Further doubts have been cast on the possibility of an "alien megastructure" around the star KIC 8462852. Researchers say the unusual dimming of the star over the past century can be attributed to the changes in the instruments used to observe it, not an alien civilisation harnessing the star's energy.

The idea of an alien megastructure blocking out light from KIC 8462852 first surfaced last October. Astronomers noticed that more than 20% of its light was being blocked out by something very large – if a Jupiter-sized planet was passing in front, for example, only about 2% of the light would be blocked.

As a result, they proposed (albeit very tentatively), that an advanced alien civilisation could have built a megastructure around the star to harness its energy. This type of hypothetical structure, also known as a Dyson sphere, could explain the dimming where natural causes (such as clouds of dust and comets) could not.

Further "evidence" supporting the alien megastructure theory came in January when astronomers said KIC 8462852 had dimmed by 20% over the past century. Their findings, accepted for publication in the Astrophysical Journal, would be extremely difficult to explain via natural causes.

However, a team of researchers from Vanderbilt University in Tennessee has now said this research is flawed. Michael Lund and colleagues looked specifically at the data used in the study – 500,000 photographic glass plates taken over the past 100 years. As these observations were made with a number of different telescopes and cameras, the team thought these instrumental variations may explain the observed changes to KIC 8462852.

"We looked at variations in the brightness of a number of comparable stars in the ... database and found that many of them experienced a similar drop in intensity in the 1960s," said Keivan Stassun, one of the authors of the study also accepted for publication in the Astrophysical Journal. "That indicates the drops were caused by changes in the instrumentation not by changes in the stars' brightness."

However, the team also said the other observed dips to KIC 8462852 are real and, at the moment, remain a mystery. For example, in 2013 a series of uneven and unnatural dips were seen lasting for 100 days. "The Kepler data contains other cases of irregular dips like these, but never in a swarm like this," Stassun said.

Michael Hippke, an author on the study, added: "What does this mean for the mystery? Are there no aliens after all? Probably not! Still, the dips found by Kepler are real. Something seems to be transiting in front of this star and we still have no idea what it is."

So far no proposed scenarios have been able to explain all of the dips in brightness. The best current explanation is that it is being caused a giant comet that has broken up into thousands of smaller pieces.