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 ${R}_{p}={3.37}_{-0.20}^{+0.15}\ {R}_{\oplus }$ , mass ${m}_{p}={16.4}_{-4.1}^{+4.1}\ {M}_{\oplus }$ , and density ${\rho }_{p}\,={2.32}_{-0.37}^{+0.38}\ {\rm{g}}\ {\mathrm{cm}}^{-3}$ 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 ${P}_{\sup }\approx 430\ \mathrm{days}$ 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 ${m}_{b}={13.3}_{-4.5}^{+4.7}\ {M}_{\oplus }$ for TOI-2015 b and ${m}_{c}={6.8}_{-2.3}^{+3.5}\ {M}_{\oplus }$ 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.