Comparison: the Sun, a young sub-brown dwarf, and Jupiter. As the sub-brown dwarf ages, it will gradually cool and shrink.
Comparison: 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 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).[1] Some researchers call them rogue planets[2] whereas others call them planetary-mass brown dwarfs.[3] 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.[4] 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.[5]

Lower mass limit

The smallest mass of gas cloud that could collapse to form a sub-brown dwarf is about 1 Jupiter mass (MJ).[6] This is because to collapse by gravitational contraction requires radiating away energy as heat and this is limited by the opacity of the gas.[7] A 3 MJ candidate is described in a 2007 paper.[8]

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.

Orbiting a brown dwarf

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


Also called rogue planets:

See also


  1. ^ Working Group on Extrasolar Planets – Definition of a "Planet" Archived 16 September 2006 at the Wayback Machine POSITION STATEMENT ON THE DEFINITION OF A "PLANET" (IAU)
  2. ^ Delorme, P.; et al. (December 2012). "CFBDSIR2149-0403: a 4–7 Jupiter-mass rogue planet in the young moving group AB Doradus ?". Astronomy & Astrophysics. 548: A26. arXiv:1210.0305. Bibcode:2012A&A...548A..26D. doi:10.1051/0004-6361/201219984. S2CID 50935950.
  3. ^ Luhman, K. L. (21 April 2014). "Discovery of a ~250 K Brown Dwarf at 2 pc from the Sun". The Astrophysical Journal Letters. 786 (2): L18. arXiv:1404.6501. Bibcode:2014ApJ...786L..18L. doi:10.1088/2041-8205/786/2/L18. S2CID 119102654.
  4. ^ What is a Planet? Debate Forces New Definition, by Robert Roy Britt, 2 November 2000
  5. ^ IAU WGESP, 'Position Statement on the Definition of "Planet"', 28 February 2003
  6. ^ Boss, Alan P.; Basri, Gibor; Kumar, Shiv S.; Liebert, James; Martín, Eduardo L.; Reipurth, Bo; Zinnecker, Hans (2003), "Nomenclature: Brown Dwarfs, Gas Giant Planets, and ?", Brown Dwarfs, 211: 529, Bibcode:2003IAUS..211..529B
  7. ^ Scholz, Alexander; Geers, Vincent; Jayawardhana, Ray; Fissel, Laura; Lee, Eve; Lafreniere, David; Tamura, Motohide (2009), "Substellar Objects in Nearby Young Clusters (Sonyc): The Bottom of the Initial Mass Function in Ngc 1333", The Astrophysical Journal, 702 (1): 805–822, arXiv:0907.2243v1, Bibcode:2009ApJ...702..805S, doi:10.1088/0004-637X/702/1/805, S2CID 5102383
  8. ^ Scholz, Aleks; Jayawardhana, Ray (2007), "Dusty disks at the bottom of the IMF", The Astrophysical Journal, 672 (1): L49–L52, arXiv:0711.2510v1, Bibcode:2008ApJ...672L..49S, doi:10.1086/526340