Trivia

Hard

Which Of These Planets Has A Moon With A Bobbing Orbit?

Jupiter
Saturn
Uranus
Neptune
A computer rendering showing the orbital patterns of Neptune's two inner moons.
NASA/JPL-Caltech

Answer: Neptune

The universe is a big place and if you look long enough, you’re bound to find some pretty strange things. Sometimes you don’t even have to look deep into the reaches of the universe, you can just look at your (relative) back yard.

The best kind of strange, in our opinion, is the kind of strange where something relatively common presents itself in an uncommon way. Take, for example, the idea of orbital resonance and the moons of Neptune. Orbital resonance, simply, is the idea that orbiting bodies exert regular, periodic gravitational influence on each other as a result of moments when their orbits align. The moons of Jupiter (Ganymede, Europa, and Io), for example, help stabilize each other by gently tugging on each other’s orbits as they pass by each other.

So there’s the relatively common part. The uncommon part presents itself with Neptune’s inner moons Naiad and Thalassa. While Thalassa maintains a relatively stable orbit (in terms of orbiting on a flat plane around the planet), Naiad moves in a motion not unlike a carousel horse bobbing up and down. To remain stable despite close proximity to Thalassa and a slightly faster orbital period, Naiad moves in “zig-zag like” pattern up and down like a wobbly hula hoop rocking on someone’s hips—this animation from NASA’s Jet Propulsion Laboratory makes it a bit easier to visualize. The odd “dancing” orbit is the only known example of two orbital bodies having up-down orbital resonance.

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