Astronomers Unknowingly Eavesdropped on Six Million Stars in Alien Search But Found Zero Signals

Astronomers have unknowingly listened in on more than 6.1 million stars during previous searches for alien signals, according to research presented by University of Manchester PhD student Louisa Mason at the Royal Astronomical Society's National Astronomy Meeting in Birmingham in July. Despite the much larger search area, no promising signal has been detected.

The news came after Mason revisited observations made by the Green Bank and Parkes radio telescopes. Existing star catalogues suggested the telescopes had viewed 288,315 stars across 1,327 observations. But when Mason compared those surveys with the Besançon Galactic Model, a simulation of the Milky Way, she found that the observations had covered millions of additional stars too faint to appear in conventional catalogues.

'One of the most exciting things about this work is realizing that we've surveyed many more stars than initially thought,' Mason said in a statement from the Royal Astronomical Society. 'Even a very small observation can contain a huge number and diversity of stars that we might never have intended to study.'

Alien Signals Were Hidden in Plain Sight

A radio telescope does not listen to only the star at the centre of its target. Its field of view also contains countless other stars, allowing researchers to conduct what is known as commensal SETI.

In practice, that means the search for extraterrestrial intelligence can piggyback on a telescope's regular observations rather than requiring every observation to be designed specifically for alien research.

The scale of Mason's finding is striking, but it does not mean those millions of stars were searched in any meaningful depth. The telescopes may have observed them only briefly, and their instruments covered a limited range of radio frequencies. A signal could easily have been missed if it was broadcast between observations or transmitted at a frequency the surveys did not examine.

That distinction matters. Counting a star in a telescope's field of view is not the same as conducting a sustained investigation of its planetary system. The headline number is huge. The listening time, by comparison, remains decidedly small.

The initial estimate came from catalogues such as those produced by the European Space Agency's Gaia mission. These catalogues are invaluable, but they primarily include stars bright enough to be detected by optical and infrared telescopes. Many more stars are too faint to be seen directly. If a radio telescope happens to point towards them, however, it could still detect a transmission from one of them.

So far, the silence tells researchers only so much.

Searching for Alien Signals Beyond the Water Hole

Mason has also examined whether SETI should look beyond the relatively narrow section of the radio spectrum that has dominated searches since modern SETI began in 1960. Much of that work has focused on the so-called 'water hole', a range between the radio emission frequencies of atomic hydrogen at 1,420 MHz and hydroxyl at 1,666 MHz.

The range has attracted attention because radio waves within it are not absorbed by Earth's atmosphere. Hydrogen is also a familiar subject of astronomical study, while the combination of hydrogen and hydroxyl forms water, a molecule associated with life as we know it. The logic is appealing, perhaps even a little neat, but the universe is under no obligation to follow human symbolism.

'For decades, SETI researchers have concentrated on a relatively small part of the radio spectrum,' Mason said. 'We wanted to ask what might happen if we looked somewhere very different.'

Her next step involved the Atacama Large Millimeter/submillimeter Array, or ALMA, in Chile. Mason did not make new observations with the facility. Instead, she searched a small amount of archived ALMA data for narrowband signals, the kind that could indicate an artificial transmission but might otherwise be overlooked.

ALMA operates at shorter wavelengths and higher frequencies than most conventional radio telescopes. Those frequencies are less affected by dispersion, a process in which electrons in space delay lower-frequency radio waves and distort a signal as it travels.

The archive search produced no promising new signals. Mason nevertheless said the higher-frequency range could open a largely unexplored area for future SETI research.

'The millimetre and submillimetre radio bands remain almost completely unexplored for SETI, so this is really about opening up a new area of parameter space to search,' she said.

Her work is described in two papers published in Monthly Notices of the Royal Astronomical Society, one examining the ALMA survey and the other applying the Besançon Galactic Model to previous telescope observations. More stars may have been overheard than anyone realised. The broadcasts, if they are out there, remain frustratingly quiet.