A Tale of Two Peas in a Pod: The Kepler-323 and Kepler-104 Systems

Abstract

In order to understand the relationship between planet multiplicity, mass, and composition, we present newly measured masses of five planets in two planetary systems: Kepler-323 and Kepler-104. We used the HIRES instrument at the W.M. Keck Observatory to collect 79 new radial velocity (RV) measurements for Kepler-323, which we combined with 48 literature RVs from TNG/HARPS-N. We also conducted a reanalysis of the Kepler-104 system, using 44 previously published RV measurements. Kepler-323 b and c have masses of 2.0−1.1+1.2 M ⊕ and 6.5±1.6 M ⊕, respectively, whereas the three Kepler-104 planets are more massive (10.0±2.8 M ⊕, 7.1−3.5+3.8 M ⊕, and 5.5−3.5+4.6 M ⊕ for planets b, c, and d, respectively). The Kepler-104 planets have densities consistent with rocky cores overlaid with gaseous envelopes ( 4.1−1.1+1.2 g cc−1, 2.9−1.5+1.7 g cc−1, and 1.6−1.1+1.5 g cc−1 respectively), whereas the Kepler-323 planets are consistent with having rocky compositions ( 4.5−2.4+2.8 g cc−1 and 9.9−2.5+2.7 g cc−1). The Kepler-104 system has among the lowest values for gap complexity ( C = 0.004) and mass partitioning ( Q = 0.03); whereas, the Kepler-323 planets have a mass partitioning similar to that of the Inner Solar System ( Q = 0.28 and Q = 0.24, respectively). For both exoplanet systems, the uncertainty in the mass partitioning is affected equally by (1) individual mass errors of the planets and (2) the possible existence of undetected low-mass planets, meaning that both improved mass characterization and improved sensitivity to low-mass planets in these systems would better elucidate the mass distribution among the planets.