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 mass below the limiting mass for thermonuclear fusion of deuterium (about 13 MJ). Some researchers call them free-floating planets whereas others call them planetary-mass 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.
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 a 2007 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.
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:
- WISE 0855–0714 3–10 MJ about 7 light years away
- S Ori 52
- UGPS J072227.51-054031.2 10–25 MJ 13 light years away
- Cha 110913-773444 5–15 MJ 163 light years away
- CFBDSIR 2149−0403 4–7 MJ 130 light years away
- OTS 44 11.5 MJ 550 light years away
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- 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)
- Delorme, P.; et al. (December 2012). "CFBDSIR2149-0403: a 4–7 Jupiter-mass free-floating 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.
- 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.
- What is a Planet? Debate Forces New Definition, by Robert Roy Britt, 2 November 2000
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- 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