It's one of the first signs of an advanced alien civilisation — an erratically blinking star. Now astronomers have found a second contender for first contact.
There aren't many signs of civilisation that can cross the void of space. Radio. Lasers. Perhaps robotic probes. But one contender could be an alien civilisation with such a hunger for energy, it has begun encasing its star in solar panels.
Such an enormous feat of engineering would cast seemingly unnatural shadows observable from the galaxy beyond.
Since the idea was put forward in the 1960s, the search for such 'Dyson Spheres' has been quietly taking place as we scan the heavens in an effort to understand our universe.
The first contender appeared in a 2015 analysis of data collected by the Kepler Planet Hunter telescope. KIC 8462852, also named 'Tabby's Star' after its discoverer, was behaving weirdly. It was 'pulsing' in an erratic, and fast, way.
Now, there's another.
VVV-WIT-07 is an old, faint star. And it's not behaving the way it should.
It's pulsing. And not in the long, slow manner as can be explained by celestial mechanics. But quickly.
Is it dust? Is it swallowing the debris of a planet? Is a cloud of comets casting a shadow as it swings past in close orbit? Or is it aliens?
Extraordinary claims require extraordinary evidence to be proven.
Problem is, it's difficult to get any evidence from such distant objects.
The old, red star which sits between our Sun and the Milky Way's galactic core was first observed to be 'flashing' back in 2012. It was part of a survey by the VISTA telescope in Chile.
The astronomers sat on their data as they sought to verify its behaviour and rule out any technical glitches.
Six years later, still not knowing what could be causing its behaviour, they're reporting the find.
In 2012, VVV-WIT-07 dimmed steadily over the course of 11 days. It then rapidly faded to almost black — by more than 80 per cent — for a period of 48 days.
That's significantly more than KIC 8462852 (Tabby's Star), which has been observed to dim by about 20 per cent.
"It's got to be over a million kilometres wide, and very dense to be able to block that much starlight," University of Rochester astrophysicist Eric Mamajek told Scientific American.
It's not that stars don't fluctuate in brightness. Their rhythmical pulsing is one of the key ways astronomers detect the presence of planets as they pass in eclipse.
Questions arise when those pulses are not rhythmical, and their magnitude is extreme and erratic. It's much harder to explain such events away through the orbital clockwork of planets, comets and other known celestial stuff.
It's a mystery.
In this case, it's a mystery that coincides with predictions about how we may observe alien civilisation from a distance. But it's also a mystery because we don't yet fully understand the workings of star systems, planets, and the clouds of comets and rubble that surround them.
Louisiana State University astronomer Tabetha Boyajian, who led a group of amateurs back in 2015 to find KIC 8462852, still isn't certain what could be the cause of such odd behaviour.
But she no longer feels 'alien megastructure' can be counted among the contenders — at least for the star that carry's her name.
"The new data shows that different colours of light are being blocked at different intensities," she recently wrote. "Therefore, whatever is passing between us and the star is not opaque, as would be expected from a planet or alien megastructure."
VVV-WIT-07's orbital objects, however, appear to be much closer to opaque.
That's a giant leap.
The mystery is something that can still be explained by wispy interstellar dust clouds. Or perhaps some catastrophic event in its solar system.
Only further observation will tell.
And those observations can come up with some extraordinary stuff: For a time the star EPIC 204278916 was considered a contender for a Dyson sphere. But closer analysis of the light patterns and behaviour make astronomers pretty sure the pulsing is caused by a planet with giant rings — some 200 times bigger than those of Saturn. Extraordinary. But feasible.
The VISTA astronomers anticipate another dimming next year. So they're casting their electronic eyes back towards VVV-WIT-07 to capture every clue they can.
Whatever they find, it will be weird.
It just probably won't be aliens.