According to space.com, remnants of the manufacturing process were left behind in shoddy, unfinished work on one of the solar system's smallest moons. I think it's called "flashing."
Another possibility not considered in the article is that we have been grossly mistaken about the nature of planets: that they are a kind of celestial fruit, with a thin peel and a juicy interior surrounding what could be called a "core." They grow over time, and eventually burst in a volcanic epiphany, releasing tiny moonlets that go one to seed other solar systems. The stars, of course, are the flowering bodies. The tree itself, larger than the size of the galaxy, is a hyper-dimensional entity known as Yggdrasil.