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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 aK= 10.3 mag M4-type dwarf with a subsolar metallicity of [Fe/H] = −0.31 ± 0.16, and an effective temperature ofT_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 ofP_b= 3.34 days, a mass ofM_p= 9.02_-0.36^+0.32M_⊕, and a radius ofR_p= 3.309_-0.011^+0.013R_⊕, resulting in a density ofρ_p= 0.25 ± 0.01ρ_⊕= 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 ofP_c= 5.583 days and a mass ofM_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 theJWSTto 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. 

Planets with radii between that of the Earth and Neptune (hereafter referred to as `sub-Neptunes') are found in close-in orbits around more than half of all Sun-like stars1,2. However, their composition, formation and evolution remain poorly understood3. The study of multiplanetary systems offers an opportunity to investigate the outcomes of planet formation and evolution while controlling for initial conditions and environment. Those in resonance (with their orbital periods related by a ratio of small integers) are particularly valuable because they imply a system architecture practically unchanged since its birth. Here we present the observations of six transiting planets around the bright nearby star HD 110067. We find that the planets follow a chain of resonant orbits. A dynamical study of the innermost planet triplet allowed the prediction and later confirmation of the orbits of the rest of the planets in the system. The six planets are found to be sub-Neptunes with radii ranging from 1.94R⊕ to 2.85R⊕. Three of the planets have measured masses, yielding low bulk densities that suggest the presence of large hydrogen-dominated atmospheres.