A class of object, long predicted by astrophysicists,
sits in the mass range between the giant gaseous planets
and the M dwarf stars.  These objects are called brown
dwarfs.  They are massive enough to burn deuterium, but they
are too light to burn hydrogen.  The first brown dwarf was
observed orbiting an M dwarf star in 1988, and since that
time, hundreds of additional brown dwarfs have been found.
They are cool, so they are primarily emitters of infrared
radiation.  In the early stages of their lives, they are
powered by deuterium fusion and gravitational potential
energy, but when they consume their deuterium, and when
the electron degeneracy pressure stops their shrinkage,
they grow cold and dark.