TOI-2015 b: A Warm Neptune with Transit Timing Variations Orbiting an Active Mid-type M Dwarf
Kobulnicky, Henry A.; Hawley, Suzanne L.; Cochran, William D.; Mahadevan, Suvrath; Wisniewski, John; Monson, Andrew; Cañas, Caleb I.; Kanodia, Shubham; Ninan, Joe P.; Hebb, Leslie; Bender, Chad F.; Diddams, Scott A.; Halverson, Samuel; Robertson, Paul; Roy, Arpita; Schwab, Christian; Terrien, Ryan C.; Libby-Roberts, Jessica E.; Lin, Andrea S. J.; Beard, Corey; Holcomb, Rae; Lubin, Jack; Fernandes, Rachel B.; Stefánsson, Guđmundur; Gupta, Arvind F.; Ramsey, Lawrence; Masuda, Kento; Jones, Sinclaire E.; Gardner, Cristilyn N.; Parker, Brock A.; Hearty, Fred R.; Kowalski, Adam
United States, Netherlands, Japan, India, Australia
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 , mass , and density 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 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 for TOI-2015 b and 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.