We report the discovery of a close-in (Porb=3.349days) warm Neptune with clear transit timing variations (TTVs)
orbiting the nearby (d=47.3pc) 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 Rp=3.37−0.20+0.15R⊕, mass mp=16.4−4.1+4.1M⊕, and
density ρp=2.32−0.37+0.38gcm−3 for TOI-2015b,
suggesting a likely volatile-rich planet. The young, active host star has a
rotation period of Prot=8.7±0.9days and
associated rotation-based age estimate of 1.1±0.1Gyr. Though
no other transiting planets are seen in the TESS data, the system shows clear
TTVs of super period Psup≈430days and
amplitude ∼100minutes. 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
mb=13.3−4.5+4.7M⊕ for TOI-2015b and
mc=6.8−2.3+3.5M⊕ 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.Comment: 28 pages, 15 figures, 6 tables. As submitted to AAS Journal