Dynamics and Formation of the Near-Resonant K2-24 System: Insights from Transit-Timing Variations and Radial Velocities

TL;DR

This study combines radial velocity and transit-timing data to analyze the formation, dynamics, and physical properties of the near-resonant exoplanets K2-24b and c, revealing insights into their low eccentricities, masses, and envelope fractions.

Contribution

It provides the first combined analysis of RV and transit-timing data for K2-24b and c, revealing their masses, eccentricities, and envelope fractions, and discusses implications for their formation.

Findings

K2-24b and c have low, non-zero eccentricities (~0.08).

K2-24b and c have masses of 19±2 and 15±2 Earth masses.

K2-24c's large envelope challenges standard core accretion models.

Abstract

While planets between the size of Uranus and Saturn are absent within the Solar System, the star K2-24 hosts two such planets, K2-24b and c, with radii equal to $5.4~R_E$ and $7.5~R_E$, respectively. The two planets have orbital periods of 20.9 days and 42.4 days, residing only 1% outside the nominal 2:1 mean-motion resonance. In this work, we present results from a coordinated observing campaign to measure planet masses and eccentricities that combines radial velocity (RV) measurements from Keck/HIRES and transit-timing measurements from K2 and Spitzer. K2-24b and c have low, but non-zero, eccentricities of $e_1 \sim e_2 \sim 0.08$. The low observed eccentricities provide clues regarding the formation and dynamical evolution of K2-24b and K2-24c, suggesting that they could be the result of stochastic gravitational interactions with a turbulent protoplanetary disk, among other mechanisms. K2-24b and c are $19\pm2~M_E$ and $15\pm2~M_E$, respectively; K2-24c is 20% less massive than K2-24b, despite being 40% larger. Their large sizes and low masses imply large envelope fractions, which we estimate at $26^{+3}_{-3}\%$ and $52^{+5}_{-3}\%$. In particular, K2-24c's large envelope presents an intriguing challenge to the standard model of core nucleated accretion that predicts the onset of runaway accretion when $f_{env} \approx 50\%$.