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

Abstract 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 ⊕ , mass m p = 16.4 − 4.1 + 4.1 M ⊕ , and density ρ p = 2.32 − 0.37 + 0.38 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 − 4.5 + 4.7 M ⊕ for TOI-2015 b and m c = 6.8 − 2.3 + 3.5 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.