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We present detections of methane in R of $\sim$1300, L band spectra of VHS1256 b and PSO 318.5, two low gravity, red, late L dwarfs that share the samecolors as the HR 8799 planets. These spectra reveal shallow methane features,which indicate VHS 1256 b and PSO 318.5 have photospheres that are out ofchemical equilibrium. Directly imaged exoplanets usually have redder nearinfrared colors than the field-age population of brown dwarfs on a colormagnitude diagram. These objects along the L to T transition show reducedmethane absorption and evidence of photospheric clouds. Compared to the H and Kbands, L band (3 micron - 4 micron) spectroscopy provides stronger constraintson the methane abundances of brown dwarfs and directly imaged exoplanets thathave similar effective temperatures as L to T transition objects. When combinedwith near infrared spectra, the L band extends our conventional wavelengthcoverage, increasing our understanding of atmospheric cloud structure. Ourmodel comparisons show relatively strong vertical mixing and photosphericclouds can explain the molecular absorption features and continua of VHS 1256 band PSO 318.5. We also discuss the implications of this work for futureexoplanet focused instruments and observations with the James Webb SpaceTelescope.

Methane in Analogs of Young Directly Imaged Exoplanets