  There are hints of a novel object ("Planet 9") with a mass $5-10$ $M_\oplus$
in the outer Solar System, at a distance of order 500 AU. If it is a relatively
conventional planet, it can be found in telescopic searches. Alternatively, it
has been suggested that this body might be a primordial black hole (PBH). In
that case, conventional searches will fail. A possible alternative is to probe
the gravitational field of this object using small, laser-launched spacecraft,
like the ones envisioned in the Breakthrough Starshot project. With a velocity
of order $.001~c$, such spacecraft can reach Planet 9 roughly a decade after
launch and can discover it if they can report timing measurements accurate to
$10^{-5}$ seconds back to Earth.
