The Second Discovery from the COCONUTS Program: A Cold Wide-orbit Exoplanet around a Young Field M Dwarf at 10.9 pc

We present the identification of the second discovery from the COol Companions ON Ultrawide orbiTS (COCONUTS) program, the COCONUTS-2 system, composed of the M3 dwarf L 34-26 and the T9 dwarf WISEPA J075108.79−763449.6. Given their common proper motions and parallaxes, these two field objects constitute a physically bound pair with a projected separation of 594'' (6471au). The primary star COCONUTS-2A has strong stellar activity (Hα, X-ray, and ultraviolet emission) and is rapidly rotating (Prot = 2.83 days), from which we estimate an age of 150–800 Myr. Comparing equatorial rotational velocity derived from the Transiting Exoplanet Survey Satellite (TESS) light curve to spectroscopic vsini, we find that COCONUTS-2A has a nearly edge-on inclination. The wide exoplanet COCONUTS-2b has an effective temperature of Teff = 434 ± 9 K, a surface gravity of log g=4.11-0.18 +0.11dex, and a mass of M = 63-1.9+1.5 MJup based on hot-start evolutionary models, leading to a mass ratio of 0.016-0.005+0.004 for the COCONUTS-2 system. COCONUTS-2b is the second coldest (after WD 0806−661B), and the second widest (after TYC 9486-927-1 b) exoplanet imaged to date. Comparison of COCONUTS-2b's infrared photometry with ultracool model atmospheres suggests the presence of both condensate clouds and non-equilibrium chemistry in its photosphere. Similar to 51 Eri b, COCONUTS-2b has a sufficiently low luminosity log(LboLodot)=-6.384± 0.028 dex) to be consistent with the cold-start process that may form gas-giant (exo)planets, though its large separation means that such formation would not have occurred in situ. Finally, at a distance of 10.9 pc, COCONUTS-2b is the nearest imaged exoplanet to Earth known to date.