Nasa

It is the astronomical mystery that keeps on deepening.

The erratic dimming of KIC 8462852, an apparently "normal" star 1,480 light-years away in the constellation Cygnus, has spawned theories ranging from clouds of comets to an exploding planet and even evidence of a vast alien megastructure known as a Dyson Sphere.


The latest observations have further confused the picture, showing that the star's brightness is decreasing progressively, but not steadily, in addition to the sharp dips that suggest objects are passing in front of it.

This newly-observed 2.5 per cent dimming over four years sits on top of a pattern of deep dips of over 20 per cent, and a third even longer-term fluctuation that also defies explanation.

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As a result of all the excitement around this strange star, scientists at the University of California, Berkeley are going on an ‘alien hunt’. The team behind the Breakthrough Listen program is devoting hours of time on the Green Bank radio telescope to see if it can detent any signals from extraterrestrials.

“Everyone, every SETI program telescope, I mean every astronomer that has any kind of telescope in any wavelength that can see Tabby’s star has looked at it,” Andrew Siemion, director of the Berkeley SETI Research Center and co-director of the program, said in a statement.


“It’s been looked at with Hubble, it’s been looked at with Keck, it’s been looked at in the infrared and radio and high energy, and every possible thing you can imagine, including a whole range of SETI experiments. Nothing has been found.”

The team have mounted powerful SETI instruments on top of the Green Bank Telescope in West Virginia and expect to gather around 1 petabyte of data, about 1000 terabytes, over hundreds of millions of individual radio channels in an attempt to find out more about the star’s unique behavior.

Though Siemion is skeptical that the star’s behavior is sign of an advanced civilisation, the Breakthrough Listen team is looking forward to finding out what’s going on. We’re in for a long wait though, the results of their observations will not be known for more than a month, because of the data analysis required to pick out patterns in the radio emissions.

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What is Tabby's star?

KIC 8462852 is also known as Tabby's Star, after Yale astronomer Tabetha Boyajian who in September 2015 first published details of its strange behaviour.


Posting details to the Planet Hunters website, which invites citizen scientists to scour reams of astronomical data in the search for exoplanets, Boyajian noted that the otherwise normal output of the type F3V star displayed an odd series of ten substantial dips in brightness.

This map shows the location of NGC 6866. The bizarre star KIC 8462852 is found northeast between NGC 6866 and Cygni Roberto Mura

Astronomers use brightness dips to identify stars being occluded by planets passing in front, and have to date found more than 3,300 exoplanets this way – many of them from Kepler spacecraft observations.

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But these dips were unlike anything seen elsewhere in the universe. A Jupiter-sized world passing in front of an F-type star would cause a dip of about one per cent, lasting a few hours and repeating every orbit, often just a few days. The first recorded dip in brightness of Tabby's Star was about one per cent, but lasted a week.

The observation that really got people taking notice was the "swarm" of ten dips in 2015 that defied explanation. They were close together, but irregular in shape, variable in depth from a fraction of a percent to over 20 per cent, and with no obvious timing pattern.

The deeper dips would require something like 1,000 Earth-sized planets to pass in front of the star at the same time.

When Pennsylvania State University astronomer Jason Wright suggested in October 2015 that an alien megastructure was one possible explanation for this erratic behaviour, the internet blew up with theories and chatter.

This led to the team at the SETI Institute turning its instruments to the star, however it found no artificial radio signals. Other astronomers rushed to offer alternative explanations, including a cluster of comet fragments and a debris field created when planets collide.

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During analysis of Kepler mission data, KIC 8462852 was seen to undergo "irregularly shaped, aperiodic dips in flux" down to below the 20% level arXiv

The picture got murkier still in January 2016 with a report comparing the star with images on astronomical plates dating back over a century. This showed a 20 per cent decrease in brightness which, if true, could support the alien megastructure theory and rule out the planetary or cometary calamities.

Other astronomers believe this century-long decline does not stand up to scrutiny because the instruments and cameras used have changed so much over the period.

The latest twist in the ongoing saga has found a new pattern of decrease in the star's brightness, but offers no explanation for this or any of the other observations.

The claims were first made in August, when Benjamin Montet of the Cahill Center for Astronomy and Astrophysics at CalTech published a letter in the astronomers' online journal arXiv. His findings have now been published in The Astrophysical Journal.

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The research paper describes a progressive decline in the star's brightness over the four years since Kepler has been watching it closely.

The Astrophysical Journal/B. Montet/J. Simon

For three years, it decreased at a rate of about 0.34 per cent per year – and then dropped by 2 per cent in just seven months before remaining steady for a few more months.

Montet, and his partner Josh Simon, compared these readings with more than 500 stars similarly observed by Kepler. A small fraction of them showed fading similar to that seen in KIC 8462852 but none exhibited such a dramatic dimming, especially not a total change in brightness of 3 percent.

Montet acknowledges he is as baffled as everybody else. A cloud of circumstellar material could be the explanation, he said, but no simple cloud model can explain the strange transit ingress and duration that has been observed.


"The steady brightness change in KIC 8462852 is astounding," said Montet. "Our highly accurate measurements over four years demonstrate that the star really is getting fainter with time. It is unprecedented for this type of star to slowly fade for years, and we don't see anything else like it in the Kepler data.

"No known or proposed stellar phenomena can fully explain all aspects of the observed light curve."

KIC 8462852, also known as Tabby's Star, has a third name among astronomers. They call it the WTF Star.