We present near-infrared long slit and multi-slit spectra of low mass brown dwarf candidates in the Orion Nebula Cluster .
The long slit data were observed in the H- & K- bands using NIRI on the Gemini North Telescope .
The multi-object spectroscopic observations were made using IRIS2 on the Anglo Australian Telescope at H- band .
We develop a spectral typing scheme based on optically calibrated , near infrared spectra of young sources in the Taurus and IC 348 star forming regions with spectral types M3.0 to M9.5 .
We apply our spectral typing scheme to 52 sources , including previously published UKIRT and GNIRS spectra .
40 objects show strong water absorption with spectral types of M3 to > M9.5 .
The latest type objects are provisionally classified as early L types .
We plot our sources on H-R diagrams overlaid with theoretical pre-main-sequence isochrones .
The majority of our objects lie close to or above the 1 Myr isochrone , leading to an average cluster age that is < 1 Myr .
We find 38 sources lie at or below the hydrogen burning limit ( 0.075 M _ { \odot } ) .
10 sources potentially have masses below the deuterium burning limit ( 0.012 M _ { \odot } ) .
We use a Monte Carlo approach to model the observed luminosity function with a variety of cluster age and mass distributions .
The lowest \chi ^ { 2 } values are produced by an age distribution centred at 1 Myr , with a mass function that declines at sub-stellar masses according to an M ^ { \alpha } power law in the range \alpha =0.3 to 0.6 .
We find that truncating the mass function at 0.012 M _ { \odot } produces luminosity functions that are starved of the faintest magnitudes , even when using bimodal age populations that contain 10 Myr old sources .
The results of these Monte Carlo simulations therefore support the existence of a planetary mass population in the ONC .