Neptune is the loneliest planet in the solar system. The ice giant, orbiting the sun at a distance of 2.8 billion miles, is the only planet that cannot be seen by the naked eye. Along with Uranus, we have only paid it a single visit, back when Voyager 2 zipped by in the late-1980s.

Even harder to see are the planet’s handful of moons. The fourteenth was only officially detected in February, and little is known about most of the others. But by using a combination of Hubble observations, Earth-based telescopes and data collected by Voyager 2, scientists have unearthed a curious quirk of its two innermost moons Naiad and Thalassa. These tiny worlds are engaged in a dance routine that has never been seen in the cosmos.

“These two things are definitely doing something weird,” said Marina Brozović, an expert in solar system dynamics at NASA’s Jet Propulsion Laboratory and the lead author of a study published in the journal Icarus last week .

Thalassa’s orbit around Neptune takes about seven and a half hours to complete; Naiad, hewing closer to the planet, takes seven. The two travel no fewer than 1,150 miles of one another. Crucially, Naiad’s orbit is tilted with respect to its partner. It zips up and down, passing by Thalassa twice from above then twice from below, a cycle that repeats whenever Naiad has lapped Thalassa four times.