Masses for the seven planets in K2-32 and K2-233. Four diverse planets in resonant chain and the first young rocky worlds
Abstract
High-precision planetary densities are key to derive robust atmospheric properties for extrasolar planets. Measuring precise masses is the most challenging part, especially in multi-planetary systems. We measure the masses and densities of a four-planet near resonant chain system (K2-32), and a young (400 Myr old) planetary system consisting of three close-in small planets (K2-233). We obtained 199 new HARPS observations for K2-32 and 124 for K2-233 covering a more than three year baseline. We find that K2-32 is a compact scaled-down version of the Solar System's architecture, with a small rocky inner planet (Me=2.1+1.3-1.1~M, Pe4.35~days) followed by an inflated Neptune-mass planet (Mb=15.0+1.8-1.7~M, Pb8.99~days) and two external sub-Neptunes (Mc=8.12.4~M, Pc20.66~days; Md=6.72.5~M, Pd31.72~days). K2-32 becomes one of the few multi-planetary systems with four or more planets known with measured masses and radii. Additionally, we constrain the masses of the three planets in K2-233. For the two inner Earth-size planets we constrain their masses to be smaller than Mb<11.3 M (Pb2.47~days), Mc<12.8 M (Pc7.06~days). The outer planet is a sub-Neptune size planet with an inferred mass of Md=8.3+5.2-4.7 M (Md<21.1 M, Pd24.36~days). Our observations of these two planetary systems confirm for the first time the rocky nature of two planets orbiting a young star, with relatively short orbital periods (<7 days). They provide key information for planet formation and evolution models of telluric planets. Additionally, the Neptune-like derived masses of the three planets K2-32 b, c, d puts them in a relatively unexplored regime of incident flux and planet mass, key for transmission spectroscopy studies.