Then there’s poor Neptune. Today, Neptune serves as the shepherd of the Kuiper Belt, the icy region of the solar system where Pluto resides. Kuiper Belt objects orbit in what astronomers call a 2:3 resonance with Neptune, completing three orbits every time Neptune makes two. This keeps the orbits fairly tidy, and prevents collisions with Neptune.But long ago, the belt had a lot more objects, and they crossed into the Neptune system a lot. The Kuiper Belt of today has only a small fraction of its original mass today. While the planets were still settling into their orbit, Neptune pulled and flung around a lot of the Kuiper Belt matter, including wholesale flinging worlds out of the solar system. But in the process, Neptune gained a giant moon—while losing a series of smaller ones.Some theories suggest that two particular objects came through Neptune, probably a pair roughly equivalent in size — as if they were each other’s moons. This double dwarf planet got separated by Neptune — the object that stayed behind orbiting Neptune backward. We know it today as Triton.“Encounters with Neptune are frequent, and Triton just happened to come in slow enough,” Ćuk says. “Leading theories are that Triton was a binary, and one half escaped.”One of those moons made its way through a region where Neptune’s original moons resided. Because it was orbiting backwards, these small moons may have hit their intruder head on. Triton may be made from as much as 10 percent made of Neptune’s original inner moons There are, of course, a few other lost moon theories floating out there — that when a protoplanet called Theia smashed into Earth to form the moon, it may have created several moons. And, until we can study the outer solar system moons in depth, we may not fully understand if any of these hypotheses hold water.Ćuk says any planets that escape the grasp of a giant planet don’t last long. A few lucky ones might, under the right conditions, get ejected out, but most will take up an orbit right outside their original planet — and eventually collide with it. Other moons may have destabilized enough to eventually fall into the Sun.But whatever the ultimate answer, our solar system may have been a lot more full of moons than we’ve ever thought — and their ghostly trail is just waiting for us to discover it.This article originally appeared on Discovermagazine.com