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

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}_\oplus ) and Kepler-444 e ({M}_{{e}}={0.034}_-0.019^+0.059 {M}_\oplus ), 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.