Vanishing stars that 'disappear' suddenly from the night sky without any scientific explanation could help us find extraterrestrial life, scientists have suggested.
Researchers in Sweden have already identified one such 'vanishing' star, and while they're not sure what made it disappear from the view of our telescopes just yet, the absence of any astrophysical explanation means extraterrestrial intervention can't be ruled out.
Stars coming and going is part of the natural flow of the Universe (our own Sun has a few billion years of life left yet), but by studying space surveys taken just a few years apart, the team wants to see if any stars appear to suddenly drop out of the sky in unusual ways.
In other words, they want to find stars logged in one survey that aren't where they should be a few years later.
"We propose to search for physically impossible effects caused by highly advanced technology, by carrying out a search for disappearing galaxies and Milky Way stars," the researchers from Uppsala University explain in their paper.
It might sound a little out-there - and it is - but it's not the first time astronomers have raised the possibility of extraterrestrials being involved when they can't reasonably explain mysterious stellar activity.
So how do you spot a disappearing star? The team compared data from the USNO-B1.0 Catalog and the Sloan Digital Sky Survey (SDSS) - two of the biggest and most comprehensive Universe maps out there - to see if they could find any unexplained discrepancies. In total, around 290,000 objects were studied across the course of 50 years.
From this data, they found one possible candidate: a star that appears to have vanished, after taking into account potential technical errors and the natural movements of objects in the Universe.
Lead researcher Beatriz Villarroel and her colleagues say there's still a lot more work to do before they can confirm for sure that this star has vanished from view, and are working on getting larger samples and more sensitive telescopes to confirm what's actually going on.
"It was a depressing case in the sense that we neither could reject it and neither could we say that it was a real candidate," Villarroel told Shannon Hall at New Scientist.
Still, scientists have long wondered about advanced alien technology such as Dyson spheres - massive structures that could surround stars and then suck energy from them. That's one hypothetical way that a star could be missing from one sky survey to the next.
A more scientific explanation could be that bright galaxy centres called quasars are known to switch off in a matter of hundreds of days – though we're not exactly sure why. These quasars are powered by supermassive black holes, sucking in huge volumes of gas and dust more rapidly than normal, and they could account for some bright spots suddenly disappearing between official surveys.
But if there are advanced lifeforms out there, it's entirely possible they may be able to manipulate stars and galaxies to harvest their energy - something Russian astrophysicist Nikolai Kardashev has already suggested - so looking for vanishing stars to check this hypothesis makes sense.
"I think it’s a very reasonable thing to do," physicist Jay Olson from Boise State University in Idaho, who wasn't involved in the study, told New Scientist.
"It's a deliberate search for something very unusual, which could be hiding in existing data across time. At this stage of the game, it's a very limited search, but it illustrates well what can be accomplished."
The study hasn't been peer-reviewed as yet, but has been published on pre-print site arXiv.org, giving the authors a chance to get feedback from other researchers before publication in a journal.