Tabby's Star
The account
A star that dims irregularly would be surrounded by a megastructure built by aliens to harvest its energy.
What investigators found
Observations showed that the dimming changes with the color of the light, which points to dust. The exact cause is still being studied.
The story
Volunteers in the Planet Hunters project, analyzing data from the Kepler space telescope, noticed something odd about the star KIC 8462852, more than a thousand light-years from Earth: its brightness dropped irregularly, sometimes by more than 20%, without the regular pattern a planet would cause. In 2015, astronomer Tabetha Boyajian published the study, and the star got the nickname "Tabby's Star."
Since no ordinary explanation fit, astronomer Jason Wright raised, as a last resort, the possibility of an alien megastructure. The idea went around the world. Later studies showed that the dimming varies with the color of the light, which is typical of dust and not of a solid structure.