Is E.T. phoning our home?
For just the second time in history, astronomers have detected a repeating fast radio burst (FRB) coming from outside the Milky Way.
Like the first, the cause of the latest signal is a mystery, but it's proof a mysterious burst detected in 2015 wasn't a one-off and has raised hopes there could be life elsewhere in the universe.
Astronomers in Canada picked up the signal using the Canadian Hydrogen Intensity Mapping Experiment (CHIME), which was built to observe the early universe but also turned out to be good at detecting FRBs.
Since 2007 when they were first discovered, about 60 FRBs have been detected - but only two have been spotted coming again and again from the same place in the sky. The latest one has appeared at least five times.
"The only other known repeating FRB is one that first appeared in 2012," Alexandra Witze wrote in journal Nature. "It seems to originate in a galaxy some 2.5 billion light-years from Earth. Finding a second repeater confirms that the first was not some kind of freak event."
The CHIME telescope isn't even fully powered up yet, so even more could be out there waiting to be discovered.
The origin of the latest signal isn't clear, except that it's likely to be from another galaxy altogether.
"Fast radio bursts are exceedingly bright given their short duration and origin at great distances, and we haven't identified a possible natural source with any confidence," theorist Avi Loeb at the Harvard-Smithsonian Center for Astrophysics said in 2017.
"An artificial origin is worth contemplating and checking."
The amount of energy required to send a FRB across the universe has been estimated at the entire output of the sun for 80 years.
Theories on possible natural origins include spinning neutron stars, black hole collisions or explosions and massive supernovas.
Prof Loeb has suggested FRBs could be used to propel alien spaceships with large sails. They're unlikely to be used for communication between galaxies, with FRBs - like everything else - bound to the speed of light.