We report the discovery of a close-in ($P_{ \mathrm{ orb}} = 3.349\:\mathrm{ days}$) warm Neptune with clear transit timing variations (TTVs) orbiting the nearby ($d=47.3\:\mathrm{ pc}$) active M4 star, TOI-2015. We characterize the planet's properties using TESS photometry, precise near-infrared radial velocities (RV) with the Habitable-zone Planet Finder (HP) Spectrograph, ground-based photometry, and high-contrast imaging. A joint photometry and RV fit yields a radius $R_p~=~3.37_{ -0.20}^{ +0.15} \:\mathrm{ R_\oplus}$, mass $m_p~=~16.4_{ -4.1}^{ +4.1}\:\mathrm{ M_\oplus}$, and density $\rho_p~=~2.32_{ -0.37}^{ +0.38} \:\mathrm{ g cm^{ -3}}$ for TOI-2015b, suggesting a likely volatile-rich planet. The young, active host star has a rotation period of $P_{ \mathrm{ rot}}~=~8.7 \pm~0.9~\mathrm{ days}$ and associated rotation-based age estimate of $1.1~\pm~0.1\:\mathrm{ Gyr}$. Though no other transiting planets are seen in the TESS data, the system shows clear TTVs of super period $P_{ \mathrm{ sup}}~\approx~430\:\mathrm{ days}$ and amplitude $\sim$$100\:\mathrm{ 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 resonance -- cannot be conclusively excluded without further observations. Assuming a 2:1 resonance in the joint TTV-RV modeling suggests a mass of $m_b~=~13.3_{ -4.5}^{ +4.7}\:\mathrm{ M_\oplus}$ for TOI-2015b and $m_c~=~6.8_{ -2.3}^{ +3.5}\:\mathrm{ M_\oplus}$ 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.
Categories: astro-ph.EP