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

April 2017 • 2017AJ....153..142P

Authors • Petigura, Erik A. • Sinukoff, Evan • Lopez, Eric D. • Crossfield, Ian J. M. • Howard, Andrew W. • Brewer, John M. • Fulton, Benjamin J. • Isaacson, Howard T. • Ciardi, David R. • Howell, Steve B. • Everett, Mark E. • Horch, Elliott P. • Hirsch, Lea A. • Weiss, Lauren M. • Schlieder, Joshua E.

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}} = 4.5-5.5 {R}\oplus ), but have dissimilar masses ({M}{{P}} = 16-60 {M}\oplus ), implying a diversity in their core and envelope masses. K2-32b is the least massive ({M}{{P}}=16.5+/- 2.7 {M}\oplus ) 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+/- 4.6 {M}\oplus and {M}{{P}}=39.8+/- 4.4 {M}\oplus , respectively, and show no signs of additional planets. K2-108b is the most massive at {M}{{P}}=59.4+/- 4.4 {M}\oplus , implying a large reservoir of heavy elements of about ≈50 {M}\oplus . 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 ≈6-60 {M}\oplus , 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.


IPAC Authors


David Ciardi

Senior Scientist


Benjamin Fulton

Assistant Scientist