2025ApJ...991..206Z

Ultra-short-period (USP) planets, defined as those with orbital periods shorter than 1 day, provide valuable insights into planetary evolution under strong stellar tidal interactions. In this work, we investigate the formation of USP planets in two-planet systems consisting of an inner terrestrial planet accompanied by an outer hot Jupiter (HJ). Our simulation results show USP planets can form through a process driven by secular perturbations from the outer companion, which induce eccentricity excitation, tidal dissipation, and subsequent orbital decay of the inner planet. The probability of USP formation is governed by key factors, including the mass ratio between two planets, their orbital eccentricities, and the tidal dissipation process. 6.7% of our simulations form USP planets, and USP planets form most efficiently when the mass ratio is around 4 M_⊕/M_J, with the inner planet less than 8 M_⊕. Furthermore, the eccentricity of the outer HJ plays a crucial role-moderate eccentricities (e_outer < 0.1) favor USP formation, whereas higher eccentricities (e_outer > 0.1) enhance the likelihood of orbital instability, often resulting in a lonely HJ. USP planets form more efficiently when the tidal dissipation function of the inner planet is comparable to the values estimated for terrestrial planets in the solar system. Comparison with observed planetary systems reveals that systems with large mass ratios or nearly circular outer planets tend to produce short-period (SP) planets instead of USP planets. Our findings offer a potential explanation for the most commonly observed system architectures, which predominantly feature either an HJ with an inner SP planet or a lonely HJ.