Prospects for localizing Planet 9 with a future Uranus mission

Past years have seen various publications attempting to explain the apparent clustering features of trans-Neptunian objects, the most popular explanation being an unconfirmed 'Planet 9'. The recently proposed Uranus Orbiter and Probe mission by NASA's Planetary Science and Astrobiology Decadal Survey could offer the opportunity to precisely determine Planet 9's sky location and mass by carefully monitoring ranging data during the interplanetary cruise. We use Monte Carlo Markov chain methods to reconstruct simulated spacecraft trajectories in a simplified Solar system model containing Planet 9, providing an estimate of the mission's localization capacity depending on sky location, Earth-spacecraft Doppler link noise level and data collection rate. We characterize the noise via the Allan deviation σ_A, scaled to the Cassini-era value $\sigma _{\rm A}^{\rm \scriptscriptstyle Cass} = 3 \times 10^{-15}$, finding that daily measurements of the spacecraft position can lead to ~0.2 deg² localization of Planet 9 (assuming M₉ = 6.3 M_⊕, d₉ = 460 au). As little as a three-fold improvement in σ_A drastically decreases the sky localization area size to ~0.01 deg². Thus, we showcase that a future Uranus mission carries a significant potential also for non-Uranian science.