60 Million Stars and Not One Alien Detected

60 Million Stars and Not One Alien Detected
A view of the galactic centre — a region of space containing an abundance of stars, superheated clouds of gas, neutron stars, white dwarfs, and a supermassive black hole parked in the core. (Image: X-Ray:NASA/CXC/UMass/D. Wang et al.; Radio:NRF/SARAO/MeerKAT)

Astrobiologists with the Breakthrough Listen project have released the preliminary results of a SETI survey, in which the team hunted for radio signals along a line of sight that extends toward the galactic centre.

The search for extraterrestrial radio signals is now in its seventh decade, and we have yet to find any trace of intelligent life. Continue the search we must, however, as no greater unsolved scientific mystery exists than the question of whether or not we’re alone in the universe.

The search for extraterrestrial intelligence, or SETI, is currently limited to searches of supposed alien technosignatures — things like optical and microwave signals and evidence of megastructures. That said, radio signals continue to be the most popular SETI target, as focused radio emissions could signal the presence of an alien civilisation, whether the leakage of those radio signals is deliberate or accidental.

The $US10 ($13) million Breakthrough Listen project — a 10-year initiative founded six years ago by Israeli-Russian billionaire Yuri Milner and the late physicist Stephen Hawking — continues in this tradition, searching the depths of space for signs of radio signals produced by aliens.

For its latest survey, the Breakthrough Listen team, based at the SETI Research Centre at the University of California, Berkeley, used the Green Bank Radio Telescope in West Virginia and CSIRO’s Parkes Radio Telescope in Australia to gather roughly 600 hours of radio observations. Their latest effort was unique in that it was the “most sensitive and deepest targeted SETI” survey ever done of the galactic centre, as the SETI scientists write in a study set to be published in the Astronomical Journal (a preprint is currently available on the arXiv).

Looking for aliens along a line of sight that extends from Earth to the supermassive black hole at the centre of the Milky Way comes with advantages and disadvantages.

The advantage is that the density of stars increases with distance to the galactic centre. Accordingly, this line of sight “offers the largest number of potentially habitable systems of any direction in the sky,” according to the study. What’s more, the relatively close proximity of these stars to each other could “accelerate development of interstellar communication and travel,” which could contribute to the rise of “advanced space-faring societies,” as the SETI scientists write in their paper.

The disadvantage is that things get a bit hairy beyond a certain point. Like the solar system, the Milky Way has its own habitable zone, beyond which life cannot emerge. Indeed, the inner region of our galaxy (i.e. the region outside the galactic habitable zone) is a high-radiation environment filled with gamma rays, exploding supernovae, and clouds of gas that reach millions of degrees. The hulking supermassive black hole at the galactic core presents another hazard entirely.

Still, the authors of the new study, led by Vishal Gajjar from the Department of Astronomy at Berkeley, decided it would be worthwhile to focus a comprehensive search near the galactic centre owing to the sheer abundance of stars from here to there. As the scientists noted in their study, “we estimated that we surveyed around 60 million stars.”

Interestingly, the team was not looking for accidental radio leakage, but rather, periodic radio transients coming from hypothetical beacons (in other words, regularly repeating radio bursts coming from machines designed to get our attention). The galactic centre, according to the scientists, “provides an ideal” central location for “advanced civilizations to place a powerful transmitter to efficiently send beacons across the entire Milky Way,” in what is yet another advantage to this strategy.

Gajjar and his colleagues swept for frequencies between 0.7 and 93 GHz. Results of the preliminary report were limited to frequencies between 1 and 8 GHz and at intervals of 7 hours (as observed by Parkes) and 11.2 hours (as observed by Green Bay Telescope). No repeating radio bursts consistent with an alien beacon were detected within these parameters.

No extraterrestrial intelligences were detected, but the scientists did manage to capture transient events consistent with magnetars, so that will be of interest to astronomers who study this type of neutron star. And again, this is a preliminary report, so we eagerly await further results.

Back in 2019, the same team came up short after analysing 1,372 nearby stars. We can’t seem to find any evidence of alien life despite our conscientious searches. It’s becoming hard to not be pessimistic about the whole SETI endeavour, but there’s something we have to keep in mind: The search for intelligent alien life has only just begun.