No Exomoons for K2-18b

K2-18b, a planet orbiting a nearby M3 dwarf within the habitable zone, has attracted attention as a possible Hycean world. Transit spectroscopy suggests that methane, carbon dioxide, and tentative sulfur-bearing molecules may exist in the planet's atmosphere, raising questions about the potential for life in its oceans. On Earth, tides driven by the Moon likely played an important role in early habitability, motivating the question of whether K2-18b could host an exomoon. We investigated this scenario using the REBOUND N-body integrator with its tides_spin module in REBOUNDx, performing thousands of simulations across a range of planetary masses, eccentricities, and tidal dissipation parameters. In every case, we find that a moon would migrate outward and escape the host planet's gravitational influence in less than ~10 Myr, with lifetimes shortened to <1 Myr under Earth-like tidal conditions. Even the most favorable scenarios produce survival times of only ~20 Myr, far shorter than the system's ~3 Gyr age. These results imply that long-lived moons cannot exist around K2-18b. If habitable conditions are present, tidal forcing of its oceans must therefore arise from the host star rather than from a moon, challenging analogies to the Earth–Moon system.