K2-19b and c transit times and radial velocities

This defies the predictions of standard formation pathways that invoke convergent or divergent migration, both of which predict {Delta}{omega}~180{deg} and eccentricities of a few percent or less. We measured masses of M_p,b_=32.4+/-1.7 M_{Earth}_ and M_p,c_=10.8+/-0.6 M_{Earth}_. 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_~15 M_{Earth}_, K2-19b is envelope-rich, with an envelope mass fraction of roughly 50%. This planet poses a challenge to standard models of core-nucleated accretion, which predict that cores >~10 M_{Earth}_ will quickly accrete gas and trigger runaway accretion when the envelope mass exceeds that of the core.