Three small planets transiting the bright young field star K2-233 (1803.05056v2)

Abstract: We report the detection of three small transiting planets around the young K3 dwarf K2-233 (2MASS J15215519-2013539) from observations during Campaign 15 of the K2 mission. The star is relatively nearby ($d$ = 69 pc) and bright ($V$ = 10.7 mag, $K_s$ = 8.4 mag), making the planetary system an attractive target for radial velocity follow-up and atmospheric characterization with the James Webb Space Telescope. The inner two planets are hot super-Earths ($R_b$ = 1.40 $\pm$ 0.06 R$\oplus$, $R_c$ = 1.34 $\pm$ 0.08 R$\oplus$), while the outer planet is a warm sub-Neptune ($R_d$ = 2.6 $\pm$ 0.1 R$\oplus$). We estimate the stellar age to be 360$^{+490}{-140}$ Myr based on rotation, activity, and kinematic indicators. The K2-233 system is particularly interesting given recent evidence for inflated radii in planets around similarly-aged stars, a trend potentially related to photo-evaporation, core-cooling, or both mechanisms.