  Most exoplanets detected by direct imaging so far have been characterized by
relatively hot (> ~1000 K) and cloudy atmospheres. A surprising feature in some
of their atmospheres has been a distinct lack of methane, possibly implying
non-equilibrium chemistry. Recently, we reported the discovery of a planetary
companion to the Sun-like star GJ 504 using Subaru/HiCIAO within the SEEDS
survey. The planet is substantially colder (<600 K) than previously imaged
planets, and has indications of fewer clouds, which implies that it represents
a new class of planetary atmospheres with expected similarities to late T-type
brown dwarfs in the same temperature range. If so, one might also expect the
presence of significant methane absorption, which is characteristic of such
objects. Here, we report the detection of deep methane absorption in the
atmosphere of GJ 504 b, using the Spectral Differential Imaging mode of HiCIAO
to distinguish the absorption feature around 1.6 um. We also report updated JHK
photometry based on new Ks-band data and a re-analysis of the existing data.
The results support the notion that GJ 504 b has atmospheric properties
distinct from other imaged exoplanets, and will become a useful reference
object for future planets in the same temperature range.
