Age dependence of the occurrence and architecture of ultra-short-period planet systems

Abstract

Ultra-short-period (USP) planets, with orbital periods shorter than 1 day, represent a unique class of exoplanets whose origin remains puzzling. Determining their age distribution and temporal evolution is vital for uncovering their formation and evolutionary pathways. Using a sample of over 1,000 short-period planets around Sun-like stars, we found that the host stars of USP planets are relatively older and have a higher prevalence in the Galactic thick disk compared to stars hosting other short-period planets. Furthermore, we found that the occurrence of USP planets increases with stellar age, and we uncovered evidence indicating that USP planetary system architectures evolve on gigayear timescales. This includes a distinct dip/pile-up in period distributions around ~1 day and an expansion of orbital spacings with time. In addition, younger USP planet systems are observed to have fewer transiting planets, implying fewer nearby companions or larger mutual orbital inclinations. Our findings indicate that USP planets continuously form through inward migration driven by tidal dissipation over gigayear timescales, and that younger and older USP planets may have originated via different specific tidal migration pathways.