Neptune’s moons are behaving strangely, and astronomers don’t know why

With 14 moons, Neptune has a lot on its plate. This surprisingly large planet on the outer edge of our solar system is big, bold, and beautiful, but recently, astronomers have discovered something even more interesting about this rarely-visited celestial body: its moons are weird.

Neptune: The basics

Even though it’s all the way out there, Neptune isn’t a planet you want to mess with. It is nearly 3 billion miles away from the sun (over 30 times the distance between Earth and the sun), and people (via spacecraft) have only visited it once: in 1989, the Voyager 2 mission flew past and snapped some sweet shots of the enormous ice giant.

Despite its icy interior (and gaseous exterior), Neptune is over four times the width of Earth. Time is also different on Neptune: were the planet capable of sustaining life, one day in Neptune — one rotation of the planet — would only take 16 hours, while one year — one rotation around the sun — would take nearly 165 years. But its physical features are far from the most interesting elements of this planetary wonder.

Neptune’s moons

Neptune has 14 moons, all named after Greek gods and sea nymphs in keeping with its patriarch. But recently, astronomers have realized that two of these moons aren’t behaving like normal moons; in fact, they’re doing a dance more akin to carousel horses than to traditional moon orbits.

“Instead of orbiting Neptune in a locked ring shape, two moons appeared to be jumping over each other

Using NASA’s Hubbel Space Telescope, powerful telescopes on Earth, and information from the Voyager 2 mission, astronomers started observing the movements of Neptune’s moons in 1981. What they found after a couple of decades of observation was startling: instead of orbiting Neptune in a locked ring shape, two moons appeared to be jumping over each other, looping each other, and moving in accordance with each other in a strange celestial dance.

Naiad and Thalassa, the main characters of this space-based performance, are relatively close neighbors. Separated by only 1,150 miles, the two moons have devised a way to avoid running into each other. While other celestial bodies have been known to move and adjust based on their neighbors (and their unspoken desire to avoid destruction), astronomers have never seen this specific type of orbit dance-movement before.

What’s up with these orbits?

Most moons, including our own, orbit their host planets in a predictable, steady loop. There are nearly 200 known moons in our solar system, and Earth’s moon is one of the largest (5th biggest, to be exact).  A couple of planets, like Mercury and Venus, don’t host moons at all, but other planets have tons — Jupiter, for example, has 79 moons orbiting its enormous self. However, more moons mean more problems: even Neptune’s 14 moons have to work together to make sure they don’t destroy each other. And that’s exactly what Naiad and Thalassa are doing.


Naiad is the dance partner doing all the work, according to astronomers‘ observations. While Thalassa has a strict, steady rotation around Neptune, Naiad seems to jump up and down and all around its partner, orbiting Neptune in about seven and a half hours to Thalassa’s seven.

Naiad doesn’t just want to show off, it is actively trying to avoid being hit by another moon. Astronomers predict that its unusual orbit was actually created by an altercation with another moon countless years ago, which knocked Naiad close to Thalassa. Mark Showalter, an astronomer, suggests that “Naiad and Thalassa have probably been locked together in this configuration for a very long time, because it makes their orbits more stable.”

Neptune’s dancing moons are just one of the recently-discovered wonders of our solar system. What will mysteries will astronomers solve next?

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