any known database. The team could not identify what it was, leaving the observation unexplained.

The study also suggested new directions for research. These include examining pre-1957 astronomical photographs for unexplained objects predating the space age and studying the spectral signatures of materials that might have been weathered by long-term exposure to space.

Though no definitive alien artefacts were found, Villarroel and her colleagues argue that their proof-of-concept demonstrates the feasibility of systematic searches using existing telescopes. They are now developing the ExoProbe project, a network of telescopes designed to capture simultaneous observations and pin down the distance to mysterious objects.