Worlds Next Door. V. A Candidate Solar System Scale Super-Jupiter in the 61 Cygni Binary System

We present new constraints on a candidate third companion in the 61 Cygni binary system-one of the closest neighbors to our solar system (3.5 pc), the first stars to have a measured parallax, and two of the most favorable targets for habitable zone exo-Earth searches with future direct imaging missions. Kervella et al. (2022) first identified the candidate based on a 4.4 σ tangential velocity anomaly in the system determined using Hipparcos and Gaia DR3 astrometry of the A and B components. We combine this astrometric evidence with constraints from Gaia DR3 RUWE, MMT/Clio Lp imaging, nearly four decades of radial velocity (RV) measurements (including about 2 years of new NEID RVs presented here), and N-body dynamical stability calculations of both stars in a Monte Carlo framework to estimate the candidate companion's mass and semi-major axis. Our analysis reveals that all of the available data are compatible with an about(8 plus or minus 3);M Jup super-Jupiter in an about(8 plus or minus 3) au orbit, with a broad range of allowed eccentricities and mutual inclinations. Using the Sonora Flame Skimmer evolutionary models, given the system's mature age of (6.0 plus or minus 1.0) Gyr, we estimate a companion effective temperature of (282 plus or minus 55) K and radius of (1.02 plus or minus 0.02);R Jup. Finally, we evaluate the stability of terrestrial planets in the habitable zone (HZ) of 61 Cygni AB in the presence of the giant planet candidate using N-body simulations. A companion semi-major axis 5 au, irrespective of the planet-planet mutual inclination, and between about 5 au and 10 au, for misaligned orbits (40 o i mut 140 o), severely disrupts the HZ. Our results highlight the need to confirm the planet candidate and characterize its orbit to inform future exo-Earth searches with missions such as SHERA, HWO, and LIFE.