Planet Nine is a hypothetical world in the far reaches of our solar system. Proposed in 2016 by Caltech astronomers Mike Brown and Konstantin Batygin, its existence would explain the unusual orbits of certain Kuiper belt objects (KBOs). But are we completely sure that Planet Nine in fact a planet?

A paper in September 2019 suggested the gravitational effects could instead be explained by the presence of a primordial black hole smaller than your fist. To get to the bottom of this mystery, there have been recent proposals to send fleets of tiny probes to the general region of this mysterious object.

In the July episode of Physics World Stories Andrew Glester gets the latest on the mystery of Planet Nine. Appearing in the podcast are Mike Brown and the University of Maryland’s Zeeve Rogoszinski, co-author of one of the mission proposals.