TOI-2015 b: A sub-Neptune in strong gravitational interaction with an outer non-transiting planet

¹ Astrobiology Research Unit, Université de Liège, Allée du 6 Août 19C, 4000 Liège, Belgium
² Department of Earth, Atmospheric and Planetary Science, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
³ Instituto de Astrofísica de Canarias (IAC), Calle Vía Láctea s/n, 38200, La Laguna, Tenerife, Spain
⁴ Lund Observatory, Division of Astrophysics, Department of Physics, Lund University, Box 118, 22100 Lund, Sweden
⁵ Department of Earth Sciences, University of Hawaii at Manoa, 1680 East-West Rd, Honolulu, HI 96822, USA
⁶ Institute for Astrophysics, University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
⁷ Department of Astronomy and Astrobiology Program, University of Washington, Box 351580, Seattle, Washington 98195, USA
⁸ NExSS Virtual Planetary Laboratory, Box 351580, University of Washington, Seattle, Washington 98195, USA
⁹ Departamento de Astrofísica, Universidad de La Laguna (ULL), E-38206 La Laguna, Tenerife, Spain
¹⁰ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
¹¹ INAF- Palermo Astronomical Observatory, Piazza del Parlamento, 1, 90134 Palermo, Italy
¹² Komaba Institute for Science, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan
¹³ Astrobiology Center, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹⁴ Research Institute for Advanced Computer Science, Universities Space Research Association, Washington, DC 20024, USA
¹⁵ Institute for Particle Physics and Astrophysics, ETH Zürich, OttoStern-Weg 5, 8093 Zürich, Switzerland
¹⁶ Department of Astrophysics, University of Zürich, Winterthurerstrasse 190 8057 Zürich, Switzerland
¹⁷ Department of Astronomy & Astrophysics, University of Chicago, Chicago, IL, USA
¹⁸ Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
¹⁹ Space Sciences, Technologies and Astrophysics Research (STAR) Institute, Université de Liège, Allée du 6 Août 19C, B-4000 Liège, Belgium
²⁰ Instituto de Alta Investigación, Universidad de Tarapacá, Casilla 7D, Arica, Chile
²¹ Gemini Observatory/NSF NOIRLab, 670 N. A’ohoku Place, Hilo, HI 96720, USA
²² Departamento de Ingeniería Topográfica y Cartografía, E.T.S.I. en Topografía, Geodesia y Cartografía, Universidad Politécnica de Madrid, 28031 Madrid, Spain
²³ Subaru Telescope, National Astronomical Observatory of Japan, 650 North A‘ohoku Place, Hilo, HI 96720, USA
²⁴ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
²⁵ Oukaimeden Observatory, High Energy Physics and Astrophysics Laboratory, Faculty of sciences Semlalia, Cadi Ayyad University, Marrakech, Morocco
²⁶ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
²⁷ Kavli Institute for Particle Astrophysics & Cosmology, Stanford University, Stanford, CA 94305, USA
²⁸ Department of Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
²⁹ Department of Astronomy & Space Sciences, Faculty of Science, Ankara University, 06100 Ankara, Türkiye
³⁰ Ankara University, Astronomy and Space Sciences Research and Application Center (Kreiken Observatory), Incek Blvd., 06837 Ahlatlıbel, Ankara, Türkiye
³¹ AIM, CEA, CNRS, Université Paris-Saclay, Université de Paris, 91191 Gif-sur-Yvette, France
³² School of Physics & Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
³³ Center for Astrophysics and Space Sciences, UC San Diego, UCSD Mail Code 0424, 9500 Gilman Drive, La Jolla, CA 92093-0424, USA
³⁴ American Association of Variable Star Observers, 185 Alewife Brook Parkway, Suite 410, Cambridge, MA 02138, USA
³⁵ George Mason University, 4400 University Drive, Fairfax, VA 22030, USA
³⁶ Department of Astronomy, University of California Berkeley, Berkeley, CA 94720, USA
³⁷ Department of Multi-Disciplinary Sciences, Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan
³⁸ Dpto. Física Teórica y del Cosmos, Universidad de Granada, 18071 Granada, Spain
³⁹ Center for Space and Habitability, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
⁴⁰ Center for Computational Astrophysics, Flatiron Institute, 162 Fifth Avenue, New York, NY 10010, USA
⁴¹ Universidad Nacional Autónoma de México, Instituto de Astronomía, AP 70-264, Ciudad de México 04510, Mexico
⁴² European Space Agency (ESA), European Space Research and Technology Centre (ESTEC), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
⁴³ SUPA Physics and Astronomy, University of St. Andrews, Fife, KY16 9SS Scotland, UK
⁴⁴ Cavendish Laboratory, JJ Thomson Avenue, Cambridge CB3 0HE, UK
⁴⁵ Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), Northwestern University, 1800 Sherman, Evanston, IL 60201, USA
⁴⁶ Okayama Observatory, Kyoto University, 3037-5 Honjo, Kamogatacho, Asakuchi, Okayama 719-0232, Japan
⁴⁷ NASA Ames Research Center, Moffett Field, CA 94035, USA
⁴⁸ Department of Physical Sciences, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan
⁴⁹ Department of Physics and Astronomy, University of Louisville, Louisville, KY 40292, USA
⁵⁰ Freie Universität Berlin, Institute of Geological Sciences, Malteserstr. 74-100, 12249 Berlin, Germany
⁵¹ Sabadell Astronomical Society, 08206 Sabadell, Barcelona, Spain
⁵² Europlanet Society, Department of Planetary Atmospheres of the Royal Belgian Institute for Space Aeronomy, B-1180 Brussels, Belgium
⁵³ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁵⁴ Astronomical Science Program, Graduate University for Advanced Studies, SOKENDAI, 2-21-1, Osawa, Mitaka, Tokyo, 181-8588, Japan
⁵⁵ Department of Physics, University of Rome “Tor Vergata”, Via della Ricerca Scientifica 1, 00133, Rome, Italy
⁵⁶ INAF – Astrophysical Observatory of Turin, via Osservatorio 20, 10025, Pino Torinese, Italy
⁵⁷ Max Planck Institute for Astronomy, Königstuhl 17, 69117, Heidelberg, Germany
⁵⁸ Villa ’39 Observatory, Landers, CA 92285, USA
⁵⁹ Wild Boar Remote Observatory, San Casciano in val di Pesa, Firenze 50026, Italy
⁶⁰ Institute for Particle Physics and Astrophysics, ETH Zürich, Wolfgang-Pauli-Strasse 2, 8093 Zürich, Switzeland
⁶¹ Department of Physics, Aristotle University of Thessaloniki, University Campus, Thessaloniki 54124, Greece
⁶² Department of Astronomy, University of Maryland, College Park, College Park, MD 20742, USA
⁶³ 7 Skies Observatory, Cypress County, Alberta, RASC (Royal Astronomical Society of Canada), Canada
⁶⁴ South African Astronomical Observatory, PO Box 9, Observatory, Cape Town 7935, South Africa
⁶⁵ Astrophysics Group, Keele University, Staffordshire ST5 5BG, UK
⁶⁶ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁶⁷ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁶⁸ Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
⁶⁹ Kotizarovci Observatory, Sarsoni 90, 51216 Viskovo, Croatia
⁷⁰ Institute of Astronomy and Astrophysics, Academia Sinica, PO Box 23-141, Taipei 10617, Taiwan, ROC
⁷¹ Department of Astrophysics, National Taiwan University, Taipei 10617, Taiwan, ROC
⁷² Department of Physics & Astronomy, Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA
⁷³ Department of Physics and Astronomy, Union College, 807 Union St., Schenectady, NY 12308, USA
⁷⁴ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
⁷⁵ Department of Physics, Engineering and Astronomy, Stephen F. Austin State University, 1936 North St, Nacogdoches, TX 75962, USA

