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

We report the discovery of a close-in ($P_{\mathrm{o}\mathrm{r}\mathrm{b}}=3.349\mathrm{d}\mathrm{a}\mathrm{y}\mathrm{s}$) warm Neptune with clear transit timing variations (TTVs) orbiting the nearby ($d=47.3\mathrm{p}\mathrm{c}$) 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 ${\rho }_p=2.32_{-0.37}^{+0.38}\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$ for TOI-2015b, suggesting a likely volatile-rich planet. The young, active host star has a rotation period of $P_{\mathrm{r}\mathrm{o}\mathrm{t}}=8.7\pm 0.9\mathrm{d}\mathrm{a}\mathrm{y}\mathrm{s}$ and associated rotation-based age estimate of $1.1\pm 0.1\mathrm{G}\mathrm{y}\mathrm{r}$. Though no other transiting planets are seen in the TESS data, the system shows clear TTVs of super period $P_{\mathrm{s}\mathrm{u}\mathrm{p}}\approx 430\mathrm{d}\mathrm{a}\mathrm{y}\mathrm{s}$ and amplitude $\sim$$100\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{u}\mathrm{t}\mathrm{e}\mathrm{s}$. 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.