Observational analysis reveals a massive outer body on a 7.7-year orbit in the TOI-201 system, suggesting TTVs are crucial for understanding exoplanet dynamics.
We report the detection and characterization of TOI-201 c, a long-period transiting companion to the warm Jupiter TOI-201 b. Its presence was first inferred from high-amplitude transit timing variations (TTVs) in TOI-201 b, pointing to a massive outer body on a 7.7+1.0-0.6-year eccentric orbit. This prediction was confirmed when TESS observed a transit of TOI-201 c, precisely constraining its orbital geometry. A joint fit to TTVs, transit photometry, and archival radial velocities yields a mass of 14.2+1.0-1.2 MJup and an eccentricity of 0.643+0.009-0.021. The mutual inclination between planets b and c is 2.9+4.8-4.4 degrees, indicating a nearly coplanar architecture. Long-term numerical integrations confirm dynamical stability over gigayear timescales and predict that transits of TOI-201 b will cease within a few thousand years. TOI-201 c ranks among the longest-period transiting planets with well-constrained properties. Its detection via TTVs, followed by a confirmed transit, represents a rare observational sequence and highlights the power of TTVs and photometric monitoring to uncover distant companions. The TOI-201 system offers a valuable laboratory for testing models of giant planet formation, migration, and secular evolution in multi-planet systems.
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