TESS Discovers A Short-Period Saturn-Mass Planet with an Inner Companion

We report the discovery of the TOI-2000 system, with a 2.7 R_E planet in a 3.10 day orbit interior to a Saturn-mass planet in a 9.13 day orbit, and confirm the latter with RV measurements from CHIRON, FEROS, and HARPS. The TOI-2000 system hosts the smallest and fourth overall hot gas giant known to have a smaller inner companion to date. Because such inner companions are not expected to remain in the system after high-eccentricity migration of the giant planet, TOI-2000 provides further evidence that some hot gas giants may have formed through other mechanisms. We discovered the smaller TOI-2000 b during a systematic BLS search of all MIT Quick Look Pipeline (QLP) light curves for ~ 800 confirmed and candidate hot Jupiter host stars brighter than the 11th TESS magnitude in the TESS prime mission. Combining TESS, Kepler, and K2 hot Jupiters, we calculate a posterior distribution for the occurrence rate of inner companions to transiting hot Jupiters. To date, TOI-2000 joins TOI-1130 as the only systems that host a hot gas giant and a smaller inner planet discovered by TESS during its two-year prime mission, along with WASP-47 and Kepler 730 systems discovered by Kepler.