Transit Timing Variations in the LHS 1678 System and Validation of the Venus-Zone Planet LHS 1678 d

The nearby LHS 1678 (TOI-696) exoplanet system contains two confirmed planets and a likely-brown-dwarf companion, which orbit an M2 dwarf with an uncommon history as a Gaia/Jao Gap star. Here we validate and characterize a third planet in the LHS 1678 system near 4:3 mean motion resonance with LHS 1678 c and first identified as a candidate by Silverstein et al. 2022. LHS 1678 d is a 0.9±0.1 R_⊕ planet in a 4.97-day orbit, with an insolation flux of 9.1 S_⊕ that places it in the Venus zone, in company with LHS 1678 c. We find that LHS 1678 d also joins its siblings as another compelling candidate for atmospheric measurements with the JWST and mass measurements using high-precision radial velocity techniques. Using new ground-based data and the full suite of TESS Cycle 1 and 3 data, we further refine the system properties and search for transit timing variations in this compact, multi-planet system.