Complex sentence: The large reduction in brightness could have happened due to a planet or brown dwarf with a circum-secondary disk of dust with a radius of 0.3 AU around a central object with a mass between 1.8 and 70 times the mass of Jupiter.
Split into simpler sentences: The large reduction in brightness happens due to a planet or brown dwarf with a circum-secondary disk of dust with a radius of 0.3 AU. The object in the center of the disk has a mass between 1.8 and 70 times the mass of Jupiter.