Sub-brown dwarfs or brown sub-dwarfs (also, less commonly, grey dwarf or gray dwarf) are cold masses smaller than the low-mass cut-off for brown dwarfs. These generally are referred to as planets.
However, a sub-brown dwarf is formed in the manner of stars, through the collapse of a gas cloud, and not through accretion or core collapse from a circumstellar disc. The distinction between a sub-brown dwarf and a planet is unclear; astronomers are divided into two camps as whether to consider the formation process of a planet as part of its division in classification. 
An alternate definition involves the same mass range (less than a brown dwarf, but in the planetary range), but is free of gravitational attachment with any star. These are generally referred to as free-floating planets.
List of suspected sub-brown dwarfs
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