Can Planet Nine Be Detected Gravitationally by a Subrelativistic Spacecraft?

Planet Nine was proposed as an explanation for the clustering of orbits for some trans-Neptunian objects. Recently, the use of a subrelativistic spacecraft was proposed to indirectly probe Planet Nine's gravitational influence. Here we study the effects of the drag and electromagnetic forces exerted on a subrelativistic spacecraft by the interstellar medium (ISM) and compare these forces with the gravitational force induced by Planet Nine. We find that the resulting noise due to density and magnetic fluctuations would dominate over Planet Nine's gravitational signal at subrelativistic speeds, v ≳ 0.001 c. We then identify the parameter space required to overcome the drag and magnetic noise from the ISM turbulence and enable the detection of Planet Nine's gravity. Finally, we discuss practical strategies to mitigate the effect of the drag and electromagnetic forces.