Teruyuki Hirano of the University of Tokyo and colleagues used data from the Kepler space telescope to probe KOI-94, a star seemingly orbited by four planets, looking for a momentary dimming of the star's light when the planets transited, or passed in front of, the star.
Two planets transiting at the same time would dim the star even more, they said, but if they also overlap there is a momentary increase in brightness as the planets cover less of the star.
This light pattern is exactly what Hirano's team saw as one planet, KOI-94.03, passed in front of the star and then the innermost planet, KOI-94.01, passed between the two.
There has been scientific debate over just what to call this never-before-witnessed occurrence, NewScientist.com reported.
Hirano said he prefers "planet-planet eclipse", but that implies the total covering of one body by another, whereas KOI-94.01 is thought to be larger than KOI-94.03 so parts of all three bodies were visible during the alignment.
Another choice is "double transit", but that can include cases where two planets pass in front of their star but don't overlap.
"Overlapping double transit" is favored by Darin Ragozzine of the Smithsonian Astrophysical Observatory in Cambridge, Mass., who first suggested two years ago a search for the light pattern Hirano's team has now seen.