K2-19b and c are in a 3:2 Commensurability but out of Resonance: A Challenge to Planet Assembly by Convergent Migration

Abstract

K2-19b and c were among the first planets discovered by NASA's K2 mission and together stand in stark contrast with the physical and orbital properties of the solar system planets. The planets are between the size of Uranus and Saturn at 7.0$\pm$0.2 R_E and 4.1$\pm$0.2 R_E, respectively, and reside a mere 0.1% outside the nominal 3:2 mean-motion resonance. They represent a different outcome of the planet formation process than the solar system, as well as the vast majority of known exoplanets. We measured the physical and orbital properties of these planets using photometry from K2, Spitzer, and ground-based telescopes, along with radial velocities from Keck/HIRES. Through a joint photodynamical model, we found that the planets have moderate eccentricities of $e\approx 0.20$ and well-aligned apsides $\Delta \varpi \approx 0$ deg. The planets occupy a strictly non-resonant configuration: the resonant angles circulate rather than librate. This defies the predictions of standard formation pathways that invoke convergent or divergent migration, both of which predict $\Delta \varpi \approx 180$ deg and eccentricities of a few percent or less. We measured masses of $M_{p,b}$ = 32.4$\pm$1.7 M_E and $M_{p,c}$ = 10.8$\pm$0.6 M_E. Our measurements, with 5% fractional uncertainties, are among the most precise of any sub-Jovian exoplanet. Mass and size reflect a planet's core/envelope structure. Despite having a relatively massive core of $M_{core}\approx 15$ $M_E$, K2-19b is envelope-rich, with an envelope mass fraction of roughly 50%. This planet poses a challenge to standard models core-nucleated accretion, which predict that cores $\gtrsim 10$ $M_E$ will quickly accrete gas and trigger runaway accretion when the envelope mass exceeds that of the core.

19 pages, 11 figures, 4 tables, accepted for publication in The Astronomical Journal

References in corpus (14)

• The K2 Mission: Characterization and Early results
• EXOFAST: A fast exoplanetary fitting suite in IDL
• The effect of red noise on planetary transit detection
• Planets and Stellar Activity: Hide and Seek in the CoRoT-7 system
• The Transit Light Curve Project. IX. Evidence for a Smaller Radius of the Exoplanet XO-3b
• Spitzer Secondary Eclipses of the Dense, Modestly-irradiated, Giant Exoplanet HAT-P-20b Using Pixel-Level Decorrelation
• A resonant chain of four transiting, sub-Neptune planets
• Resonant Repulsion of Kepler Planet Pairs
• TTVFast: An efficient and accurate code for transit timing inversion problems
• Spitzer Observations Confirm and Rescue the Habitable-Zone Super-Earth K2-18b for Future Characterization
• Properties of analytic transit light curve models
• Measurement of planet masses with transit timing variations due to synodic "chopping" effects
• Four Sub-Saturns with Dissimilar Densities: Windows into Planetary Cores and Envelopes
• Spitzer transit follow-up of planet candidates from the K2 mission

Cited by in corpus (15)

• ExoClock Project III: 450 new exoplanet ephemerides from ground and space observations
• Tidal Inflation Reconciles Low-Density Sub-Saturns with Core Accretion
• Overfitting Affects the Reliability of Radial Velocity Mass Estimates of the V1298 Tau Planets
• Scaling K2. VI. Reduced Small Planet Occurrence in High Galactic Amplitude Stars
• TOI-216: Resonant Constraints on Planet Migration
• Dynamics and Origins of the Near-Resonant Kepler Planets
• Resonance in the K2-19 system is at odds with its high reported eccentricities
• A Tidal Origin for a 3-body Resonance in Kepler-221
• TOI-4010: A System of Three Large Short-Period Planets With a Massive Long-Period Companion
• Exciting the TTV Phases of Resonant Sub-Neptunes
• Near Mean Motion Resonance of Terrestrial Planet Pair Induced by Giant Planet: Application to Kepler-68 System
• Apsidal Alignment and Anti-Alignment of Planets in Mean-Motion Resonance: Disk-Driven Migration and Eccentricity Driving
• TOI-5108 b and TOI 5786 b: Two transiting sub-Saturns detected and characterized with TESS, MaHPS and SOPHIE
• The K2-24 planetary system revisited by CHEOPS
• Evidence of a gap in the envelope mass fraction of sub-Saturns