These Are The Most Likely Places To Detect Signals From An Extraterrestrial Intelligence

A new study on the search for extraterrestrial intelligence (SETI) has proposed the best places to search for a signal from alien intelligences, suggesting it could significantly improve our chances of making such a detection.

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As well as searching for potentially habitable exoplanets, and searching the skies for signs of alien megastructures, scientists looking for alien life have focused on searching for electromagnetic transmissions. These could be intermittent but intentionally directed signals (aliens saying "howdy") or simply leakage of persistent signals.

We are a little short on data on alien civilizations, given that we have found a grand total of zero in our century of searching. As such, researchers have attempted to look at our own signatures of intelligence and how they propagate across the cosmos. 

"Previous studies have proposed various kinds of radio technosignatures emitted by humankind, assessed their detectability, and have often described the search for these technosignatures as searching for 'Earth-level civilizations'," the team explains in their new paper, adding that studies have found that the most detectable of our signals come from intermittent, celestially targeted radio transmissions, and persistent, celestially targeted radio transmissions.

"Intermittent, celestially targeted radio transmissions include intentional transmissions such as the Arecibo message (NAIC Staff 1975) or planetary radar. However, these transmissions are highly sporadic in timing and directionality. By contrast, persistent, celestially targeted radio transmissions include much more frequent transmissions between ground stations and deep-space spacecraft, such as those from the NASA Deep Space Network (DSN)."

It is NASA's Deep Space Network that is the focus of the new study. This is a network of antennas used to communicate with spacecraft, including the furthest spacecraft from Earth, currently over 25 million kilometers (15.5 million miles) away. According to the team, if aliens use similar methods to communicate with their own probes in their solar systems, this could significantly improve our chances of detecting a stray alien signal, because it gives us some precise places to look.

“Humans are predominantly communicating with the spacecraft and probes we have sent to study other planets like Mars,” Pinchen Fan, graduate student in astronomy and astrophysics at the Penn State Eberly College of Science, and first author of the paper, 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. This suggests that we should look for alignment of planets outside of our solar system when searching for extraterrestrial communications.”

The idea is similar to another proposal, which suggests that during supernova events, aliens may choose to direct their own signal at the star, knowing that others may want to take a closer look. Then anyone looking at the supernova from behind it could pick up the signal. But in this case, we might not have to wait for a supernova event, and should instead try and observe exoplanets around other stars when they are nicely aligned with Earth's telescopes.

The team analyzed data from NASA's DSN communications over the past two decades, as well as the position in the Solar System of the spacecraft they were communicating with. 

“NASA's Deep Space Network provides the crucial link between Earth and its interplanetary missions like the New Horizons spacecraft, which is now outbound from the Solar System, and the James Webb Space Telescope,” Joseph Lazio, project scientist at JPL and an author of the paper, added. “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.”

Focusing on the most powerful communications – rather than low-power communication with satellites orbiting the Earth – the team found that the signals were predominantly directed at Mars, with some going to further out spacecraft, for example, placed at the Sun-Earth Lagrange points convenient for parking our space telescopes.

”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% chance that they would be in the path of one of our transmissions — orders of magnitude more likely than being in a random position at a random time," Fan explained. 

“If they could view an alignment with another solar system planet, there is a 12% chance they would be in the path of our transmissions. When not observing a planet alignment, however, these chances are minuscule.”

In this instance, the universe (or gravitational interactions) does appear to be giving us a little help. With a few oddball exceptions, planets in mature systems tend to orbit within a fairly narrow plane around their host stars, making alignment much more probable than if they were to orbit in random orientations. The team suggests that by looking for planetary alignments around other stars, we could significantly increase our chances of detecting stray alien signals. 

According to the team's work, the signals from the DSN could be detected from about 23 light-years away using telescopes similar to ours. The team is now planning to identify exoplanet systems that are "edge on" to our own Solar System's plane, and figure out how frequently they could have received signals as we communicate with our spacecraft. 

“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 added. “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.”

If any aliens have had this same idea, there's a (still very slim) chance that they could have detected us already. However, there is another problem with the idea, which could make the detection of aliens unlikely. NASA and others are already experimenting with potential laser transmissions. If aliens do the same and are successful, these are much less prone to "spilling" their signal out for all behind the target to see. It could be that communication using radio waves is only very brief on astronomical timescales, if aliens use it to communicate with their own spacecraft at all.

But, with no obvious signs of life out there in a vast galaxy, this could certainly be a way of narrowing down where to look.

The study is published in The Astrophysical Journal Letters.