Our spectroscopic method using laser trapping and heating has demonstrated that thermal emission from a metal oxide microsphere is enhanced at frequencies resonant with the whispering gallery modes of the spherical resonator. Only a mode series of a specific order effectively emits thermal photons, and spectral peaks shift from higher-order whispering gallery modes to fundamental whispering gallery modes as the size parameter decreases. These spectral profiles are analyzed with the Mie scattering theory and a semiclassical rate-equation model. The observed mode selectivity in thermal radiation is attributed to a matching between the rates of cavity damping and internal absorption. Excellent reproducibility of the observed spectral profiles leads to a precise determination of optical constants of extremely hot materials.