Astronomers have found two planet systems with pairs of gas giant planets locked in an orbital embrace.
The intimate dance is closer around a dying star in one of the systems containing a planetary pair.
"A planetary system with such closely spaced giant planets would be destroyed quickly if the planets weren't doing such a well synchronized dance," said Eric Ford of the University of Florida in Gainsville.
"This makes it a real puzzle how the planets could have found their rhythm."
All of the four newly discovered exoplanets are gas giants more massive than Jupiter and the members of each pair are located remarkably close to one another.
Because of their large masses and close proximity, the exoplanet pairs exert a large gravitational force on each other.
Planets often move around after they form, in a process known as migration - but the process isn't orderly.
Planets located farther out in the protoplanetary disk can migrate faster than those closer in, "so planets will cross paths and jostle each other around," said John A. Johnson, an assistant professor of astronomy at the California Institute of Technology (Caltech).
"The only way they can 'get along' and become stable is if they enter an orbital resonance."
"This is the tightest system that's ever been discovered and we're at a loss to explain why this happened. Each time we think we can explain them, something else comes along," said Johnson.
The astronomers conclude that eventually the subgiant stars will grow into red giants, throwing off their outer atmospheres, swelling to the point where they could engulf the inner planet of their dancing pair.
"The planets will then move out, and their orbits will become unstable," Johnson says.
"Most likely one of the planets will get flung out of the system completely" - and the dance will end.