DIRECT IMAGING OF A COLD JOVIAN EXOPLANET IN ORBIT AROUND THE SUN-LIKE STAR GJ 504

Abstract

Several exoplanets have recently been imaged at wide separations of >10 AU\nfrom their parent stars. These span a limited range of ages (<50 Myr) and\natmospheric properties, with temperatures of 800--1800 K and very red colors (J\n- H > 0.5 mag), implying thick cloud covers. Furthermore, substantial model\nuncertainties exist at these young ages due to the unknown initial conditions\nat formation, which can lead to an order of magnitude of uncertainty in the\nmodeled planet mass. Here, we report the direct imaging discovery of a Jovian\nexoplanet around the Sun-like star GJ 504, detected as part of the SEEDS\nsurvey. The system is older than all other known directly-imaged planets; as a\nresult, its estimated mass remains in the planetary regime independent of\nuncertainties related to choices of initial conditions in the exoplanet\nmodeling. Using the most common exoplanet cooling model, and given the system\nage of 160 [+350, -60] Myr, GJ 504 b has an estimated mass of 4 [+4.5, -1.0]\nJupiter masses, among the lowest of directly imaged planets. Its projected\nseparation of 43.5 AU exceeds the typical outer boundary of ~30 AU predicted\nfor the core accretion mechanism. GJ 504 b is also significantly cooler (510\n[+30, -20] K) and has a bluer color (J-H = -0.23 mag) than previously imaged\nexoplanets, suggesting a largely cloud-free atmosphere accessible to\nspectroscopic characterization. Thus, it has the potential of providing novel\ninsights into the origins of giant planets, as well as their atmospheric\nproperties.\n