Four Sub-Saturns with Dissimilar Densities: Windows into Planetary Cores and Envelopes

Abstract

We present results from a Keck/HIRES radial velocity campaign to study four sub-Saturn-sized planets, K2-27b, K2-32b, K2-39b, and K2-108b, with the goal of understanding their masses, orbits, and heavy element enrichment. The planets have similar sizes $(R_P=4.5-5.5R_E)$, but have dissimilar masses $(M_P=16-60M_E)$, implying a diversity in their core and envelope masses. K2-32b is the least massive $(M_P=16.5\pm 2.7M_E)$ and orbits in close proximity to two sub-Neptunes near a 3:2:1 period commensurability. K2-27b and K2-39b are significantly more massive at $M_P=30.9\pm 4.6M_E$ and $M_P=39.8\pm 4.4M_E$, respectively, and show no signs of additional planets. K2-108b is the most massive at $M_P=59.4\pm 4.4M_E$, implying a large reservoir of heavy elements of about $\approx 50M_E$. Sub-Saturns as a population have a large diversity in planet mass at a given size. They exhibit remarkably little correlation between mass and size; sub-Saturns range from $\approx 6-60M_E$, regardless of size. We find a strong correlation between planet mass and host star metallicity, suggesting that metal-rich disks form more massive planet cores. The most massive sub-Saturns tend to lack detected companions and have moderately eccentric orbits, perhaps as a result of a previous epoch of dynamical instability. Finally, we observe only a weak correlation between the planet envelope fraction and present-day equilibrium temperature, suggesting that photo-evaporation does not play a dominant role in determining the amount of gas sub-Saturns accrete from their protoplanetary disks.

25 pages, 15 figures, accepted for publication in AJ