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A size comparison between the Sun, a young sub-brown dwarf, and Jupiter. As the sub-brown dwarf ages, it will gradually cool and shrink.

A sub-brown dwarf or planetary-mass brown dwarf is an astronomical object formed in the same manner as stars and brown dwarfs (i.e. through the collapse of a gas cloud) but that has a mass below the limiting mass for thermonuclear fusion of deuterium (about 13 Jupiter mass).

Sub-brown dwarf or free-floating planet

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 that originally formed around a star and were ejected from orbit, and on the other hand a sub-brown dwarf formed free-floating in a star cluster may get captured into orbit around a star. 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.

Lower mass limit

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 the paper.

List of possible sub-brown dwarfs

Orbiting one or more stars

There is no consensus whether these companions of stars should be considered sub-brown dwarfs or planets.

  • WD 0806-661 B
  • DT Virginis c
  • FW Tauri b
  • ROXs 42b b

Orbiting a brown dwarf

There is no consensus whether these companions of brown dwarfs should be considered sub-brown dwarfs or planets.

  • The 5–10MJ companion of 2MASS J04414489+2301513
  • 2M1207b


  • WISE 0855–0714 between 3-10 MJ about 7 light years from Earth.
  • S Ori 52
  • UGPS J072227.51-054031.2
  • Cha 110913-773444
  • CFBDSIR2149-0403
  • OTS 44
  • PSO J318.5-22

See also