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 ($\sim 400$ 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 (M${}_e=2.1_{-1.1}^{+1.3}$~M${}_{\oplus }$, P${}_e\sim 4.35$~days) followed by an inflated Neptune-mass planet (M${}_b=15.0_{-1.7}^{+1.8}$~M${}_{\oplus }$, P${}_b\sim 8.99$~days) and two external sub-Neptunes (M${}_c=8.1\pm 2.4$~M${}_{\oplus }$, P${}_c\sim 20.66$~days; M${}_d=6.7\pm 2.5$~M${}_{\oplus }$, P${}_d\sim 31.72$~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 M${}_b<11.3$ M${}_{\oplus }$ (P${}_b\sim 2.47$~days), M${}_c<12.8$ M${}_{\oplus }$ (P${}_c\sim 7.06$~days). The outer planet is a sub-Neptune size planet with an inferred mass of M${}_d=8.3_{-4.7}^{+5.2}$ M${}_{\oplus }$ (M${}_d<21.1$ M${}_{\oplus }$, P${}_d\sim 24.36$~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.

arXiv commentAccepted for publication in A&A. 21 pages, 12 figures, 11 Tables

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