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, in the late 1980s when Voyager 2 zipped by.
Even harder to see are its moons. The 14th was only officially detected in February, and little is known about most of the others. But by using Hubble observations, Earth-based telescopes and data collected by Voyager 2, scientists have noticed a curious quirk of its two innermost moons, Naiad and Thalassa.
"These two things are definitely doing something weird," said Marina Brozovic, an expert in solar system dynamics at NASA's Jet Propulsion Laboratory and the lead author of a study published in the journal Icarus.
Thalassa's orbit around Neptune takes about 7 ½ hours. Naiad, hewing closer to the planet, takes seven hours. The two travel no fewer than 1,150 miles from 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.
Naiad's undulations provide orbital stability, Brozovic said. Every time these 60-mile-long, pill-shaped, icy moons line up, they are as far apart as they can get. If they moved too close to each other their gravity fields could entangle, leading to a fatal collision or expulsion from Neptune's orbit.
Naiad and Thalassa's periodic 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.
Style on 12/09/2019