TOI-2015 b: a sub-Neptune in strong gravitational interaction with an outer non-transiting planet
Authors: Barkaoui K. , Korth J. , Gaidos E. , Agol E. , Parviainen H. , Pozuelos F.J. , Palle E. , Narita N. , Grimm S. , Brady M. , Bean J.L. , Morello G. , Rackham B.V. , Burgasser A.J. , Van Grootel V. , Rojas-Ayala B. , Seifahrt A. , Marfil E. , Passegger V.M. , Stalport M. , Gillon M. , Collins K.A. , Shporer A. , Giacalone S. , Yalçinkaya S. , Ducrot E. , Timmermans M. , Triaud A.H.M.J. , de Wit J. , Soubkiou A. , Watkins C.N. , Aganze C. , Alonso R. , Amado P.J. , Basant R. , Başturk O. , Benkhaldoun Z. , Burdanov A. , Calatayud-Borras Y. , Chouqar J. , Conti D.M. , Collins K.I. , Davoudi F. , Delrez L. , Dressing C.D. , de Leon J. , Devora-Pajares M. , Demory B.O. , Dransfield G. , Esparza-Borges E. , Fern'andez-Rodriguez G. , Fukuda I. , Fukui A. , Gallardo P.P.M. , Garcia L. , Garcia N.A. , Ghachoui M. , Geraldia-González S. , Gomez Maqueo Chew Y. , Gonzalez-Rodriguez J. , Gunther M.N. , Hayashi Y. , Horne K. , Hooton M.J. , Hsu C.C. , Ikuta K. , Isogai K. , Jehin E. , Jenkins J.M. , Kawauchi K. , Kagetani T. , Kawai Y. , Kasper D. , Kielkopf J.F. , Klagyivik P. , Lacedelli G. , Latham D.W. , Libotte F. , Luque R. , Livingston J.H. , Mancini L. , Massey B. , Mori M. , Muñoz Torres S. , Murgas F. , Niraula P. , Orell-Miquel J. , Rapetti David , Rebolo-Lopez R. , Ricker G. , Papini R. , Pedersen P.P. , Peláez-Torres A. , Pérez-Prieto J.A. , Poultourtzidis E. , Rodriguez P.M. , Queloz D. , Savel A.B. , Schanche N. , Sanchez-Benavente M. , Sibbald L. , Sefako R. , Sohy S. , Sota A. , Schwarz R.P. , Seager S. , Sebastian D. , Southworth J. , Stangret M. , Stefansson G. , Sturmer J. , Srdoc G. , Thompson S.J. , Terada Y. , Vanderspek R. , Wang G. , Watanabe N. , Wilkin F.P. , Winn J. , Wells R.D. , Ziegler C. , Zuniga-Fernandez S. 
Published: February 2025

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 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 re-characterized the target star by combining optical spectra obtained by the 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_⊕, and a radius of R_⊕, 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 = M_⊕. 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.