A Dance That Stops 2 of Neptune’s Moons From Colliding

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.

This may appear chaotic at first. But Naiad’s perfectly timed undulations provide orbital stability, said Dr. Brozović. Every time these 60-mile-long, pill-shaped, icy moons line up, they are as far apart as they can get. Another, perhaps less meandering configuration could see the two diminutive moons move too close to each other and find their gravity fields entangled. This could irreversibly disturb their orbits, leading to a fatal collision or a dramatic expulsion from Neptune’s orbit.

Naiad and Thalassa’s strange salsa — a type of repeating orbital pattern known as an orbital resonance — was likely set up in the distant astronomical past.

Long ago, Neptune captured its largest moon, Triton. The large icy object was most likely stolen from the Kuiper belt beyond Neptune’s orbit, home to Pluto, Eris and many other distant worlds. Whatever moons already orbited Neptune at that time were severely perturbed by this gate-crasher. Some were annihilated through collisions or from being torn apart in Neptune’s gravitational well. This produced Neptune’s rings and its innermost moons, including Naiad and Thalassa, which serendipitously fell into their odd yet steady orbits.

This unexpected finding is a reminder that we still know so little about Neptune and its moons, said Paul Schenk, a planetary geologist at the Lunar and Planetary Institute in Houston who wasn’t involved with the study. Only if we decide to pay it a second visit — an idea recently proposed to NASA — will its bounty of secrets be revealed, he said.

Naiad and Thalassa’s periodic, repetitive cha-cha is not the only noteworthy orbital resonance in the solar system.

The resonance between the Jovian moons of Io, Europa and Ganymede causes tides inside Io, creating friction, heat and the solar system’s most active volcanic system. Jupiter itself is in resonance with the asteroid belt, with the gas giant’s immense gravitational pull keeping lanes within the belt conspicuously free of asteroids. Neptune and Pluto are also in a resonance, with the dwarf planet completing two orbits of the sun for Neptune’s three, a groove that keeps both orbits stable.

“Resonances have shaped the solar system,” said Dr. Brozović, who added that it was fascinating to discover a brand-new one so far from home.

“It’s neat when things surprise you.”