CH₄ index (F1/F3) as a function of the spectral type (SpT). Values higher than 1 mean that CH₄ is present in the atmosphere. The index is computed for field dwarfs from the SpeX Prism library (gray squares), references field dwarfs are plotted with larger squares, the peculiar red dwarfs from Stephens et al. (2009; open circles), young free-floating planetary-mass objects (triangles), and young companions (alternative symbols). The typical uncertainty on the index is 10%.