Spectroscopic properties of ultra-cool dwarfs and brown dwarfs

In looking at the census of coolest objects now recognized, we see a menagerie of spectra falling in the gap between low-mass stars and high-mass planets. These spectra can be used not only to extract details about the physics of substellar atmospheres, but also to define observationally the continuum between stellar and planetary spectra. As we uncover more examples in this menagerie, we also find ourselves facing a new era in spectral classification. In this paper, the spectral evidence is used to highlight our current understanding of low-temperature atmospheres, and the possibilities of extending spectral classifications beyond the MK system are also addressed.