2020A&A...640A..48L

Context. High-precision planetary densities are key pieces of information necessary to derive robust atmospheric properties for extrasolar planets. Measuring precise masses is the most challenging part of this task, especially in multi-planetary systems. The ESO-K2 collaboration focuses on the follow-up of a selection of multi-planetary systems detected by the K2 mission using the HARPS instrument with this goal in mind.
Aims. In this work, we measure the masses and densities of two multi-planetary systems: a four-planet near resonant chain system (K2-32) and a young (∼400Myr old) planetary system consisting of three close-in small planets (K2-233).
Methods. We obtained 199 new HARPS observations for K2-32 and 124 for K2-233 covering a long baseline of more than three years. We performed a joint analysis of the radial velocities and K2 photometry with PASTIS to precisely measure and constrained the properties of these planets, focusing on their masses and orbital properties.
Results. 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^+1.3M_⊕, P_e∼4.35-days) followed by an inflated Neptune-mass planet (M_b=15.0_–1.7^+1.8M_⊕, P_b∼8.99-days) and two external sub-Neptunes (M_c=8.1±2.4M_⊕, P_c∼20.66-days; M_d=6.7±2.5M_⊕, P_d∼31.72-days). K2-32 becomes one of the few multi-planetary systems with four or more planets known where all have measured masses and radii. Additionally, we constrain the masses of the three planets in the K2-233 system through marginal detection of their induced radial velocity variations. For the two inner Earth-size planets we constrain their masses at a 95% confidence level to be smaller than M_b<11.3M_⊕ (P_b∼2.47-days), M_c<12.8M_⊕ (P_c∼7.06-days). The outer planet is a sub-Neptune size planet with an inferred mass of M_d=8.3_–4.7^+5.2M_⊕ (M_d<21.1M_⊕, P_d∼24.36-days).
Conclusions. 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, which is key for transmission spectroscopy studies in the near future.