TOI-618: A benchmark multi-planet system discovered using TESS photometry and long-term RV monitoring

The formation and evolution of short-period gas-giant exoplanets have been a persistent enigma for planetary science. Most theoretical frameworks describing short-period gas-giant formation can be broadly divided into two classes - disk migration and high-eccentricity migration - and each of these model paradigms imprints a characteristic (though not necessarily mutually exclusive) set of orbital and atmospheric properties onto the resultant planetary systems. Multi-planet systems consisting of a close-in transiting gas giant and a massive bound outer companion provide a particularly insightful perspective on the short-period gas-giants formation and evolution. The outer companion can have a direct bearing on the present-day orbital architecture, e.g., via Kozai-Lidov oscillations. Meanwhile, the presence of the short-period transiting gas giant makes these multi-planet systems exceptionally valuable for detailed atmospheric characterization, as the compositional information gleaned from spectroscopic observations of the inner planet can be contextualized alongside dynamical modeling of the entire system, thereby providing a direct probe of the interplay between past migration history and atmospheric properties that may be broadly applicable across the exoplanet population. To date, there are fewer than 30 such multi-gas-giant systems in which the orbits of all planets are well-characterized. We present the discovery of a new two-planet system - TOI-618. This system features an inner transiting sub-Saturn (0.20±0.02 M_Jup, 0.79±0.02 R_Jup) on a 7.76-day orbit around a moderately bright (V = 10.7 mag) F-type star that was initially uncovered using TESS photometry. Subsequent long-term radial velocity follow-up spanning more than 3 years revealed the presence of a massive outer companion (Msini = 6.8±0.7 M_Jup) with an orbital period of 1071±6 days. Notably, both planets are moderately eccentric (e ~ 0.15), indicating a dynamically excited orbital configuration that is consistent with the high-eccentricity migration scenario. We present a global fit of the transit light curves and RV datasets and discuss prospects for future atmospheric characterization and detailed dynamical studies.