  Direct imaging allows for the detection and characterization of exoplanets
via their thermal emission. We report the discovery via imaging of a young
Jovian planet in a triple star system and characterize its atmospheric
properties through near-infrared spectroscopy. The semi-major axis of the
planet is closer relative to that of its hierarchical triple star system than
for any known exoplanet within a stellar binary or triple, making HD 131399
dynamically unlike any other known system. The location of HD 131399Ab on a
wide orbit in a triple system demonstrates that massive planets may be found on
long and possibly unstable orbits in multi-star systems. HD 131399Ab is one of
the lowest mass (4+/-1 MJup) and coldest (850+/-50 K) exoplanets to have been
directly imaged.
