Leading astronomers are hunting for alien messages in supernova explosions. Here’s why

Imagine you're an alien on a distant planet. You're desperate to let everyone know you're there. Instead of howling aimlessly into the void, how can you give yourself the best chance of being heard? According to new research, extra-terrestrial civilisations could piggyback onto one of the brightest beacons in the known Universe: a supernova.

These cataclysmic explosions detonate as the biggest stars die. For a time, they shine as brightly as 10 billion Suns and release as much energy as the Sun will emit in its entire 10-billion-year lifetime. When a supernova goes off, astronomers sit up and take notice.

In May this year, astronomers exploded in a flurry of excitement when they spotted the supernova SN 2023ixf detonate in the nearby Pinwheel Galaxy (also known as M101).

“It was the closest supernova in a decade,” says James Davenport, an assistant professor of astronomy from the University of Washington.

It was bright enough to see with amateur telescopes, even though M101 is 21 million light years away.

A team led by Davenport has since examined the possibility that an alien civilisation may choose to flag us down by co-ordinating a message alongside the light from this supernova in a ploy known as ‘signal synchronisation’. “They'll shout, ‘We are here!’ in a conspicuous way,” Davenport explains.

The idea is that that civilisation would know astronomers from other inhabited worlds like ours will probably be looking in the direction of the supernova, making the chances of that message being spotted far higher.

So where exactly do we look? The sweet spot to search is defined by a rugby ball-shaped region within our Milky Way galaxy called the ‘SETI ellipsoid’ (SETI stands for the Search for Extra-Terrestrial Intelligence).

But there is one caveat. The aliens in question have to be nearer to the supernova than us in order for them to be able to see its light first and then have time to quickly send on an accompanying message that will arrive on Earth close to when we see the supernova.

“It's the same maths that goes into figuring out echoes,” Davenport says.

It's civilisations on the perimeter of the ellipsoid that we're most interested in. We'd have already missed the signals from those stars far inside it and stars outside may have seen the supernova, but any synchronised message wouldn't have had time to reach the Earth yet.

Narrowing down the search

Working out which stars are in the right place has only recently become possible, thanks to modern space technology.

“Missions like Gaia have given us really precise locations of stars,” Davenport says. “There are around 100 stars currently in the sweet spot.”

Armed with that knowledge, Davenport pointed the Allen Telescope Array in California at each of them over the course of a single weekend.

Did he hear anything? Not yet. But all is not lost.

“We are going to revisit them a few times over the next couple of months because there is some uncertainty over when the signals would arrive,” Davenport says.

That uncertainty stems from how quickly after the supernovae the alien civilisation would send on their message.

“Would they have to go to alien congress and ask permission to send it?” Davenport asks.

So, there could be a few weeks' delay. Revisiting the target stars also opens the door to being able to detect faint signals that slowly change, rather than taking just one snatched five-minute glimpse and saying nothing is there.

“They could be sending a wave of information,” Davenport says.

The SETI ellipsoid is also constantly shifting to incorporate new stars as the passage of time allows both the light from the supernova and the relayed signals to travel further.

So, we could be scanning stars on the perimeter of the ellipsoid for years to come. Plus, SN 2023ixf is just one supernova. Every nearby supernova has its own ellipsoid to comb through.

In fact, Davenport's work predates this most recent explosion – he originally focussed on the supernova SN 1987A, which exploded in 1987.

“Even though it happened nearly 40 years ago there are still stars where the signal would only just be reaching us,” he says.

So, how does Davenport rate the chances of success?

“They are low on any given day,” he says. “My guess is that it could take us a thousand years, but that's okay.”

A millennium may sound like a long time, but humans have routinely carried out projects on such timescales. The University of Bologna, the world's oldest, has been teaching students ceaselessly since 1088, for example.

“You have to start somewhere,” Davenport says. “It's about leaving a legacy of data and of methodology.”

The best things take time and good things come to those who wait. One day measurements of a star on the edge of a supernova SETI ellipsoid might just go down in history as one of the most important observations ever made.