The CARMENES search for exoplanets around M dwarfs

Two terrestrial planets orbiting G 264–012 and one terrestrial planet orbiting Gl 393^★

¹ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
e-mail: [email protected]
² Instituto de Astrofísica de Canarias, Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
³ Departamento de Astrofísica, Universidad de La Laguna, 38026 La Laguna, Tenerife, Spain
⁴ Centro de Astrobiología (CSIC-INTA), ESAC, camino bajo del castillo s/n, 28049 Villanueva de la Cañada, Madrid, Spain
⁵ Departamento de Física Teórica y del Cosmos, Universidad de Granada, 18071 Granada, Spain
⁶ Institut de Ciéncies de l’Espai (ICE, CSIC), Campus UAB, c/de Can Magrans s/n, 08193 Bellaterra, Barcelona, Spain
⁷ Institut d’Estudis Espacials de Catalunya (IEEC), C/Gran Capità 2-4, 08034 Barcelona, Spain
⁸ Astrobiology Research Unit, University of Liège, Allée du 6 août, 19, 4000 Liège (Sart-Tilman), Belgium
⁹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹⁰ Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
¹¹ Institut für Astrophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
¹² Université de Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹³ Departamento de Matemática y Física Aplicadas, Universidad Católica de la Santísima Concepción, Alonso de Rivera 2850, Concepción, Chile
¹⁴ Centro Astronómico Hispano-Alemán (CSIC-Junta de Andalucía), Observatorio Astronómico de Calar Alto, Sierra de los Filabres, 04550 Gérgal, Almería, Spain
¹⁵ Astronomical Observatory of the University of Geneva, 51 ch. Pegasi, 1290 Versoix, Switzerland
¹⁶ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
¹⁷ Facultad de Ciencias Físicas, Departamento de Física de la Tierra y Astrofísica & IPARCOS-UCM (Instituto de Física de Partículas y del Cosmos de la UCM), Universidad Complutense de Madrid, 28040 Madrid, Spain
¹⁸ Department of Physics, Ariel University, Ariel 40700, Israel
¹⁹ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
²⁰ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
²¹ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
²² Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
²³ Centro de Astrobiología (CSIC-INTA), Carretera de Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
²⁴ Max Planck Institute for Solar System Research, Justus-von-Liebig-weg 3, 37077 Göttingen, Germany
²⁵ Space sciences, Technologies and Astrophysics Research (STAR) Institute, Université de Liège, 19C Allée du 6 Août, 4000 Liège, Belgium

Received: 22 February 2021
Accepted: 18 May 2021

Abstract

We report the discovery of two planetary systems, namely G 264–012, an M 4.0 dwarf with two terrestrial planets (M_b sin i=2.50_−0.30^+0.29 M_⊕ and M_c sin i=3.75_−0.47^+0.48 M_⊕), and Gl 393, a bright M 2.0 dwarf with one terrestrial planet (M_b sini = 1.71 ± 0.24M_⊕). Although both stars were proposed to belong to young stellar kinematic groups, we estimate their ages to be older than about 700 Ma. The two planets around G 264–012 were discovered using only radial-velocity (RV) data from the CARMENES exoplanet survey, with estimated orbital periods of 2.30 d and 8.05 d, respectively.Photometric monitoring and analysis of activity indicators reveal a third signal present in the RV measurements, at about 100 d,caused by stellar rotation. The planet Gl 393 b was discovered in the RV data from the HARPS, CARMENES, and HIRES instruments. Its identification was only possible after modelling, with a Gaussian process (GP), the variability produced by the magnetic activity of the star. For the earliest observations, this variability produced a forest of peaks in the periodogram of the RVs at around the 34 d rotation period determined from Kepler data, which disappeared in the latestepochs. After correcting for them with this GP model, a significant signal showed at a period of 7.03 d. No significant signals in any of our spectral activity indicators or contemporaneous photometry were found at any of the planetary periods. Given the orbital and stellar properties, the equilibrium temperatures of the three planets are all higher than that for Earth. Current planet formation theories suggest that these two systems represent a common type of architecture. This is consistent with formation following the core accretion paradigm.