Vol. 640
10. Planets and planetary systems

Masses for the seven planets in K2-32 and K2-233. Four diverse planets in resonant chain and the first young rocky worlds

by J. Lillo-Box, T. A. Lopez, A. Santerne, et al. 2020, A&A, 640, A48 alt

K2-32 and K2-233 are two planetary systems which host four and three planets, respectively. They were detected by the K2 mission transiting their host star. In this study, the authors measured their masses and bulk densities for the first time by using a long-term radial velocity follow-up with the HARPS instrument. K2-32 displays a similar architecture as our Solar System but in a scaled-down version, with a small rocky world in the inner regions, followed by a transitional sub-Saturn planet and two mini-Neptunes in the external region. All of them are in a compact resonant configuration inside half of Mercury’s orbit, which makes them key targets for atmospheric studies. With regard to K2-233, it is a young 600 Myr old star. Lillo-Box et al. confirmed in this study the rocky nature of the two inner planets in the system, thus representing the first instances of rocky worlds with measured masses around a young star. The two planets K2-233 b and c show masses and radii corresponding to a telluric composition. The results of this study allow for further investigations on the atmosphere of lower-mass planets, the early evolution of multi-planetary systems with rocky worlds, and the planet-planet interactions in compact multi-planetary systems in near resonant chains. The transiting nature of all of these planets and the precise bulk densities established in this work put them at the forefront of future atmospheric studies.