The phrase "a chip off the old block" apparently also applies to the outer moons of our solar system.

A tiny moon whirling around Neptune that was uncovered in Hubble Space Telescope photographs taken in 2013 has puzzled astronomers ever since then because it is very close to a much larger moon named Proteus. The orbits of the two moons are presently 7,500 miles apart.

Proteus, at 260 miles in diameter, is roughly the size of the state of Ohio. By contrast, Hippocamp is just 20 miles across, or the size of metropolitan Columbus, Ohio. Proteus should have gravitationally swept aside or swallowed the moon while clearing out its orbital path.

Smoking-gun evidence for Hippocamp's origin comes from NASA Voyager 2 images from 1989 that show a large impact crater on Proteus, almost large enough to have shattered the moon. Apparently, a little piece of Proteus got kicked off and has slowly migrated away from the parent body.

Neptune's satellite system has a violent and tortured history. Many billions of years ago, Neptune captured the large moon Triton from the Kuiper Belt. Triton's gravity would have torn up Neptune's original satellite system. Triton settled into a circular orbit and the debris from shattered Neptunian moons re-coalesced into a second generation of natural satellites. However, comet bombardment continued to tear things up, leading to the birth of Hippocamp, which might be considered a third-generation satellite.

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