“Over the first 1,000 days, KIC 8462852 faded approximately linearly at a rate of 0.341 ± 0.041 percent per year, for a total decline of 0.9 percent. KIC 8462852 then dimmed much more rapidly in the next 200 days, with its flux [total light emitted] dropping by more than 2 percent,” the authors report.
The final 200 days of Kepler's observations may have involved a further small decrease in brightness, but the uncertainty in measurements is too large to be sure. A fall of around 3 percent is completely unprecedented for a star so similar to the Sun, though.
“Kepler was designed to detect and characterize events with timescales of minutes to hours,” the authors note, explaining why these long-term trends were not detected previously.
Montet and Simon analyzed 548 comparable stars in case the changes were an artifact of some unknown change to Kepler's sensitivity. Although 0.6 percent showed a decline in brightness similar to that seen during the first 1,000 days, none came close to the subsequent, much more rapid, drop.
“The part that really surprised me was just how rapid and non-linear it was,” Montet said to Gizmodo. “We spent a long time trying to convince ourselves this wasn’t real. We just weren’t able to.”
So, for now, the mystery of KIC 8462852 continues to deepen.
KIC 8462852's brightness with time. Kepler rotates every three months for protection against the Sun. Observations are color-coded by Kepler's orientation to ensure this was not affecting the dimming. Montet and Simon