TOI-2015 b: A Warm Neptune with Transit Timing Variations Orbiting an Active Mid-type M Dwarf

We report the discovery of a close-in (P_orb = 3.349 days) warm Neptune with clear transit timing variations (TTVs) orbiting the nearby (d = 47.3 pc) active M4 star, TOI-2015. We characterize the planet's properties using Transiting Exoplanet Survey Satellite (TESS) photometry, precise near-infrared radial velocities (RVs) with the Habitable-zone Planet Finder Spectrograph, ground-based photometry, and high-contrast imaging. A joint photometry and RV fit yields a radius R_p = 3.37^+0.15_-0.20 R_⊕, mass m_p = 16.4^+4.1_-4.1 M_⊕, and density p_p  = 2.32^+0.38_-0.37 g cm^-3 for TOI-2015 b, suggesting a likely volatile-rich planet. The young, active host star has a rotation period of P_rot = 8.7 ± 0.9 days and associated rotation-based age estimate of 1.1 ± 0.1 Gyr. Though no other transiting planets are seen in the TESS data, the system shows clear TTVs of super-period P_sup ≈ 430 days and amplitude ∼100 minutes. After considering multiple likely period-ratio models, we show an outer planet candidate near a 2:1 resonance can explain the observed TTVs while offering a dynamically stable solution. However, other possible two-planet solutions—including 3:2 and 4:3 resonances—cannot be conclusively excluded without further observations. Assuming a 2:1 resonance in the joint TTV-RV modeling suggests a mass of m_b = 13.3^+4.7_-4.5 M_⊕ for TOI-2015 b and m_c = 6.8^+3.5_-2.3 M_⊕ for the outer candidate. Additional transit and RV observations will be beneficial to explicitly identify the resonance and further characterize the properties of the system.