ORBITAL ARCHITECTURES of PLANET-HOSTING BINARIES. I. FORMING FIVE SMALL PLANETS in the TRUNCATED DISK of KEPLER-444A

Abstract

We present the first results from our Keck program investigating the orbital architectures of planet-hosting multiple star systems. Kepler-444 is a metal-poor triple star system that hosts five sub-Earth-sized planets orbiting the primary star (Kepler-444A), as well as a spatially unresolved pair of M dwarfs (Kepler-444BC) at a projected distance of $1\buildrel{\prime\prime}\over{.} 8$ (66 AU). We combine our Keck/NIRC2 adaptive optics astrometry with multi-epoch Keck/HIRES RVs of all three stars to determine a precise orbit for the BC pair around A, given their empirically constrained masses. We measure minimal astrometric motion (1.0 ± 0.6 mas yr−1, or 0.17 ± 0.10 km s−1), but our RVs reveal significant orbital velocity (1.7 ± 0.2 km s−1) and acceleration (7.8 ± 0.5 m s−1 yr−1). We determine a highly eccentric stellar orbit ($e=0.864\pm 0.023$) that brings the tight M dwarf pair within ${5.0}_{-1.0}^{+0.9}$ AU of the planetary system. We validate that the system is dynamically stable in its present configuration via n-body simulations. We find that the A–BC orbit and planetary orbits are likely aligned (98%) given that they both have edge-on orbits and misalignment induces precession of the planets out of transit. We conclude that the stars were likely on their current orbits during the epoch of planet formation, truncating the protoplanetary disk at ≈2 AU. This truncated disk would have been severely depleted of solid material from which to form the total ≈1.5 M⊕ of planets. We thereby strongly constrain the efficiency of the conversion of dust into planets and suggest that the Kepler-444 system is consistent with models that explain the formation of most close-in Kepler planets in more typical, not truncated, disks.