TOI-663: A newly discovered multi-planet system with three transiting mini-Neptunes orbiting an early M star

¹ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
e-mail: marion.cointepas@unige.ch
² Observatoire de Genève, Département d’Astronomie, Université de Genève, Chemin Pegasi 51b, 1290 Versoix, Switzerland
³ Departamento de Matemática y Física Aplicadas, Universidad Católica de la Santísima Concepción, Alonso de Rivera 2850, Concepción, Chile
⁴ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
⁵ George Mason University, 4400 University Drive, Fairfax, VA, 22030 USA
⁶ Department of Astronomy and Tsinghua Centre for Astrophysics, Tsinghua University, Beijing 100084, PR China
⁷ Institute for Computational Science, University of Zurich, Winterthurerstr. 190, 8057 Zurich, Switzerland
⁸ Institute for Particle Physics and Astrophysics, ETH Zürich, Wolfgang-Pauli-Strasse 27, 8093 Zurich, Switzerland
⁹ Departamento de Física, Universidade Federal do Rio Grande do Norte, 59072-970 Natal, RN, Brazil
¹⁰ Instituto de Astrofísica de Canarias (IAC), 38200 La Laguna, Tenerife, Spain
¹¹ Dept. Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
¹² Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
¹³ Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
¹⁴ Department of Aeronautics and Astronautics, MIT, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
¹⁵ NASA Sagan Fellow; Department of Astrophysical Sciences, Princeton University, NJ 08544, USA
¹⁶ NASA Ames Research Center, Moffett Field, CA 94035, USA
¹⁷ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove drive, Pasadena, CA 91109, USA
¹⁸ U.S. Naval Observatory, 3450 Massachusetts Avenue NW, Washington, DC 20392, USA
¹⁹ Villa ’39 Observatory, Landers, CA 92285, USA
²⁰ Department of Physics and Astronomy, Union College, 807 Union St., Schenectady, NY 12308, USA
²¹ SUPA Physics and Astronomy, University of St. Andrews, Fife, KY16 9SS Scotland, UK
²² Space sciences, Technologies and Astrophysics Research (STAR) Institute, University of Liège, Allée du 6 Août 19C, 4000 Liège, Belgium
²³ Perth Exoplanet Survey Telescope, Perth, Western Australia, Australia
²⁴ Komaba Institute for Science, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan
²⁵ Okayama Observatory, Kyoto University, 3037-5 Honjo, Kamogatacho, Asakuchi, Okayama 719-0232, Japan
²⁶ Department of Multi-Disciplinary Sciences, Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo, Japan
²⁷ Department of Physical Sciences, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan
²⁸ Astrobiology Center, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
²⁹ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
³⁰ Department of Astronomical Science, The Graduated University for Advanced Studies, SOKENDAI, 2-21-1, Osawa, Mitaka, Tokyo, 181-8588, Japan
³¹ Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
³² Department of Astronomy and Astrophysics, The University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA
³³ Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
³⁴ Center for Space and Habitability, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
³⁵ Universidad Nacional Autónoma de Mexico, Instituto de Astronomía, AP 70-264, CDMX 04510, Mexico
³⁶ Universidad Nacional Autónoma de Mexico, Instituto de Astronomía, AP 106, Ensenada 22800, BC, Mexico
³⁷ SETI Institute, Mountain View, CA 94043, USA
³⁸ Department of Astronomy, University of Maryland, College Park, MD 20742, USA

Received: 14 May 2023
Accepted: 2 November 2023

Abstract

We present the detection of three exoplanets orbiting the early M dwarf TOI-663 (TIC 54962195; V = 13.7 mag, J = 10.4 mag, R_★ = 0.512 ± 0.015 R_⊙, M_★ = 0.514 ± 0.012 M_⊙, d = 64 pc). TOI-663 b, c, and d, with respective radii of 2.27 ± 0.10 R_⊕, 2.26 ± 0.10 R_⊕, and 1.92 ± 0.13 R_⊕ and masses of 4.45 ± 0.65 M_⊕, 3.65 ± 0.97 M_⊕, and <5.2 M_⊕ at 99%, are located just above the radius valley that separates rocky and volatile-rich exoplanets. The planet candidates are identified in two TESS sectors and are validated with ground-based photometric follow-up, precise radial-velocity measurements, and high-resolution imaging. We used the software package juliet to jointly model the photometric and radial-velocity datasets, with Gaussian processes applied to correct for systematics. The three planets discovered in the TOI-663 system are low-mass mini-Neptunes with radii significantly larger than those of rocky analogs, implying that volatiles, such as water, must predominate. In addition to this internal structure analysis, we also performed a dynamical analysis that confirmed the stability of the system. The three exoplanets in the TOI-663 system, similarly to other sub-Neptunes orbiting M dwarfs, have been found to have lower densities than planets of similar sizes orbiting stars of different spectral types.