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.
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
At around 2022 the IAU working definition of an exoplanet excludes these objects as planets. The only fitting label would be as sub-brown dwarfs, but they are more often referred as planetary mass objects. Other definitions, like from the NASA Exoplanet Archive or the Extrasolar Planets Encyclopaedia would include these as exoplanets. There is no consensus whether these companions of brown dwarfs should be considered sub-brown dwarfs or planets.
WISE J0336−0143B, orbits a brown dwarf or sub-brown dwarf. The primary has a mass of 8.5 to 18 MJ and secondary has a mass of 5-11.5 MJ. This object does not fit the IAU working definition of an exoplanet. This definition requires a mass ratio of about q<0.04, but the mass ratio of WISE J0336−0143AB is q=0.61±0.05. It also does not fit the definition of a rogue planet, because it is gravitationally bound to a brown dwarf (or possibly sub-brown dwarf). Therefore it only fits the definition of sub-brown dwarf.
2M1207b orbits around a young brown dwarf with a circumstellar disk and itself is likely surrounded by a circumstellar disk. The mass ratio is well above the upper limit of q=0.04 for exoplanets.
Other examples of planetary-mass objects orbiting brown dwarfs and with MB<13 MJ and q>0.04:
- CFHTWIR-Oph 98B, with MB=11.6+0.4
−0.8 MJ and q=0.509+0.017
- 2MASS J04414489+2301513Bb, with MBb=9.8±1.8 MJ and 
- 2MASS J0249-0557ABc, with Mc=11.6+1.3
−1.0 MJ and 
Also called rogue planets:
- WISE 0855–0714 3–10 MJ about 7 light years away
- S Ori 52
- UGPS 0722-05 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
- Brown dwarf
- Fusor (astronomy)
- Giant planet
- Hot Jupiter
- Red dwarf
- Rogue planet
- Substellar object
- List of planet types
- Lists of astronomical objects
- 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 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.
- 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
- IAU WGESP, 'Position Statement on the Definition of "Planet"', 28 February 2003
- 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
- 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
- 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
- Etangs, A. Lecavelier des; Lissauer, Jack J. (17 March 2022). "The IAU Working Definition of an Exoplanet". New Astronomy Reviews. 94: 101641. arXiv:2203.09520. doi:10.1016/j.newar.2022.101641.
- Calissendorff, Per; De Furio, Matthew; Meyer, Michael; Albert, Loïc; Aganze, Christian; Ali-Dib, Mohamad; Bardalez Gagliuffi, Daniella C.; Baron, Frederique; Beichman, Charles A.; Burgasser, Adam J.; Cushing, Michael C.; Faherty, Jacqueline Kelly; Fontanive, Clémence; Gelino, Christopher R.; Gizis, John E. (1 April 2023). "JWST/NIRCam Discovery of the First Y+Y Brown Dwarf Binary: WISE J033605.05-014350.4". The Astrophysical Journal. 947: L30. doi:10.3847/2041-8213/acc86d. ISSN 0004-637X.
- Luhman, K. L.; Tremblin, P.; Birkmann, S. M.; Manjavacas, E.; Valenti, J.; Alves de Oliveira, C.; Beck, T. L.; Giardino, G.; Lützgendorf, N.; Rauscher, B. J.; Sirianni, M. (1 June 2023). "JWST/NIRSpec Observations of the Planetary Mass Companion TWA 27B". The Astrophysical Journal. 949: L36. doi:10.3847/2041-8213/acd635. ISSN 0004-637X.
- Fontanive, Clémence; Allers, Katelyn N.; Pantoja, Blake; Biller, Beth; Dubber, Sophie; Zhang, Zhoujian; Dupuy, Trent; Liu, Michael C.; Albert, Loïc (1 December 2020). "A Wide Planetary-mass Companion to a Young Low-mass Brown Dwarf in Ophiuchus". The Astrophysical Journal. 905: L14. doi:10.3847/2041-8213/abcaf8. ISSN 0004-637X.
- Bowler, Brendan P.; Hillenbrand, Lynne A. (1 October 2015). "Near-infrared Spectroscopy of 2M0441+2301 AabBab: A Quadruple System Spanning the Stellar to Planetary Mass Regimes". The Astrophysical Journal. 811: L30. doi:10.1088/2041-8205/811/2/L30. ISSN 0004-637X.
- Dupuy, Trent J.; Liu, Michael C.; Allers, Katelyn N.; Biller, Beth A.; Kratter, Kaitlin M.; Mann, Andrew W.; Shkolnik, Evgenya L.; Kraus, Adam L.; Best, William M. J. (1 August 2018). "The Hawaii Infrared Parallax Program. III. 2MASS J0249-0557 c: A Wide Planetary-mass Companion to a Low-mass Binary in the β Pic Moving Group". The Astronomical Journal. 156: 57. doi:10.3847/1538-3881/aacbc2. ISSN 0004-6256.