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

Abstract

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$\sigma$ 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 $\sim$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 $\approx (8\pm 3)M_{\mathrm{Jup}}$ super-Jupiter in an $\approx (8\pm 3)$ 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 $(6.0\pm 1.0)$ Gyr, we estimate a companion effective temperature of $(282\pm 55)$ K and radius of $(1.02\pm 0.02)R_{\mathrm{Jup}}$. 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 $\lesssim$5 au, irrespective of the planet-planet mutual inclination, and between $\approx$5 au and 10 au, for misaligned orbits ($40^o\lesssim i_{\mathrm{mut}}\lesssim 140^o$), 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.

arXiv comment19 pages, 8 figures, 3 tables. Comments and feedback welcome!