I'm not sure that was the conclusion at all. They modeled the dust particles in the solar system at various stages of the solar system's development. They saw

I'm not sure that was the conclusion at all. They modeled the dust particles in the solar system at various stages of the solar system's development. They saw that today, Neptune's gravitational pull creates a noticeable footprint in the Kuiper belt (a massive body of dust and rocks forming a ring outside the orbit of Neptune). When looking at other star systems they could potentially see similar footprints, which would imply the presence of Neptune-sized planets. They also saw that when modelling the young solar system, the result looked very similar to the Fomalhaut system. This system is known to be young and also known to have at least one large planet. So from that the technique could also be used to identify young star systems that may contain planets.