The 'alien megastructure' star KIC 8462852 is continuing to baffle scientists, with the latest data showing a bizarre pattern of dimming over the four years it was observed by the Kepler Space Telescope.

The star, located over 1,400 light-years from Earth, faded at a rate of 0.34% for the first 1,000 days of observation before dropping by 2% for the next 200 days. It then levelled off at a constant rate. Astronomers Ben Montet and Joshua Simon, who published their findings (not yet peer reviewed) on the preprint server, say no known or proposed stellar phenomena can explain the unusual dimming observed.

KIC 8462852 made global headlines in October last year after astronomers noticed "huge light dips," with something blocking over 20% of the star's light. To put that into perspective, A Jupiter-sized planet travelling at roughly the same distance from the star would block less than 1% of the light. Whatever is blocking the light, therefore, must be massive.

At the time, several theories were put forward about what was causing the dimming, including a massive dust cloud or loads of comets. Another idea was that it was evidence of an advanced alien civilisation that had built a huge structure through which it could harness energy from the star. The idea for such a structure, (the so-called Dyson sphere), was first proposed by Freeman Dyson in the 1960s, who argued any civilisation advanced enough would eventually turn to its star for its energy requirements.

By analysing the data from Kepler, Montet and Simon were able to derive a rate of dimming for the star. They then compared the data to 193 nearby comparison stars, along with 355 stars with similar parameters to see if any others showed signs of KIC 8462852's unusual behaviour.

Their findings showed none exhibited the fast decline in brightness or the continual fading observed at KIC 8462852. Furthermore, the astronomers looked at whether the huge dips in brightness could be the result of a cloud of circumstellar material that had evaded detection via sub-mm observations. Again, they found this was not the case.

"Such a cloud could evade detection in sub-mm observations, the transit ingress and duration cannot be explained by a simple cloud model," they wrote. "Moreover, this model cannot account for the observed longer-term dimming. No known or proposed stellar phenomena can fully explain all aspects of the observed light curve."

In an interview with Gizmodo, Montet said he was not expecting this result: "The part that really surprised me was just how rapid and non-linear it was," he said. "We spent a long time trying to convince ourselves this wasn't real. We just weren't able to."

To find out exactly what is causing the dimming of KIC 8462852, astronomers will need to observe the star during one of its big dipping events. Tabetha Boyajian, who discovered the star, recently won funding to do just that. Speaking to Popular Science about the latest study, she said: "hese results introduce us to another delightfully unexpected piece of the puzzle."