GJ 806 (TOI-4481): A bright nearby multi-planetary system with a transiting hot low-density super-Earth

¹ Instituto de Astrofísica de Canarias (IAC), 38200 La Laguna, Tenerife, Spain
e-mail: epalle@iac.es
² Deptartamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
³ Department of Astronomy and Astrophysics, University of Chicago, Chicago, IL 60637, USA
⁴ Thüringer Landessternwarte Tautenburg, 07778 Tautenburg, Germany
⁵ Institut de Ciències de l’Espai (CSIC), Campus UAB, c/ de Can Magrans s/n, 08193 Bellaterra, Barcelona, Spain
⁶ Institut d'Estudis Espacials de Catalunya, 08034 Barcelona, Spain
⁷ Universitäts-Sternwarte, Ludwig-Maximilians-Universität München, Scheinerstrasse 1, 81679 München, Germany
⁸ Exzellenzcluster Origins, Boltzmannstraße 2, 85748 Garching, Germany
⁹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹⁰ Centro de Astrobiología (CAB, CSIC-INTA), Depto. de Astrofísica, ESAC campus, 28692, Villanueva de la Cañada (Madrid), Spain
¹¹ Department of Physics, Ariel University, Israel
¹² Department of Astronomy/Steward Observatory, The University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
¹³ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
¹⁴ Institut für Astrophysik und Geophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
¹⁵ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
¹⁶ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
¹⁷ SETI Institute/NASA Ames Research Center, 339 Bernardo Ave, Suite 200 Mountain View, CA 94043, USA
¹⁸ Center for Astrophysics – Harvard Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
¹⁹ Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
²⁰ Vereniging Voor Sterrenkunde (VVS), Oostmeers 122 C, 8000 Brugge, Belgium
²¹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
²² Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
²³ Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
²⁴ Komaba Institute for Science, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan
²⁵ Universidad Nacional Autónoma de México, Instituto de Astronomía, AP 70-264, CDMX 04510, Mexico
²⁶ Universidad Nacional Autónoma de México, Instituto de Astronomía, AP 106, Ensenada 22800, BC, Mexico
²⁷ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
²⁸ Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
²⁹ NASA Ames Research Center, Moffett Field, CA 94035, USA
³⁰ Zentrum fur Astronomie der Universitat Heidelberg, Landessternwarte Konigstuhl 12, 69117 Heidelberg, Germany
³¹ Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
³² Departamento de Física de la Tierra y Astrofísica & IPARCOSUCM (Instituto de Física de Partículas y del Cosmos de la UCM), Facultad de Ciencias Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
³³ Astrobiology Center, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
³⁴ Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
³⁵ Center for Space and Habitability, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
³⁶ Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ 08540, USA
³⁷ Centre for Mathematical Plasma-Astrophysics, Department of Mathematics, KU Leuven, Celestijnenlaan 200B, 3001 Heverlee, Belgium
³⁸ Belgium ASTROLAB IRIS, Provinciaal Domein “De Palingbeek”, Verbrandemolenstraat 5, 8902 Zillebeke, Ieper, Belgium

Received: 14 June 2022
Accepted: 21 April 2023

Abstract

One of the main scientific goals of the TESS mission is the discovery of transiting small planets around the closest and brightest stars in the sky. Here, using data from the CARMENES, MAROON-X, and HIRES spectrographs together with TESS, we report the discovery and mass determination of aplanetary system around the M1.5 V star GJ 806 (TOI-4481). GJ 806 is a bright (V ≈ 10.8mag, J ≈ 7.3 mag) and nearby (d = 12 pc) M dwarf that hosts at least two planets. The innermost planet, GJ 806 b, is transiting and has an ultra-short orbital period of 0.93 d, a radius of 1.331 ± 0.023 R_⊕, a mass of 1.90 ± 0.17 M_⊕, a mean density of 4.40 ± 0.45 g cm⁻³, and an equilibrium temperature of 940 ± 10 K. We detect a second, non-transiting, super-Earth planet in the system, GJ 806 c, with an orbital period of 6.6 d, a minimum mass of 5.80 ± 0.30 M_⊕, and an equilibrium temperature of 490 ± 5 K. The radial velocity data also shows evidence for a third periodicity at 13.6 d, although the current dataset does not provide sufficient evidence to unambiguously distinguish between a third super-Earth mass (M sin i = 8.50 ± 0.45 M_⊕) planet or stellar activity. Additionally, we report one transit observation of GJ 806 b taken with CARMENES in search of a possible extended atmosphere of H or He, but we can only place upper limits to its existence. This is not surprising as our evolutionary models support the idea that any possible primordial H/He atmosphere that GJ 806 b might have had would be long lost. However, the bulk density of GJ 806 b makes it likely that the planet hosts some type of volatile atmosphere. With transmission spectroscopy metrics (TSM) of 44 and emission spectroscopy metrics (ESM) of 24, GJ 806 b is to date the third-ranked terrestrial planet around an M dwarf suitable for transmission spectroscopy studies using JWST, and the most promising terrestrial planet for emission spectroscopy studies. GJ 806b is also an excellent target for the detection of radio emission via star-planet interactions.