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As gas giant planets and brown dwarfs radiate away the residual heat fromtheir formation, they cool through a spectral type transition from L to T,which encompasses the dissipation of cloud opacity and the appearance of strongmethane absorption. While there are hundreds of known T-type brown dwarfs, thefirst generation of directly-imaged exoplanets were all L-type. Recently,Kuzuhara et al. (2013) announced the discovery of GJ 504 b, the first T dwarfexoplanet. GJ 504 b provides a unique opportunity to study the atmosphere of anew type of exoplanet with a ~500 K temperature that bridges the gap betweenthe first directly imaged planets (~1000 K) and our own Solar System's Jupiter(~130 K). We observed GJ 504 b in three narrow L-band filters (3.71, 3.88, and4.00 microns), spanning the red end of the broad methane fundamental absorptionfeature (3.3 microns) as part of the LEECH exoplanet imaging survey. Bycomparing our new photometry and literature photometry to a grid of custommodel atmospheres, we were able to fit GJ 504 b's unusual spectral energydistribution for the first time. We find that GJ 504 b is well-fit by modelswith the following parameters: T_eff=544+/-10 K, g<600 m/s^2,[M/H]=0.60+/-0.12, cloud opacity parameter of f_sed=2-5, R=0.96+/-0.07 R_Jup,and log(L)=-6.13+/-0.03 L_Sun, implying a hot start mass of 3-30 M_jup for aconservative age range of 0.1-6.5 Gyr. Of particular interest, our model fitssuggest that GJ 504 b has a super-stellar metallicity. Since planet formationcan create objects with non-stellar metallicities, while binary star formationcannot, this result suggests that GJ 504 b formed like a planet, not like abinary companion.

The LEECH Exoplanet Imaging Survey: Characterization of the Coldest Directly Imaged Exoplanet, GJ 504 b, and Evidence for Super-Stellar Metallicity