  Directly detecting thermal emission from young extrasolar planets allows
measurement of their atmospheric composition and luminosity, which is
influenced by their formation mechanism. Using the Gemini Planet Imager, we
discovered a planet orbiting the \$sim$20 Myr-old star 51 Eridani at a
projected separation of 13 astronomical units. Near-infrared observations show
a spectrum with strong methane and water vapor absorption. Modeling of the
spectra and photometry yields a luminosity of L/LS=1.6-4.0 x 10-6 and an
effective temperature of 600-750 K. For this age and luminosity, "hot-start"
formation models indicate a mass twice that of Jupiter. This planet also has a
sufficiently low luminosity to be consistent with the "cold- start" core
accretion process that may have formed Jupiter.
