A sub-brown dwarf is an astronomical object of planetary mass that is not orbiting a star and is not considered to be a brown dwarf

Brown dwarf

Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...

 because its mass is below the limiting mass for thermonuclear fusion of deuterium (about 13 Jupiter masses

Jupiter mass

Jupiter mass , is the unit of mass equal to the total mass of the planet Jupiter . Jupiter mass is used to describe masses of the gas giants, such as the outer planets and extrasolar planets. It is also used in describing brown dwarfs....

).
Sub-brown dwarfs are formed in the manner of star

Star

A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s, through the collapse of a gas cloud

Nebula

A nebula is an interstellar cloud of dust, hydrogen gas, helium gas and other ionized gases...

 (perhaps with the help of photo-erosion

Photo-erosion

Photo-erosion is the dispersion of the outer layers of a prestellar core by ionizing radiation of a nearby star, thus preventing these layers from accreting onto the protostar at the centre of the core, and thereby reducing the mass of the developing object so instead of forming a star, it may form...

), and not through accretion or core collapse from a circumstellar disc

Protoplanetary disk

A protoplanetary disk is a rotating circumstellar disk of dense gas surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star...

.