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 measurements (RVs) 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${}_{\oplus }$ and 6.5$\pm 1.6$ M${}_{\oplus }$, respectively, whereas the three Kepler-104 planets are more massive (10.0$\pm 2.8$ M${}_{\oplus }$, $7.1_{-3.5}^{+3.8}$ M${}_{\oplus }$, and $5.5_{-3.5}^{+4.6}$ M${}_{\oplus }$ 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, $2.9_{-1.5}^{+1.7}$ g/cc, and $1.6_{-1.1}^{+1.5}$ g/cc respectively), whereas the Kepler-323 planets are consistent with having rocky compositions ($4.5_{-2.4}^{+2.8}$ g/cc and $9.9_{-2.5}^{+2.7}$ g/cc). The Kepler-104 system has among the lowest values for gap complexity ($\mathcal{C}$ = 0.004) and mass partitioning ($\mathcal{Q}$ = 0.03); whereas, the Kepler-323 planets have a mass partitioning similar to that of the Inner Solar System ($\mathcal{Q}$ = 0.28 and $\mathcal{Q}$ = 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.

Accepted for publication in Astronomical Journal