Three planets transiting the evolved star EPIC 249893012: a hot 8.8-M_⊕ super-Earth and two warm 14.7 and 10.2-M_⊕ sub-Neptunes^★,★★

¹ Instituto de Astrofisica de Canarias, Tenerife, Spain
e-mail: dhidalgo@iac.es
² Departamento de Astrofísica Universidad de La Laguna, Tenerife, Spain
³ Dipartimento di Fisica, Università degli Studi di Torino, Torino, Italy
⁴ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
⁵ Leiden Observatory, University of Leiden, Leiden, The Netherlands
⁶ Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan
⁷ Department of Astronomy, Graduate School of Science, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-0033, Japan
⁸ JST, PRESTO, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁹ Astrobiology Center, NINS, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹⁰ National Astronomical Observatory of Japan, NINS, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹¹ Stellar Astrophysics Centre, Deparment of Physics and Astronomy, Aarhus University, Ny Munkegrade 120, 8000 Aarhus C, Denmark
¹² Department of Astronomy and McDonald Observatory, University of Texas at Austin, 2515 Speedway, Stop C1400, Austin, TX 78712, USA
¹³ Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA, 02139, USA
¹⁴ Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ, 08544, USA
¹⁵ Astronomy Department and Van Vleck Observatory, Wesleyan University, Middletown, CT 06459, USA
¹⁶ Institute of Planetary Research, German Aerospace Center, Rutherfordstrasse 2, 12489 Berlin, Germany
¹⁷ Center for Astronomy and Astrophysics, TU Berlin, Hardenbergstr. 36, 10623 Berlin, Germany
¹⁸ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
¹⁹ Rheinisches Institut für Umweltforschung, Abteilung Planetenforschung an der Universität zu Köln, Aachener Strasse 209, 50931 Köln, Germany
²⁰ Institute of Geological Sciences, Freie Universität Berlin, Malteserstr. 74-100, 12249 Berlin, Germany
²¹ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
²² Institut de Ciències de l’Espai (ICE, CSIC), Campus UAB, C/ de Can Magrans s/n, 08193 Bellaterra, Spain
²³ Institut d’Estudis Espacials de Catalunya (IEEC), C/ Gran Capità 2-4, 08034 Barcelona, Spain
²⁴ Monash Centre for Astrophysics, School of Physics and Astronomy, Monash University, VIC 3800, Australia
²⁵ European Southern Observatory (ESO), Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago de Chile, Chile
²⁶ Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK
²⁷ Sub-department of Astrophysics, Department of Physics, University of Oxford, Oxford OX1 3RH, UK
²⁸ Astronomical Institute of the Czech Academy of Sciences, Fričova 298, 25165 Ondřejov, Czech Republic
²⁹ Astronomical Institute of Charles University, V Holešovičkách 2, 180 00 Praha, Czech Republic
³⁰ Centro de Astrobiología (CSIC-INTA), ESAC, camino bajo del castillo s/n, 28692 Villanueva de la Cañada, Madrid Spain
³¹ Institut für Astrophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
³² Zentrum für Astronomie der Universtät Heidelberg, Landessternwarte, Königstuhl 12, 69117 Heidelberg, Germany
³³ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
³⁴ Centre for Exoplanets and Habitability, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, UK
³⁵ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, UK
³⁶ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago 19, Chile
³⁷ Instituto de Astrofísica e Ciências do Espaço, CAUP, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
³⁸ Departamento de Física, Universidade Federal do Rio Grande do Norte, 59078-970 Natal, RN, Brazil
³⁹ Geneva Observatory, University of Geneva, Chemin des Mailettes 51, 1290 Versoix, Switzerland
⁴⁰ Center for Space and Habitability, Universität Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland

Received: 7 November 2019
Accepted: 29 January 2020

Abstract

We report the discovery of a new planetary system with three transiting planets, one super-Earth and two sub-Neptunes, that orbit EPIC 249893012, a G8 IV-V evolved star (M_⋆ = 1.05 ± 0.05 M_⊙, R_⋆ = 1.71 ± 0.04 R_⊙, T_eff = 5430 ± 85 K). The star is just leaving the main sequence. We combined K2 photometry with IRCS adaptive-optics imaging and HARPS, HARPS-N, and CARMENES high-precision radial velocity measurements to confirm the planetary system, determine the stellar parameters, and measure radii, masses, and densities of the three planets. With an orbital period of 3.5949_−0.0007^+0.0007 days, a mass of 8.75_−1.08^+1.09 M_⊕, and a radius of 1.95_−0.08^+0.09 R_⊕, the inner planet b is compatible with nickel-iron core and a silicate mantle (ρ_b = 6.39_−1.04^+1.19 g cm⁻³). Planets c and d with orbital periods of 15.624_−0.001^+0.001 and 35.747_−0.005^+0.005 days, respectively, have masses and radii of 14.67_−1.89^+1,84 M_⊕ and 3.67_−0.14^+0.17 R_⊕ and 10.18_−2.42^+2.46 M_⊕ and 3.94_−0.12^+0.13 R_⊕, respectively, yielding a mean density of 1.62_−0.29^+0.30 and 0.91_−0.23^+0.25 g cm⁻³, respectively. The radius of planet b lies in the transition region between rocky and gaseous planets, but its density is consistent with a rocky composition. Its semimajor axis and the corresponding photoevaporation levels to which the planet has been exposed might explain its measured density today. In contrast, the densities and semimajor axes of planets c and d suggest a very thick atmosphere. The singularity of this system, which orbits a slightly evolved star that is just leaving the main sequence, makes it a good candidate for a deeper study from a dynamical point of view.