The mystery of the so-called “alien megastructure” star just deepened.

KIC 8462852, as it is more properly known, flickers so erratically that one astronomer has speculated that nothing other than a massive extraterrestrial construction project could explain its weird behaviour. A further look showed it has been fading for a century. Now, fresh analysis suggests the star has also dimmed more rapidly over the past four years – only adding to the enigma.

“It seems that every time someone looks at the star, it gets weirder and weirder,” says Benjamin Montet at the California Institute of Technology, who led the study.

This space oddity was first spotted by NASA’s Kepler space telescope, which continually monitored 100,000 stars from 2009 to 2013. Any dip observed in a star’s light is a sign that an exoplanet has passed in front of it. These dips, which occur regularly, block at most 1 per cent of the star’s light and have revealed thousands of exoplanets.

But KIC 8462852, also known as Tabby’s star after its discoverer Tabetha Boyajian of Yale University, was an outlier. Its light dipped by as much as 20 per cent and didn’t conform to any regular time intervals – so the signature couldn’t have been caused by a planet.

Astronomers came up with an array of potential explanations, from the mundane to the bizarre. The star made headlines when Jason Wright, an astronomer at Pennsylvania State University, announced that an advanced extraterrestrial civilization could be responsible for the signal.

Curiouser and curiouser
But the plot thickened when Bradley Schaefer, at Louisiana State University in Baton Rouge, probed the star’s behaviour over the past century by looking at old photographic plates from 1890 to 1989. More than 1200 images revealed that Tabby’s star gradually dimmed by as much as 15 per cent over the course of a century.

Schaefer’s work was immediately called into question. However, with so few astronomers who have an expertise in these plates, no one seemed able to settle the debate. That is until Montet and his advisor Josh Simon realised that an answer might be hidden within the Kepler data.

They found that for the first 1000 days of the Kepler mission, Tabby’s star decreased in brightness at roughly 0.34 per cent a year – twice as fast as measured by Schaefer. What’s more, over the next 200 days, the star’s brightness dropped another 2.5 per cent before beginning to level out. It was a much more rapid change than before.

That means the star undergoes three types of dimming: the deep dips that first made it famous, the relatively slow decline observed by Schaefer and verified by Montet and Simon, and the intermediate rapid decline that occurred over a few hundred days.

“We can come up with scenarios that explain one or maybe two of these, but there’s nothing that nicely explains all three,” says Montet.

And the team doesn’t want to resort to creating three separate scenarios. “It would be much more satisfying to think of a single physical cause that could be responsible for all of the brightness variations that we observe,” says Simon. “But we’re still struggling to come up with what that might be.”

And Wright couldn’t be more thrilled. “I was always worried that the mystery would be solved with some really mundane explanation, like some overlooked instrumental effect, and that it would turn out to be a wild goose chase,” he says.

Explanations range from a swarm of comets orbiting the star to an intervening cloud in the interstellar medium – but none fit all the data.

An alien concept
What about that advanced alien megastructure? “Once you’re invoking arbitrary advanced aliens doing something with technology far beyond ours, then there isn’t very much that can’t be explained,” says Simon. “But we don’t really want to resort to that until we exhaust all of the possible natural explanations we can think of.”

Even Wright, the astronomer who postulated the alien megastructure in the first place, admits that it’s a last resort.

In the meantime, astronomers will continue to monitor the star. A successful crowdfunding campaign earlier this year raised over $100,000, allowing astronomers to secure time at the Las Cumbres Observatory Global Telescope Network, where they can observe the star for a year.

The hope is that Tabby’s star will soon drastically dim and they will be able to swing different ground-based and space-based observatories towards it. Catching a transit in as many wavelengths as possible should help pin down what is interfering with the star – be it a swarm of comets, an alien megastructure, or something else entirely.

Reference: http://arxiv.org/abs/1608.01316