^★ Corresponding author; khalid.barkaoui@uliege.be

Received: 7 November 2024
Accepted: 10 February 2025

Abstract

TOI-2015 is a known exoplanetary system around an M4 dwarf star, consisting of a transiting sub-Neptune planet in a 3.35-day orbital period, TOI-2015 b, accompanied by a non-transiting companion, TOI-2015 c. High-precision radial-velocity measurements were taken with the MAROON-X spectrograph, and high-precision photometric data were collected, primarily using the SPECULOOS, MUSCAT, TRAPPIST and LCOGT networks. We collected 63 transit light curves and 49 different transit epochs for TOI-2015 b. We recharacterized the target star by combining optical spectra obtained by the MAROON-X, Shane/KAST and IRTF/SpeX spectrographs, Bayesian model averaging (BMA) and spectral energy distribution (SED) analysis. The TOI-2015 host star is a K = 10.3 mag M4-type dwarf with a subsolar metallicity of [Fe/H] = −0.31 ± 0.16, and an effective temperature of T_eff ≈ 3200 K. Our photodynamical analysis of the system strongly favors the 5:3 mean-motion resonance and in this scenario the planet b (TOI-2015 b) has an orbital period of P_b = 3.34 days, a mass of M_p = 9.02_-0.36^+0.32M_⊕ , and a radius of R_p = 3.309_-0.011^+0.013R_⊕ , resulting in a density of ρ_p = 0.25 ± 0.01 ρ_⊕ = 1.40 ± 0.06 g cm⁻³; this is indicative of a Neptune-like composition. Its transits exhibit large (> 1 hr) timing variations characteristic of an outer perturber in the system. We performed a global analysis of the high-resolution radial-velocity measurements, the photometric data, and the TTVs, and inferred that TOI-2015 hosts a second planet, TOI-2015 c, in a non-transiting configuration. Our analysis places it near a 5:3 resonance with an orbital period of P_c = 5.583 days and a mass of M_p = 8.91_-0.40^+0.38M_⊕. The dynamical configuration of TOI-2015 b and TOI-2015 c can be used to constrain the system’s planetary formation and migration history. Based on the mass-radius composition models, TOI-2015 b is a water-rich or rocky planet with a hydrogen-helium envelope. Moreover, TOI-2015 b has a high transmission-spectroscopic metric (TSM=149), making it a favorable target for future transmission spectroscopic observations with the JWST to constrain the atmospheric composition of the planet. Such observations would also help to break the degeneracies in theoretical models of the planet’s interior structure.