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}} = 4.5-5.5 {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.
- Publication:
-
The Astronomical Journal
- Pub Date:
- April 2017
- DOI:
- arXiv:
- arXiv:1702.00013
- Bibcode:
- 2017AJ....153..142P
- Keywords:
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- planets and satellites: composition;
- planets and satellites: detection;
- planets and satellites: dynamical evolution and stability;
- planets and satellites: formation;
- planets and satellites: gaseous planets;
- planets and satellites: interiors;
- Astrophysics - Earth and Planetary Astrophysics
- E-Print:
- 25 pages, 15 figures, accepted for publication in AJ