Volume 640, August 2020
|Number of page(s)||18|
|Section||Planets and planetary systems|
|Published online||11 August 2020|
Masses for the seven planets in K2-32 and K2-233
Four diverse planets in resonant chain and the first young rocky worlds★,★★
Centro de Astrobiología (CAB, CSIC-INTA), Depto. de Astrofísica, ESAC campus 28692 Villanueva de la Cañada (Madrid), Spain
2 Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France
3 Geneva Observatory, University of Geneva, Chemin des Mailettes 51, 1290 Versoix, Switzerland
4 Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
5 Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
6 Centre for Exoplanets and Habitability, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, UK
7 Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
8 NCCR/Planet-S, Centre for Space and Habitability, University of Bern, Bern 3012, Switzerland
9 Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
10 European Southern Observatory, Alonso de Cordova 3107, Vitacura, Region Metropolitana, Chile
Accepted: 27 May 2020
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 (~400 Myr 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 (Me = 2.1−1.1+1.3 M⊕, Pe ~ 4.35 days) followed by an inflated Neptune-mass planet (Mb = 15.0−1.7+1.8 M⊕, Pb ~ 8.99 days) and two external sub-Neptunes (Mc = 8.1 ± 2.4 M⊕, Pc ~ 20.66 days; Md = 6.7 ± 2.5 M⊕, Pd ~ 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 Mb < 11.3 M⊕ (Pb ~ 2.47 days), Mc < 12.8 M⊕ (Pc ~ 7.06 days). The outer planet is a sub-Neptune size planet with an inferred mass of Md = 8.3−4.7+5.2 M⊕ (Md < 21.1 M⊕, Pd ~ 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.
Key words: planets and satellites: terrestrial planets / planets and satellites: composition / techniques: radial velocities / techniques: photometric
Full Tables A.1–A.4 are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (220.127.116.11) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/640/A48
© ESO 2020
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