  A chiral absorber of light can emit spin-polarized (circularly polarized)
thermal radiation based on Kirchhoff's law which equates spin-resolved
emissivity with spin-resolved absorptivity for reciprocal media at thermal
equilibrium. No such law is known for nonreciprocal media. In this work, we
discover three spin-resolved Kirchhoff's laws of thermal radiation applicable
for both reciprocal and nonreciprocal planar media. In particular, these laws
are applicable to multi-layered or composite slabs of generic bianisotropic
material classes which include (uniaxial or biaxial) birefringent crystals,
(gyrotropic) Weyl semimetals, magnetized semiconductors, plasmas, ferromagnets
and ferrites, (magnetoelectric) topological insulators, metamaterials and
multiferroic media. We also propose an experiment to verify these laws using a
single system of doped Indium Antimonide (InSb) thin film in an external
magnetic field. Furthermore, we reveal a surprising result that the planar
slabs of all these material classes can emit partially circularly polarized
thermal light without requiring any surface patterning, and identify planar
configurations which can experience nontrivial thermal optomechanical forces
and torques upon thermal emission into the external environment at lower
temperature (nonequilibrium). Our work also provides a new fundamental insight
of detailed balance of angular momentum (in addition to energy) of equilibrium
thermal radiation, and paves the way for practical functionalities based on
thermal radiation using nonreciprocal bianisotropic materials.
