Validation of TOI-1221 b: A Warm Sub-Neptune Exhibiting Transit Timing Variations around a Sun-like Star

We present a validation of a long-period (91.68278 (+0.00032)(-0.00041) days) transiting sub-Neptune planet, TOI-1221 b (TIC 349095149.01), around a Sun-like (m(V)= 10.5) star. This is one of the few known exoplanets with a period >50 days, and belongs to the even smaller subset of which have bright enough hosts for detailed spectroscopic follow-up. We combine Transiting Exoplanet Survey Satellite light curves and ground-based time-series photometry from the Perth Exoplanet Survey Telescope (0.3m) and Las Cumbres Observatory global telescope network (1.0m) to analyze the transit signals and rule out nearby stars as potential false-positive sources. High-contrast imaging from the Southern Astrophysical Research Telescope and Gemini/Zorro rule out nearby stellar contaminants. Reconnaissance spectroscopy from CHIRON sets a planetary scale upper mass limit on the transiting object (1.1 and 3.5 M-Jup at 1 sigma and 3 sigma, respectively) and shows no sign of a spectroscopic binary companion. We determine a planetary radius of R-p 2.91 (+0.13)(-0.12) R-circle plus, placing it in the sub-Neptune regime. With a stellar insolation of S 6.06 (-0.77) (+0.85) S-circle plus, we calculate a moderate equilibrium temperature of T-eq= 440 K, assuming no albedo and perfect heat redistribution. We find a falsepositive probability from the TRICERATOPS tool of FPP= 0.0014 +/- 0.0003 as well as other qualitative and quantitative evidence to support the statistical validation of TOI-1221 b. We find significant evidence (>5 sigma) of oscillatory transit timing variations, likely indicative of an additional nontransiting planet.

Automated summary

We validated the existence of a long-period transiting sub-Neptune planet, TOI-1221 b (TIC 349095149.01), with a period of 91.68278 (+0.00032)(-0.00041) days, orbiting a Sun-like star. We used various observations and analysis methods to confirm its existence and rule out false-positive sources. Additionally, we found evidence of potential non-transiting planets through oscillatory transit timing variations.