Planetary alignments may hold the key to detecting alien life

If an extraterrestrial civilization went hunting for human-made signals, the best time to listen would be during lineups between Earth and other planets. The best places to look would be solar systems oriented edge-on to us, within a few dozen light-years.

That’s the message from a new analysis of our strongest, most persistent transmissions into deep space. The study was led by researchers at Penn State and NASA’s Jet Propulsion Laboratory.

The experts mapped 20 years of human deep-space communications to show when and where they would be most detectable by observers beyond the solar system. The findings could improve our own search strategies for technosignatures.

Detecting extraterrestrial communications

“Humans are predominantly communicating with the spacecraft and probes we have sent to study other planets like Mars,” said lead author Pinchen Fan, a graduate student in astronomy and astrophysics at Penn State.

Because Mars does not fully block transmissions, a distant spacecraft or planet aligned with Earth and another Solar System body could detect the spillover.

“This suggests that we should look for alignment of planets outside of our solar system when searching for extraterrestrial communications,” she said.

SETI scientists sift the sky for technosignatures – signs of past or present technology – as evidence of intelligent life.

“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.”

Tracking decades of space transmissions

The team analyzed public logs from NASA’s Deep Space Network (DSN). This trio of powerful antenna complexes in California, Spain, and Australia that uplinks commands and downlinks data to and from interplanetary spacecraft and space telescopes.

The researchers matched each DSN transmission to the targets’ positions, reconstructing the timing and directionality of Earth’s most potent radio beams.

“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 co-author Joseph Lazio, a project scientist at JPL.

“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.”

Most of those beams point toward Mars missions. Others target spacecraft elsewhere in the Solar System and observatories parked at the Sun–Earth Lagrange points. These are gravitational sweet spots where telescopes like the JWST hover in a stable alignment as seen from Earth.

Earth signals during planetary alignments

Because the Solar System is essentially flat – think planets tracing paths on a dinner plate – DSN transmissions hug Earth’s orbital plane, with the vast majority within five degrees of it. Eavesdroppers are most likely to catch our signals when Earth aligns with another planet.

”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 percent chance that they would be in the path of one of our transmissions,” Fan said.

“If they could view an alignment with another solar-system planet, there is a 12 percent chance they would be in the path of our transmissions.”

In practical terms, an average DSN signal would be detectable with Earth-class radio telescopes out to roughly 23 light-years, the team calculated. Focusing on edge-on star systems and timing observations with planetary alignments could greatly improve success odds.

Using Earth’s signals as a guide

Astronomers already watch exoplanets during transits, when a world crosses its star and dims it from our perspective. That same planetary alignment is when spillover from another civilization’s interplanetary chatter would most likely pass our 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 hundreds of thousands previously undetected exoplanets, so our potential search area should increase greatly.”

The team noted that this alignment-first strategy also applies to searches for optical technosignatures. Future spacefaring societies may prefer lasers for deep-space links, which spill less than radio beams. These signals would still be most detectable along the transmitter’s line to its target.

Searching for alien signals

“Humans are pretty early in our spacefaring journey,” said Professor Jason Wright, the director of the Penn State Extraterrestrial Intelligence Center and co-author of the study. “As we reach further into our Solar System, our transmissions to other planets will only increase.”

The next step is to identify nearby star systems that match those criteria, focusing on planetary alignments, and estimate how often any given system would have been swept by Earth’s beams over the past two decades.

In a field where vast search spaces collide with limited telescope time, aiming at the cosmic equivalents of our own “Mars call” geometry could be the efficiency gain that finally turns a faint whisper into a first hello.

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