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K2-19b and c were among the first planets discovered by NASA's K2 mission andtogether stand in stark contrast with the physical and orbital properties ofthe solar system planets. The planets are between the size of Uranus and Saturnat 7.0$\pm$0.2 R_E and 4.1$\pm$0.2 R_E, respectively, and reside a mere 0.1%outside the nominal 3:2 mean-motion resonance. They represent a differentoutcome of the planet formation process than the solar system, as well as thevast majority of known exoplanets. We measured the physical and orbitalproperties of these planets using photometry from K2, Spitzer, and ground-basedtelescopes, along with radial velocities from Keck/HIRES. Through a jointphotodynamical model, we found that the planets have moderate eccentricities of$e \approx0.20$ and well-aligned apsides $\Delta \varpi \approx 0$ deg. Theplanets occupy a strictly non-resonant configuration: the resonant anglescirculate rather than librate. This defies the predictions of standardformation pathways that invoke convergent or divergent migration, both of whichpredict $\Delta \varpi \approx 180$ deg and eccentricities of a few percent orless. 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 amongthe most precise of any sub-Jovian exoplanet. Mass and size reflect a planet'score/envelope structure. Despite having a relatively massive core of $M_{core}\approx15$ $M_E$, K2-19b is envelope-rich, with an envelope mass fraction ofroughly 50%. This planet poses a challenge to standard models core-nucleatedaccretion, which predict that cores $\gtrsim 10$ $M_E$ will quickly accrete gasand trigger runaway accretion when the envelope mass exceeds that of the core.

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