Validation of a Third Planet in the LHS 1678 System

The nearby LHS 1678 (TOI-696) system contains two confirmed planets and awide-orbit, likely-brown-dwarf companion, which orbit an M2 dwarf with a uniqueevolutionary history. The host star occupies a narrow "gap" in the HR diagramlower main sequence, associated with the M dwarf fully convective boundary andlong-term luminosity fluctuations. This system is one of only about a dozen Mdwarf multi-planet systems to date that hosts an ultra-short period planet(USP). Here we validate and characterize a third planet in the LHS 1678 systemusing TESS Cycle 1 and 3 data and a new ensemble of ground-based light curves.LHS 1678 d is a 0.98 +/-0.07 Earth radii planet in a 4.97-day orbit, with aninsolation flux of 9.1 +0.9/-0.8 Earth insolations. These properties place itnear 4:3 mean motion resonance with LHS 1678 c and in company with LHS 1678 cin the Venus zone. LHS 1678 c and d are also twins in size and predicted mass,making them a powerful duo for comparative exoplanet studies. LHS 1678 d joinsits siblings as another compelling candidate for atmospheric measurements withthe JWST and mass measurements using high-precision radial velocity techniques.Additionally, USP LHS 1678 b breaks the "peas-in-a-pod" trend in this system,although additional planets could fill in the "pod" beyond its orbit. LHS1678's unique combination of system properties and their relative rarity amongthe ubiquity of compact multi-planet systems around M dwarfs makes the system avaluable benchmark for testing theories of planet formation and evolution.