A sub-brown dwarf or planetary-mass brown dwarf is an astronomical object that formed in the same manner as stars and brown dwarfs (i.e. through the collapse of a gas cloud) but that has a planetary mass, therefore by definition below the limiting mass for thermonuclear fusion of deuterium (about 13 MJ). Some researchers call them rogue planets whereas others call them planetary-mass brown dwarfs. They are sometimes categorized as Y spectral class brown dwarfs.
Sub-brown dwarfs are formed in the manner of stars, through the collapse of a gas cloud (perhaps with the help of photo-erosion) but there is no consensus amongst astronomers on whether the formation process should be taken into account when classifying an object as a planet. Free-floating sub-brown dwarfs can be observationally indistinguishable from rogue planets, which originally formed around a star and were ejected from orbit. Similarly, a sub-brown dwarf formed free-floating in a star cluster may be captured into orbit around a star, making distinguishing sub-brown dwarfs and large planets also difficult. A definition for the term "sub-brown dwarf" was put forward by the IAU Working Group on Extra-Solar Planets (WGESP), which defined it as a free-floating body found in young star clusters below the lower mass cut-off of brown dwarfs.
The smallest mass of gas cloud that could collapse to form a sub-brown dwarf is about 1 Jupiter mass (MJ). This is because to collapse by gravitational contraction requires radiating away energy as heat and this is limited by the opacity of the gas. A 3 MJ candidate is described in a 2007 paper.
There is no consensus whether these companions of stars should be considered sub-brown dwarfs or planets.
There is no consensus whether these companions of brown dwarfs should be considered sub-brown dwarfs or planets.
Also called rogue planets: