Predicted transit times of Kepler-51b, c, and d

Transit timing variations (TTVs) measured with past Kepler and Hubble Space Telescope (HST) observations have been successfully modeled by considering gravitational interactions between the three transiting planets, yielding low masses and low mean densities (<=0.1g/cm^3^) for all three planets.

However, the transit time of the outermost transiting planet Kepler-51d recently measured by the James Webb Space Telescope 10yr after the Kepler observations is significantly discrepant from the prediction made by the three-planet TTV model, which we confirmed with ground-based and follow-up HST observations. We show that the departure from the three-planet model is explained by including a fourth outer planet, Kepler-51e, in the TTV model.