https://www.cnet.com/news/planet-9-might-really-be-a-gang-of-asteroids-shooting-comets-at-us/

At the edge of the solar system, small dwarf planets like Sedna move in strange ways that suggest an unseen ninth planet may be lurking in deep space beyond Neptune, asserting its gravitational pull. 

But new research offers a different explanation: the collective gravity of smaller bits of space debris like asteroids might be responsible for the behavior of so-called "detached objects" like Sedna, which can seem disconnected from the rest of our solar system as they orbit the sun.