Another Superdense Sub-Neptune in K2-182 b and Refined Mass Measurements for K2-199 b and c

Abstract

We combine multiple campaigns of K2 photometry with precision radial velocity measurements from Keck-HIRES to measure the masses of three sub-Neptune-sized planets. We confirm the planetary nature of the massive sub-Neptune K2-182 b (P_b = 4.7 days, R_b = 2.69 R_⊕) and derive refined parameters for K2-199 b and c (P_b = 3.2 days, R_b = 1.73 R_⊕ and P_c = 7.4 days, R_c = 2.85 R_⊕). These planets provide valuable data points in the mass–radius plane, especially as TESS continues to reveal an increasingly diverse sample of sub-Neptunes. The moderately bright (V = 12.0 mag) early K dwarf K2-182 (EPIC 211359660) was observed during K2 campaigns 5 and 18. We find that K2-182 b is potentially one of the densest sub-Neptunes known to date (20 ± 5 M_⊕ and 5.6 ± 1.4 g cm⁻³). The K5V dwarf K2-199 (EPIC 212779596; V = 12.3 mag), observed in K2 campaigns 6 and 17, hosts two recently confirmed planets. We refine the orbital and planetary parameters for K2-199 b and c by modeling both campaigns of K2 photometry and adding 12 Keck-HIRES measurements to the existing radial velocity data set (N = 33). We find that K2-199 b is likely rocky, at 6.9 ± 1.8 M_⊕ and 7.2_(-2.0)^(+2.1) g cm⁻³, and that K2-199 c has an intermediate density at 12.4 ± 2.3 M_⊕ and 2.9_(-0.6)^(+0.7) g cm⁻³. We contextualize these planets on the mass–radius plane, discuss a small but intriguing population of "superdense" sub-Neptunes (R_p < 3 R_⊕, M_p >20 M_⊕), and consider our prospects for the planets' atmospheric characterization.