  This review focuses on numerical approaches to deducing the light-scattering
and thermal-emission properties of primitive dust particles in planetary
systems from astronomical observations. The particles are agglomerates of small
grains with sizes comparable to visible wavelength and compositions being
mainly magnesium-rich silicates, iron-bearing metals, and organic refractory
materials in pristine phases. These unique characteristics of primitive dust
particles reflect their formation and evolution around main-sequence stars of
essentially solar composition. The development of light-scattering theories has
been offering powerful tools to make a thorough investigation of light
scattering and thermal emission by primitive dust agglomerates in such a
circumstellar environment. In particular, the discrete dipole approximation,
the T-matrix method, and effective medium approximations are the most popular
techniques for practical use in astronomy. Numerical simulations of light
scattering and thermal emission by dust agglomerates of submicrometer-sized
constituent grains have a great potential to provide new state-of-the-art
knowledge of primitive dust particles in planetary systems. What is essential
to this end is to combine the simulations with comprehensive collections of
relevant results from not only astronomical observations, but also in-situ data
analyses, laboratory sample analyses, laboratory analogue experiments, and
theoretical studies on the origin and evolution of the particles.
