The search for extraterrestrial intelligence (SETI) has yet to detect alien technosignatures like radio waves, but the cosmos is vast, and there are plenty of places left to look. New research suggests refining our search parameters by using our own broadcasts into deep space as a helpful guide.
Research published earlier this week in Astrophysical Journal Letters suggests we search for alien signals by studying how we beam strong, directed transmissions during two-way communication with our deep space missions.
The research team, which included scientists from Penn State and NASA’s Jet Propulsion Laboratory (JPL), studied how alien observers might detect our deep space radio transmissions. They reasoned that if extraterrestrials engage in similar space-faring activities, their signals are most likely to appear under the same conditions, thereby narrowing where and when we should look.
SETI has been sniffing around for extraterrestrial radio signals since the 1960s but has so far come up empty. Part of the challenge is that we’re not entirely sure what we’re looking for, aside from something discernibly artificial in origin.
That could mean a deliberate, focused signal beamed out to grab our attention, or it might be unintentional: residual transmissions bleeding out from a civilization just going about its business. In fact, our own civilization has been leaking radio signals for over a century. We haven’t exactly been quiet—our airports, for example, are particularly leaky when it comes to radio waves.
Finding patterns in our own habits
In an effort to improve our chances of detecting extraterrestrial radio signals, the researchers, including astronomer Pinchen Fan from Penn State, investigated NASA’s deep space transmissions as a way to predict how alien civilizations might use radio themselves.
“Humans are predominantly communicating with the spacecraft and probes we have sent to study other planets like Mars,” Fan explained in a statement. “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.”
We should therefore focus on these types of key moments, when exoplanets in distant star systems fall into alignment—that’s when SETI should be on the lookout for possible radio signals, Fan said.
For the study, the researchers analyzed logs from NASA’s Deep Space Network (DSN), a system of ground-based facilities that track and communicate with human-made objects in space—from satellites in low Earth orbit to far-flung missions like the Voyager probes and the New Horizons spacecraft. As Joseph Lazio, a JPL project scientist and co-author of the study, explained, the DSN “sends some of humanity’s strongest and most persistent radio signals into space.” By matching DSN radio broadcasts with data on spacecraft locations, the team was able to determine both the timing and direction of Earth’s deep space transmissions.
“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,” said Jason Wright, a professor of astronomy and astrophysics at Penn State.
Look for the alignments
The researchers found that deep space radio signals are predominantly aimed toward Mars but are also directed at other solar system planets and at telescopes positioned at the Sun-Earth Lagrange points (the James Webb Space Telescope being a good example). Two decades’ worth of DSN data showed that, if aliens were in a position to observe an Earth-Mars alignment, there’s a 77% chance they’d be in the path of our transmissions, as compared to a 12% chance for other planetary alignments. “When not observing a planet alignment, however, these chances are minuscule,” Fan said.
The research team says we should apply these insights to SETI, as that’ll likely improve our chances of finding wayward radio signals. Specifically, we should focus on moments when exoplanets fall into alignment with each other or with their host star, as seen from our perspective on Earth.
Our solar system is relatively flat, with most planets orbiting on the same plane, so most human transmissions travel along that same plane. Aliens are likely to use similar communication strategies, so it makes sense for SETI to target the same kinds of alignments.
Given the strength of DSN transmissions and assuming aliens use similar communications technology, the study’s co-authors recommend searching within 23 light-years. Importantly, they say we should also focus on nearby systems with orbital planes edge-on to Earth. They also note that these patterns could aid in the search for laser signals, which spill less than radio waves and may be preferred by alien civilizations; and in fact, we are moving in that exact direction.
This new proposal makes a lot of sense. Hopefully SETI scientists will take note and launch campaigns with these insights in mind. The cosmos has the potential to be a very noisy place—we just have to know when and where to look.
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