Mass, Density, and Formation Constraints in the Compact, Sub-Earth Kepler-444 System including Two Mars-mass Planets

摘要

Kepler-444 is a five-planet system around a host star approximately 11 billion years old. The five transiting planets all have sub-Earth radii and are in a compact configuration with orbital periods between 3 and 10 days. Here, we present a transit-timing analysis of the system using the full Kepler data set in order to determine the masses of the planets. Two planets, Kepler-444 d (M-d = 0.036(-0.020)(+0.065)M(circle plus)) and Kepler-444 e (M-e = 0.034(-0.019)(+0.059)M(circle plus)), have confidently detected masses due to their proximity to resonance that creates transit-timing variations. The mass ratio of these planets combined with the magnitude of possible star-planet tidal effects suggests that smooth disk migration over a significant distance is unlikely to have brought the system to its currently observed orbital architecture without significant post-formation perturbations.