Silicon microspheres with a diameter in the range of 2–3 micrometers constitute photonic nanocavities that emit light through their Mie resonances when heated at high temperatures. At 500–600 °C these microresonators show a particular mid-infrared (MIR) emission dominated by the lowest order modes. Such resonances feature a large free spectral range, about 600 cm−1, and a high proximity to the critical coupling condition. In fact, resonances with high-quality factor, around 160 are found. It corresponds to the limit of detection of their measuring setup, being 600 the theoretical value. Most importantly, several modes emit light above the calculated black body limit because they feature an optical absorption cross-section larger than their geometric one. All these characteristics set silicon microspheres as very promising zero-dimensional materials for developing micrometric and sub-wavelength light sources in the MIR.