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.

arXiv comment25 pages, 15 figures, accepted for publication in AJ

References in corpus (56)

• Astropy: A Community Python Package for Astronomy
• emcee: The MCMC Hammer
• The Dartmouth Stellar Evolution Database
• The K2 Mission: Characterization and Early results
• Planet Occurrence within 0.25 AU of Solar-type Stars from Kepler
• The false positive rate of Kepler and the occurrence of planets
• batman: BAsic Transit Model cAlculatioN in Python
• The Keck Planet Search: Detectability and the Minimum Mass and Orbital Period Distribution of Extrasolar Planets
• The NASA Exoplanet Archive: Data and Tools for Exoplanet Research
• Prevalence of Earth-size planets orbiting Sun-like stars
• Kepler planets: a tale of evaporation
• Most 1.6 Earth-Radius Planets are not Rocky
• EXOFAST: A fast exoplanetary fitting suite in IDL
• Understanding the Mass-Radius Relation for Sub-Neptunes: Radius as a Proxy for Composition
• A Technique for Extracting Highly Precise Photometry for the Two-Wheeled Kepler Mission
• The Mass-Radius Relation for 65 Exoplanets Smaller than 4 Earth Radii
• Chromospheric Activity and Jitter Measurements for 2630 Stars on the California Planet Search
• Masses, Radii, and Orbits of Small Kepler Planets: The Transition from Gaseous to Rocky Planets
• A Neptune-Mass Planet Orbiting the Nearby M Dwarf GJ 436
• The Mass-Metallicity Relation for Giant Planets
• HAT-P-11b: A Super-Neptune Planet Transiting a Bright K Star in the Kepler Field
• The California Planet Survey I. Four New Giant Exoplanets
• How Thermal Evolution and Mass Loss Sculpt Populations of Super-Earths and Sub-Neptunes: Application to the Kepler-11 System and Beyond
• Models of the in situ formation of detected extrasolar giant planets
• Spectral Properties of Cool Stars: Extended Abundance Analysis of 1617 Planet Search Stars
• Stellar Spin-Orbit Misalignment in a Multiplanet System
• Almost All of Kepler's Multiple Planet Candidates are Planets
• The K2 Ecliptic Plane Input Catalog (EPIC) and Stellar Classifications of 138,600 Targets in Campaigns 1-8
• Understanding The Effects Of Stellar Multiplicity On The Derived Planet Radii From Transit Surveys: Implications for Kepler, K2, and TESS
• The NASA-UC-UH Eta-Earth Program: IV. A Low-mass Planet Orbiting an M Dwarf 3.6 PC from Earth
• Limits on Planetary Companions from Doppler Surveys of Nearby Stars
• A diagram for the evaporation status of extrasolar planets
• To Cool is to Accrete: Analytic Scalings for Nebular Accretion of Planetary Atmospheres
• A resonant chain of four transiting, sub-Neptune planets
• 197 Candidates and 104 Validated Planets in K2's First Five Fields
• Spitzer Transit and Secondary Eclipse Photometry of GJ 436b
• Kepler-413b: a slightly misaligned, Neptune-size transiting circumbinary planet
• New constraints on Saturn's interior from Cassini astrometric data
• Kepler 18-b, c, and d: A System Of Three Planets Confirmed by Transit Timing Variations, Lightcurve Validation, Spitzer Photometry and Radial Velocity Measurements
• All Six Planets Known to Orbit Kepler-11 Have Low Densities
• HAT-P-26b: A Low-Density Neptune-Mass Planet Transiting a K Star
• Precise time-series photometry for the Kepler-2.0 mission
• Eleven Multi-planet Systems from K2 Campaigns 1 & 2 and the Masses of Two Hot Super-Earths
• Kepler-4b: Hot Neptune-Like Planet of a G0 Star Near Main-Sequence Turnoff
• KOI-152's Low Density Planets
• Detection of Stars within $\sim$0.8 arcseconds of Kepler Objects of Interest
• On the Survival of Short-Period Terrestrial Planets
• An Independent Planet Search In The Kepler Dataset. II. An extremely low-density super-Earth mass planet around Kepler-87
• Transiting exoplanets from the CoRoT space mission. XI. CoRoT-8b: a hot and dense sub-Saturn around a K1 dwarf
• Accurate Gravities of F, G, and K stars from High Resolution Spectra Without External Constraints
• The Frequency of Low-mass Exoplanets
• A Quasi-Stationary Solution to Gliese 436b's Eccentricity
• Doppler Monitoring of five K2 Transiting Planetary Systems
• HATS-7b: A Hot Super Neptune Transiting a Quiet K Dwarf Star
• Two Transiting Low Density Sub-Saturns from K2
• The K2-ESPRINT Project II: Spectroscopic follow-up of three exoplanet systems from Campaign 1 of K2

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• The Occurrence-weighted Median Planets Discovered by Transit Surveys Orbiting Solar-type Stars and Their Implications for Planet Formation and Evolution
• The TESS-Keck Survey. XIX. A Warm Transiting Sub-Saturn Mass Planet and a non-Transiting Saturn Mass Planet Orbiting a Solar Analog
• Evidence of a gap in the envelope mass fraction of sub-Saturns