Confirming the Discovery of a Dense USP Neptune with HARPS and ESPRESSO

The core-nucleated accretion model is a long-accepted framework for explaining the formation of giant planets. In this theory, a critical mass marks the boundary between giant versus terrestrial planets, delineating between inefficient versus runaway gas accretion. However, this critical mass is not well constrained, typically spanning between 5-10 ME. This discrepancy is due to the difficulty of characterizing the core of a giant planet observationally. Recent discoveries of ultra-dense, Neptune-sized ultra-short period planets (USPs) reveal a population of giant planet core analogues. These former giant planets are thought to have migrated too close to their star, triggering mass loss via tidal effects and exposing their cores. This allows rare opportunities to directly study the nascent stages of giant planet formation and test theoretical requirements for producing a Neptune-sized USP. Using ESPRESSO and HARPS radial velocities, we report the discovery of TOI-3261b (R = 4RE, M = 30ME), the fourth known Neptune-sized USP. We characterize the end product of an extreme mass loss mechanism, and directly test the predicted critical core mass that forms a giant planet.